Cell_Biology_Alberts

Cell_Biology_Alberts. It is estimated that the human brain contains about 1011 neurons and 1014 synaptic connections. To make matters more complex, neural circuitry is continuously sculpted in response to experience, modified as we learn and store memories, and irreversibly altered by the gradual loss of neurons and their connections as we age. How can a system so complex be subject to such change and yet continue to function stably? One emerging theory suggests that individual neurons are self-tuning devices, constantly adjusting the expression of ion channels and neurotransmitter receptors in order to maintain a stable function. How might this work?


Cell_Biology_Alberts

Cell_Biology_Alberts. Neurons can be categorized into functionally different types, based in part on their propensity to fire action potentials and their pattern of firing. For example, some neurons fire action potentials at high frequencies, while others fire rarely. The firing properties of each neuron type are determined to a large extent by the ion channels that the cell expresses. The number of ion channels in a neuron’s membrane is not fixed: as conditions change, a neuron can modify the numbers of depolarizing (Na+ and Ca2+) and hyperpolarizing (K+) channels and keep their proportions adjusted so as to maintain its characteristic firing behavior—a remarkable example of homeostatic control. The molecular mechanisms involved remain an important mystery. Neuronal signals are transmitted from cell to cell at specialized sites of contact known as synapses. The usual mechanism of transmission is indirect. The cells are


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 11–35 Patch-clamp measurements for a single voltage-gated Na+ channel. A tiny patch of plasma membrane was detached from an embryonic rat muscle cell, as in Figure 11–34. (A) The membrane was depolarized by an abrupt shift of potential from –90 to about –40 mV. (B) Three current records from three experiments performed on the same patch of membrane. Each major current step in (B) represents the opening and closing of a single channel. A comparison of the three records shows that, whereas the durations of channel opening and closing vary greatly, the rate at which current flows through an open channel (its conductance) is practically constant. The minor fluctuations in the current records arise largely from electrical noise in the recording apparatus. Current flowing into the cell, measured in picoamperes (pA), is shown as a downward deflection of the curve. By convention, the electrical potential on the outside of the cell is defined as zero.


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Cell_Biology_Alberts. (C) The sum of the currents measured in 144 repetitions of the same experiment. This aggregate current is equivalent to the usual Na+ current that would be observed flowing through a relatively large region of membrane containing 144 channels. A comparison of (B) and (C) reveals that the time course of the aggregate current reflects the probability that any individual channel will be in the open state; this probability decreases with time as the channels in the depolarized membrane adopt their inactivated conformation. (Data from J. Patlak and R. Horn, J. Gen. Physiol. 79:333–351, 1982. With permission from The Rockefeller University Press.) nerve terminal of presynaptic cell neurotransmitter in synaptic vesicle


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 11–36 a chemical synapse. (A) When an action potential reaches the nerve terminal in a presynaptic cell, it stimulates the terminal to release its neurotransmitter. The neurotransmitter molecules are contained in synaptic vesicles and are released to the cell exterior when the vesicles fuse with the plasma membrane of the nerve terminal. The released neurotransmitter binds to and opens the transmitter-gated ion channels concentrated in the plasma membrane of the postsynaptic target cell at the synapse. The resulting ion flows alter the membrane potential of the postsynaptic membrane, thereby transmitting a signal from the excited nerve (B) (Movie 11.12). (B) A thin-section electron micrograph of two nerve terminal synapses on a dendrite of a postsynaptic cell. (B, courtesy of Cedric Raine.) electrically isolated from one another, the presynaptic cell being separated from the postsynaptic cell by a narrow synaptic cleft. When an action potential arrives at the presynaptic site,


Cell_Biology_Alberts

Cell_Biology_Alberts. Raine.) electrically isolated from one another, the presynaptic cell being separated from the postsynaptic cell by a narrow synaptic cleft. When an action potential arrives at the presynaptic site, the depolarization of the membrane opens voltage-gated Ca2+ channels that are clustered in the presynaptic membrane. Ca2+ influx triggers the release into the cleft of small signal molecules known as neurotransmitters, which are stored in membrane-enclosed synaptic vesicles and released by exocytosis (discussed in Chapter 13). The neurotransmitter diffuses rapidly across the synaptic cleft and provokes an electrical change in the postsynaptic cell by binding to and opening transmitter-gated ion channels (Figure 11–36). After the neurotransmitter has been secreted, it is rapidly removed: it is either destroyed by specific enzymes in the synaptic cleft or taken up by the presynaptic nerve terminal or by surrounding glial cells. Reuptake is mediated by a variety of Na+-dependent


Cell_Biology_Alberts

Cell_Biology_Alberts. it is either destroyed by specific enzymes in the synaptic cleft or taken up by the presynaptic nerve terminal or by surrounding glial cells. Reuptake is mediated by a variety of Na+-dependent neurotransmitter symporters (see Figure 11–8); in this way, neurotransmitters are recycled, allowing cells to keep up with high rates of release. Rapid removal ensures both spatial and temporal precision of signaling at a synapse. It decreases the chances that the neurotransmitter will influence neighboring cells, and it clears the synaptic cleft before the next pulse of neurotransmitter is released, so that the timing of repeated, rapid signaling events can be accurately communicated to the postsynaptic cell. As we shall see, signaling via such chemical synapses is far more versatile and adaptable than direct electrical coupling via gap junctions at electrical synapses (discussed in Chapter 19), which are also used by neurons but to a much smaller extent.


Cell_Biology_Alberts

Cell_Biology_Alberts. Transmitter-gated ion channels, also called ionotropic receptors, are built for rapidly converting extracellular chemical signals into electrical signals at chemical synapses. The channels are concentrated in a specialized region of the postsynaptic plasma membrane at the synapse and open transiently in response to the binding of neurotransmitter molecules, thereby producing a brief permeability change in the membrane (see Figure 11–36A). Unlike the voltage-gated channels responsible for action potentials, transmitter-gated channels are relatively insensitive to the membrane potential and therefore cannot by themselves produce a self-amplifying excitation. Instead, they produce local permeability increases, and hence changes of membrane potential, that are graded according to the amount of neurotransmitter released at the synapse and how long it persists there. Only if the summation of small depolarizations at this site opens sufficient numbers of nearby voltage-gated cation channels


Cell_Biology_Alberts

Cell_Biology_Alberts. neurotransmitter released at the synapse and how long it persists there. Only if the summation of small depolarizations at this site opens sufficient numbers of nearby voltage-gated cation channels can an action potential be triggered. This may require the opening of transmitter-gated ion channels at numerous synapses in close proximity on the target nerve cell.


Cell_Biology_Alberts

Cell_Biology_Alberts. Transmitter-gated ion channels differ from one another in several important ways. First, as receptors, they have highly selective binding sites for the neurotransmitter that is released from the presynaptic nerve terminal. Second, as channels, they are selective in the type of ions that they let pass across the plasma membrane; this determines the nature of the postsynaptic response. Excitatory neurotransmitters open cation channels, causing an influx of Na+, and in many cases Ca2+, that depolarizes the postsynaptic membrane toward the threshold potential for firing an action potential. Inhibitory neurotransmitters, by contrast, open either Cl– channels or K+ channels, and this suppresses firing by making it harder for excitatory neurotransmitters to depolarize the postsynaptic membrane. Many transmitters can be either excitatory or inhibitory, depending on where they are released, what receptors they bind to, and the ionic conditions that they encounter. Acetylcholine, for example,


Cell_Biology_Alberts

Cell_Biology_Alberts. Many transmitters can be either excitatory or inhibitory, depending on where they are released, what receptors they bind to, and the ionic conditions that they encounter. Acetylcholine, for example, can either excite or inhibit, depending on the type of acetylcholine receptors it binds to. Usually, however, acetylcholine, glutamate, and serotonin are used as excitatory transmitters, and γ-aminobutyric acid (GABA) and glycine are used as inhibitory transmitters. Glutamate, for instance, mediates most of the excitatory signaling in the vertebrate brain.


Cell_Biology_Alberts

Cell_Biology_Alberts. We have already discussed how the opening of Na+ or Ca2+ channels depolarizes a membrane. The opening of K+ channels has the opposite effect because the K+ concentration gradient is in the opposite direction—high concentration inside the cell, low outside. Opening K+ channels tends to keep the cell close to the equilibrium potential for K+, which, as we discussed earlier, is normally close to the resting membrane potential because at rest K+ channels are the main type of channel that is open. When additional K+ channels open, it becomes harder to drive the cell away from the resting state. We can understand the effect of opening Cl– channels similarly. The concentration of Cl– is much higher outside the cell than inside (see Table 11–1, p. 598), but the membrane potential opposes its influx. In fact, for many neurons, the equilibrium potential for Cl– is close to the resting potential—or even more negative. For this reason, opening Cl– channels tends to buffer the membrane potential;


Cell_Biology_Alberts

Cell_Biology_Alberts. In fact, for many neurons, the equilibrium potential for Cl– is close to the resting potential—or even more negative. For this reason, opening Cl– channels tends to buffer the membrane potential; as the membrane starts to depolarize, more negatively charged Cl– ions enter the cell and counteract the depolarization. Thus, the opening of Cl– channels makes it more difficult to depolarize the membrane and hence to excite the cell. Some powerful toxins act by blocking the action of inhibitory neurotransmitters: strychnine, for example, binds to glycine receptors and prevents their inhibitory action, causing muscle spasms, convulsions, and death.


Cell_Biology_Alberts

Cell_Biology_Alberts. However, not all chemical signaling in the nervous system operates through these ionotropic ligand-gated ion channels. In fact, most neurotransmitter molecules that are secreted by nerve terminals, including a large variety of neuropeptides, bind to metabotropic receptors, which regulate ion channels only indirectly through the action of small intracellular signal molecules (discussed in Chapter 15). All neurotransmitter receptors fall into one or other of these two major classes—ionotropic or metabotropic—on the basis of their signaling mechanisms: 1. Ionotropic receptors are ion channels and feature at fast chemical synapses. Acetylcholine, glycine, glutamate, and GABA all act on transmitter-gated ion channels, mediating excitatory or inhibitory signaling that is generally immediate, simple, and brief. 2.


Cell_Biology_Alberts

Cell_Biology_Alberts. 2. Metabotropic receptors are G-protein-coupled receptors (discussed in Chapter 15) that bind to all other neurotransmitters (and, confusingly, also acetylcholine, glutamate, and GABA). Signaling mediated by ligand-binding to metabotropic receptors tends to be far slower and more complex than that at ionotropic receptors, and longer-lasting in its consequences. The Acetylcholine Receptors at the Neuromuscular Junction Are Excitatory Transmitter-Gated Cation Channels


Cell_Biology_Alberts

Cell_Biology_Alberts. The Acetylcholine Receptors at the Neuromuscular Junction Are Excitatory Transmitter-Gated Cation Channels A well-studied example of a transmitter-gated ion channel is the acetylcholine receptor of skeletal muscle cells. This channel is opened transiently by acetylcholine released from the nerve terminal at a neuromuscular junction—the specialized chemical synapse between a motor neuron and a skeletal muscle cell (Figure 11–37). This synapse has been intensively investigated because it is readily accessible to electrophysiological study, unlike most of the synapses in the central nervous system, that is, the brain and spinal cord in vertebrates. Moreover, the acetylcholine receptors are densely packed in the muscle cell plasma membrane at a neuromuscular junction (about 20,000 such receptors per μm2), with relatively few receptors elsewhere in the same membrane.


Cell_Biology_Alberts

Cell_Biology_Alberts. The receptors are composed of five transmembrane polypeptides, two of one kind and three others, encoded by four separate genes (Figure 11–38A). The four genes are strikingly similar in sequence, implying that they evolved from a single ancestral gene. The two identical polypeptides in the pentamer each contribute one acetylcholine-binding site. When two acetylcholine molecules bind to the pentameric complex, they induce a conformational change that opens the channel. With ligand bound, the channel still flickers between open and closed states, but now it has a 90% probability of being open. This state continues—with acetylcholine binding and unbinding—until hydrolysis of the free acetylcholine by the enzyme acetylcholinesterase lowers its concentration at the neuromuscular junction sufficiently. Once freed of its bound neurotransmitter, the acetylcholine receptor reverts to its initial resting state. If the presence of acetylcholine persists for a prolonged time as a result of


Cell_Biology_Alberts

Cell_Biology_Alberts. sufficiently. Once freed of its bound neurotransmitter, the acetylcholine receptor reverts to its initial resting state. If the presence of acetylcholine persists for a prolonged time as a result of excessive nerve stimulation, the channel inactivates. Normally, the acetylcholine is rapidly hydrolyzed and the channel closes within about 1 millisecond, well before significant desensitization occurs. Desensitization would occur after about 20 milliseconds in the continued presence of acetylcholine.


Cell_Biology_Alberts

Cell_Biology_Alberts. The five subunits of the acetylcholine receptor are arranged in a ring, forming a water-filled transmembrane channel that consists of a narrow pore through the lipid bilayer, which widens into vestibules at both ends. Acetylcholine binding opens the channel by causing the helices that line the pore to rotate outward, thus disrupting a ring of hydrophobic amino acids that blocks ion flow in the closed state. Clusters of negatively charged amino acids at either end of the pore help to exclude negative ions and encourage any positive ion of diameter less than 0.65 nm to pass through (Figure 11–38B). The normal through-traffic consists chiefly of Na+ and K+, together with some Ca2+. Thus, unlike voltage-gated cation channels, such as the K+ channel discussed earlier, there is little selectivity among cations, and the relative contributions of the different cations to the current through the channel depend chiefly on their concentrations and on the electrochemical driving forces. When the


Cell_Biology_Alberts

Cell_Biology_Alberts. among cations, and the relative contributions of the different cations to the current through the channel depend chiefly on their concentrations and on the electrochemical driving forces. When the muscle cell membrane is at its resting potential, the net driving force for K+ is near zero, since the voltage gradient nearly balances the K+ concentration gradient across the membrane (see Panel 11–1, p. 616). For Na+, in contrast, the voltage gradient and the concentration gradient both act in the same direction to drive the ion into the cell. (The same is true for Ca2+, but the


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 11–37 a low-magnification scanning electron micrograph of a neuromuscular junction in a frog. The termination of a single axon on a skeletal muscle cell is shown. (From J. Desaki and Y. Uehara, J. Neurocytol. 10:101–110, 1981. With permission from Kluwer Academic Publishers.) extracellular concentration of Ca2+ is so much lower than that of Na+ that Ca2+ makes only a small contribution to the total inward current.) Therefore, the opening of the acetylcholine-receptor channels leads to a large net influx of Na+ (a peak rate of about 30,000 ions per channel each millisecond). This influx causes a membrane depolarization that signals the muscle to contract, as discussed below. Neurons Contain Many Types of Transmitter-Gated Channels


Cell_Biology_Alberts

Cell_Biology_Alberts. Neurons Contain Many Types of Transmitter-Gated Channels The ion channels that open directly in response to the neurotransmitters acetylcholine, serotonin, GABA, and glycine contain subunits that are structurally similar and probably form transmembrane pores in the same way as the ionotropic acetylcholine receptor, even though they have distinct neurotransmitter-binding specificities and ion selectivities. These channels are all built from homologous polypeptide subunits, which assemble as a pentamer. Glutamate-gated ion channels are an exception, in that they are constructed from a distinct family of subunits and form tetramers resembling the K+ channels discussed earlier (see Figure 11–24A).


Cell_Biology_Alberts

Cell_Biology_Alberts. For each class of transmitter-gated ion channel, there are alternative forms of each type of subunit, which may be encoded by distinct genes or else generated by alternative RNA splicing of a single gene product. The subunits assemble in different combinations to form an extremely diverse set of distinct channel subtypes, with different ligand affinities, different channel conductances, different rates of opening and closing, and different sensitivities to drugs and toxins. Some vertebrate neurons, for example, have acetylcholine-gated ion channels that differ from those of muscle cells in that they are formed from two subunits of one type and three of another; but there are at least nine genes coding for different versions of the first type of subunit and at least three coding for different versions of the second. Subsets of such neurons performing different functions in the brain express different combinations of the genes for these subunits. In principle, and already to some extent


Cell_Biology_Alberts

Cell_Biology_Alberts. versions of the second. Subsets of such neurons performing different functions in the brain express different combinations of the genes for these subunits. In principle, and already to some extent in practice, it is possible to design drugs targeted against these narrowly defined subsets, thereby specifically influencing particular brain functions.


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Cell_Biology_Alberts. Transmitter-gated ion channels have for a long time been important drug targets. A surgeon, for example, can relax muscles for the duration of an operation


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Cell_Biology_Alberts. Figure 11–38 a model for the structure of the skeletal muscle acetylcholine receptor. (A) Five homologous subunits (α, α, β, γ, δ) combine to form a transmembrane pore. Both of the α subunits contribute an acetylcholinebinding site nestled between adjoining subunits. (B) The pore is lined by a ring of five transmembrane α helices, one contributed by each subunit (just the two α subunits are shown). In its closed conformation, the pore is occluded by the hydrophobic side chains of five leucines (green), one from each α helix, which form a gate near the middle of the lipid bilayer. When acetylcholine binds to both α subunits, the channel undergoes a conformational change that opens the gate by an outward rotation of the helices containing the occluding leucines. Negatively charged side chains (indicated by the “–“ signs) at either end of the pore ensure that only positively charged ions pass through the channel. (PDB code: 2BG9.) by blocking the acetylcholine receptors on skeletal


Cell_Biology_Alberts

Cell_Biology_Alberts. chains (indicated by the “–“ signs) at either end of the pore ensure that only positively charged ions pass through the channel. (PDB code: 2BG9.) by blocking the acetylcholine receptors on skeletal muscle cells with curare, a plant-derived drug that was originally used by South American Indians to make poison arrows. Most drugs used to treat insomnia, anxiety, depression, and schizophrenia exert their effects at chemical synapses, and many of these act by binding to transmitter-gated channels. Barbiturates, tranquilizers such as Valium, and sleeping pills such as Ambien, for example, bind to GABA receptors, potentiating the inhibitory action of GABA by allowing lower concentrations of this neurotransmitter to open Cl– channels. Our increasing understanding of the molecular biology of ion channels should allow us to design a new generation of psychoactive drugs that will act still more selectively to alleviate the miseries of mental illness.


Cell_Biology_Alberts

Cell_Biology_Alberts. In addition to ion channels, many other components of the synaptic signaling machinery are potential targets for psychoactive drugs. As mentioned earlier, after release into the synaptic cleft, many neurotransmitters are cleared by reuptake mechanisms mediated by Na+-driven symports. Inhibiting such transporters prolongs the effect of the neurotransmitter, thereby strengthening synaptic transmission. Many antidepressant drugs, including Prozac, inhibit the reuptake of serotonin; others inhibit the reuptake of both serotonin and norepinephrine. Ion channels are the basic molecular units from which neuronal devices for signaling and computation are built. To provide a glimpse of how sophisticated these devices can be, we consider several examples that demonstrate how the coordinated activities of groups of ion channels allow you to move, feel, and remember. Neuromuscular Transmission Involves the Sequential Activation of Five Different Sets of Ion Channels


Cell_Biology_Alberts

Cell_Biology_Alberts. Neuromuscular Transmission Involves the Sequential Activation of Five Different Sets of Ion Channels The following process, in which a nerve impulse stimulates a muscle cell to contract, illustrates the importance of ion channels to electrically excitable cells. This apparently simple response requires the sequential activation of at least five different sets of ion channels, all within a few milliseconds (Figure 11–39). 1. The process is initiated when a nerve impulse reaches the nerve terminal and depolarizes the plasma membrane of the terminal. The depolarization transiently opens voltage-gated Ca2+ channels in this presynaptic membrane. As the Ca2+ concentration outside cells is more than 1000 times Figure 11–39 The system of ion channels at a neuromuscular junction. These gated ion channels are essential for the stimulation of muscle contraction by a nerve impulse. The various channels are numbered in the sequence in which they are activated, as described in the text.


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Cell_Biology_Alberts. greater than the free Ca2+ concentration inside, Ca2+ flows into the nerve terminal. The increase in Ca2+ concentration in the cytosol of the nerve terminal triggers the local release of acetylcholine by exocytosis into the synaptic cleft. 2. The released acetylcholine binds to acetylcholine receptors in the muscle cell plasma membrane, transiently opening the cation channels associated with them. The resulting influx of Na+ causes a local membrane depolarization. 3. The local depolarization opens voltage-gated Na+ channels in this membrane, allowing more Na+ to enter, which further depolarizes the membrane. This, in turn, opens neighboring voltage-gated Na+ channels and results in a self-propagating depolarization (an action potential) that spreads to involve the entire plasma membrane (see Figure 11–31). 4.


Cell_Biology_Alberts

Cell_Biology_Alberts. 4. The generalized depolarization of the muscle cell plasma membrane activates voltage-gated Ca2+ channels in the transverse tubules (T tubules— discussed in Chapter 16) of this membrane. 5. This in turn causes Ca2+-release channels in an adjacent region of the sarcoplasmic reticulum (SR) membrane to open transiently and release Ca2+ stored in the SR into the cytosol. The T-tubule and SR membranes are closely apposed with the two types of channel joined together in a specialized structure, in which activation of the voltage-sensitive Ca2+ channel in the T-tubule plasma membrane causes a channel conformational change that is mechanically transmitted to the Ca2+-release channel in the SR membrane, opening it and allowing Ca2+ to flow from the SR lumen into the cytoplasm (see Figure 16–35). The sudden increase in the cytosolic Ca2+ concentration causes the myofibrils in the muscle cell to contract.


Cell_Biology_Alberts

Cell_Biology_Alberts. Whereas the initiation of muscle contraction by a motor neuron is complex, an even more sophisticated interplay of ion channels is required for a neuron to integrate a large number of input signals at its synapses and compute an appropriate output, as we now discuss. In the central nervous system, a single neuron can receive inputs from thousands of other neurons, and it can in turn form synapses with many thousands of other cells. Several thousand nerve terminals, for example, make synapses on an average motor neuron in the spinal cord, almost completely covering its cell body and dendrites (Figure 11–40). Some of these synapses transmit signals from the brain or spinal cord; others bring sensory information from muscles or from the skin. The motor neuron must combine the information received from all these sources and react, either by firing action potentials along its axon or by remaining quiet.


Cell_Biology_Alberts

Cell_Biology_Alberts. Of the many synapses on a neuron, some tend to excite it, while others inhibit it. Neurotransmitter released at an excitatory synapse causes a small depolarization in the postsynaptic membrane called an excitatory postsynaptic potential (excitatory PSP), whereas neurotransmitter released at an inhibitory synapse generally causes a small hyperpolarization called an inhibitory PSP. The plasma membrane of the dendrites and cell body of most neurons contains a relatively low density of voltage-gated Na+ channels, and so an individual excitatory PSP is generally too small to trigger an action potential. Instead, each incoming signal initiates a local PSP, which decreases with distance from the site of the synapse. If signals arrive simultaneously at several synapses in the same region of the dendritic tree, the total PSP in that neighborhood will be roughly the sum of the individual PSPs, with inhibitory PSPs making a negative contribution to the total. The PSPs from each neighborhood


Cell_Biology_Alberts

Cell_Biology_Alberts. dendritic tree, the total PSP in that neighborhood will be roughly the sum of the individual PSPs, with inhibitory PSPs making a negative contribution to the total. The PSPs from each neighborhood spread passively and converge on the cell body. For long-distance transmission, the combined magnitude of the PSP is then translated, or encoded, into the frequency of firing of action potentials: the greater the stimulation (depolarization), the higher the frequency of action potentials.


Cell_Biology_Alberts

Cell_Biology_Alberts. 0.1 mm Neuronal Computation Requires a Combination of at Least Three Kinds of K+ Channels The intensity of stimulation that a neuron receives is encoded by that neuron into action potential frequency for long-distance transmission. The encoding takes place at a specialized region of the axonal membrane known as the initial segment, or axon hillock, at the junction of the axon and the cell body (see Figure 11–40). This membrane is rich in voltage-gated Na+ channels; but it also contains at least four other classes of ion channels—three selective for K+ and one selective for Ca2+—all of which contribute to the axon hillock’s encoding function. The three varieties of K+ channels have different properties; we shall refer to them as delayed, rapidly inactivating, and Ca2+-activated K+ channels.


Cell_Biology_Alberts

Cell_Biology_Alberts. To understand the need for multiple types of channels, consider first what would happen if the only voltage-gated ion channels present in the nerve cell were the Na+ channels. Below a certain threshold level of synaptic stimulation, the depolarization of the initial-segment membrane would be insufficient to trigger an action potential. With gradually increasing stimulation, the threshold would be crossed, the Na+ channels would open, and an action potential would fire. The action potential would be terminated by inactivation of the Na+ channels. Before another action potential could fire, these channels would have to recover from their inactivation. But that would require a return of the membrane voltage to a very negative value, which would not occur as long as the strong depolarizing stimulus (from PSPs) was maintained. An additional channel type is needed, therefore, to repolarize the membrane after each action potential to prepare the cell to fire again.


Cell_Biology_Alberts

Cell_Biology_Alberts. The delayed K+ channels perform this task, as discussed previously in relation to the propagation of the action potential (see Figure 11–31). They are voltage-gated, but because of their slower kinetics they open only during the falling phase of the action potential, when the Na+ channels are inactive. Their opening permits an efflux of K+ that drives the membrane back toward the K+ equilibrium potential, which is so negative that the Na+ channels rapidly recover from their inactivated state. Repolarization of the membrane also closes the delayed K+ channels. The initial segment is now reset so that the depolarizing stimulus from


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 11–40 a motor neuron in the spinal cord. (A) Many thousands of nerve terminals synapse on the cell body and dendrites. These deliver signals from other parts of the organism to control the firing of action potentials along the single axon of this large cell. (B) Fluorescence micrograph showing a nerve cell body and its dendrites stained with a fluorescent antibody that recognizes a cytoskeletal protein (green) that is not present in axons. Thousands of axon terminals (red) from other nerve cells (not visible) make synapses on the cell body and dendrites; the terminals are stained with a fluorescent antibody that recognizes a protein in synaptic vesicles. (B, courtesy of Olaf Mundigl and Pietro de Camilli.) frequency of fring(action potentials per second) magnitude of combined PSP synaptic inputs can fire another action potential. In this way, sustained stimulation of the dendrites and cell body leads to repetitive firing of the axon.


Cell_Biology_Alberts

Cell_Biology_Alberts. Repetitive firing in itself, however, is not enough. The frequency of firing has to reflect the intensity of stimulation, and a simple system of Na+ channels and delayed K+ channels is inadequate for this purpose. Below a certain threshold level of steady stimulation, the cell will not fire at all; above that threshold level, it will abruptly begin to fire at a relatively rapid rate. The rapidly inactivating K+ channels solve the problem. These, too, are voltage-gated and open when the membrane is depolarized, but their specific voltage sensitivity and kinetics of inactivation are such that they act to reduce the rate of firing at levels of stimulation that are only just above the threshold required for firing. Thus, they remove the discontinuity in the relationship between the firing rate and the intensity of stimulation. The result is a firing rate that is proportional to the strength of the depolarizing stimulus over a very broad range (Figure 11–41).


Cell_Biology_Alberts

Cell_Biology_Alberts. The process of encoding is usually further modulated by the two other types of ion channels in the initial segment that were mentioned earlier—voltage-gated Ca2+ channels and Ca2+-activated K+ channels. They act together to decrease the response of the cell to an unchanging, prolonged stimulation—a process called adaptation. These Ca2+ channels are similar to the Ca2+ channels that mediate the release of neurotransmitter from presynaptic axon terminals; they open when an action potential fires, transiently allowing Ca2+ into the axon cytosol at the initial segment.


Cell_Biology_Alberts

Cell_Biology_Alberts. The Ca2+-activated K+ channel opens in response to a raised concentration of Ca2+ at the channel’s cytoplasmic face (Figure 11–42). Prolonged, strong depolarizing stimuli will trigger a long train of action potentials, each of which permits a brief influx of Ca2+ through the voltage-gated Ca2+ channels, so that local cytosolic Ca2+ concentration gradually builds up to a level high enough to open the Ca2+-activated K+ channels. Because the resulting increased permeability of the membrane to K+ makes the membrane harder to depolarize, the delay between one action potential and the next is increased. In this way, a neuron that is stimulated continuously for a prolonged period becomes gradually less responsive to the constant stimulus.


Cell_Biology_Alberts

Cell_Biology_Alberts. Such adaptation, which can also occur by other mechanisms, allows a neuron—indeed, the nervous system generally—to react sensitively to change, even against a high background level of steady stimulation. It is one of the computational strategies that help us, for example, to feel a light touch on the shoulder and yet ignore the constant pressure of our clothing. We discuss adaptation as a general feature in cell signaling processes in more detail in Chapter 15. Other neurons do different computations, reacting to their synaptic inputs in myriad ways, reflecting the different assortments of ion channels in their membrane. There are several hundred genes that code for ion channels in the human genome, with over 150 encoding voltage-gated channels alone. Further complexity is introduced by alternative splicing of RNA transcripts and assembling channel subunits in different combinations. Moreover, ion channels are selectively


Cell_Biology_Alberts

Cell_Biology_Alberts. Figure 11–41 The magnitude of the combined postsynaptic potential (PSP) is reflected in the frequency of firing of action potentials. The mix of excitatory and inhibitory PSPs produces a combined PSP at the initial segment. A comparison of (A) and (B) shows how the firing frequency of an axon increases with an increase in the combined PSP, while (C) summarizes the general relationship.


Cell_Biology_Alberts

Cell_Biology_Alberts. localized to different sites in the plasma membrane of a neuron. Some K+ and Ca2+ channels are concentrated in the dendrites and participate in processing the input that a neuron receives. As we have seen, other ion channels are located at the axon’s initial segment, where they control action potential firing; and some ligand-gated channels are distributed over the cell body and, depending on their ligand occupancy, modulate the cell’s general sensitivity to synaptic inputs. The multiplicity of ion channels and their locations evidently allows each of the many types of neurons to tune the electrical behavior to the particular tasks they perform.


Cell_Biology_Alberts

Cell_Biology_Alberts. One of the crucial properties of the nervous system is its ability to learn and remember. This property depends in part on the ability of individual synapses to strengthen or weaken depending on their use—a process called synaptic plasticity. We end this chapter by considering a remarkable type of ion channel that has a special role in some forms of synaptic plasticity. It is located at many excitatory synapses in the central nervous system, where it is gated by both voltage and the excitatory neurotransmitter glutamate. It is also the site of action of the psychoactive drug phencyclidine, or angel dust. Long-Term Potentiation (LTP) in the Mammalian Hippocampus Depends on Ca2+ Entry Through NMDA-Receptor Channels


Cell_Biology_Alberts

Cell_Biology_Alberts. Practically all animals can learn, but mammals seem to learn exceptionally well (or so we like to think). In a mammal’s brain, the region called the hippocampus has a special role in learning. When it is destroyed on both sides of the brain, the ability to form new memories is largely lost, although previous long-established memories remain. Some synapses in the hippocampus show a striking form of synaptic plasticity with repeated use: whereas occasional single action potentials in the presynaptic cells leave no lasting trace, a short burst of repetitive firing causes long-term potentiation (LTP), such that subsequent single action potentials in the presynaptic cells evoke a greatly enhanced response in the postsynaptic cells. The effect lasts hours, days, or weeks, according to the number and intensity of the bursts of repetitive firing. Only the synapses that were activated exhibit LTP; synapses that have remained quiet on the same postsynaptic cell are not affected. However, while


Cell_Biology_Alberts

Cell_Biology_Alberts. and intensity of the bursts of repetitive firing. Only the synapses that were activated exhibit LTP; synapses that have remained quiet on the same postsynaptic cell are not affected. However, while the cell is receiving a burst of repetitive stimulation via one set of synapses, if a single action potential is delivered at another synapse on its surface, that latter synapse also will undergo LTP, even though a single action potential delivered there at another time would leave no such lasting trace.


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Cell_Biology_Alberts. The underlying rule in such events seems to be that LTP occurs on any occasion when a presynaptic cell fires (once or more) at a time when the postsynaptic membrane is strongly depolarized (either through recent repetitive firing of the same presynaptic cell or by other means). This rule reflects the behavior of a particular class of ion channels in the postsynaptic membrane. Glutamate is the main excitatory neurotransmitter in the mammalian central nervous system, and glutamate-gated ion channels are the most common of all transmitter-gated channels in the brain. In the hippocampus, as elsewhere, most of the depolarizing current responsible for excitatory PSPs is carried by glutamate-gated ion channels called AMPA receptors, which operate in the standard way (Figure 11–43). But the current has, in addition, a second and more intriguing component, which is mediated by a separate subclass of glutamate-gated ion channels known as NMDA receptors, so named because they are selectively


Cell_Biology_Alberts

Cell_Biology_Alberts. has, in addition, a second and more intriguing component, which is mediated by a separate subclass of glutamate-gated ion channels known as NMDA receptors, so named because they are selectively activated by the artificial glutamate analog N-methyl-D-aspartate. The NMDA-receptor channels are doubly gated, opening only when two conditions are satisfied simultaneously: glutamate must be bound to the receptor, and the membrane must be strongly depolarized. The second condition is required for releasing the Mg2+ that normally blocks the resting channel. This means that NMDA receptors are normally activated only when AMPA receptors are activated as well and depolarize the membrane. The NMDA receptors are critical for LTP. When they are selectively blocked with a specific inhibitor or inactivated genetically, LTP does not occur, even though ordinary synaptic transmission continues, indicating the importance of NMDA receptors


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Cell_Biology_Alberts. Figure 11–42 Structure of a Ca2+activated K+ channel. The channel contains four identical subunits (which are shown in different colors for clarity). It is both voltageand Ca2+-gated. The structure shown is a composite of the cytosolic and membrane portions of the channel that were separately crystallized. (PDB codes: 2R99, 1LNQ.) Figure 11–43 The structure of the aMPa receptor. This ionotropic glutamate receptor (named after the glutamate analog α-Amino 3-hydroxy 5-Methyl 4-isoxazole Propionic Acid) is the most common mediator of fast, excitatory synaptic transmission in the central nervous system (CNS). (PDB code: 3KG2.) AMPA-receptor channels, depolarization removes Mg2+ block from NMDA-allowing Na+ infux receptor channel, which that depolarizes (with glutamate bound) the postsynaptic allows Ca2+ to enter the membrane postsynaptic cell for LTP induction. Such animals exhibit specific deficits in their learning abilities but behave almost normally otherwise.


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Cell_Biology_Alberts. How do NMDA receptors mediate LTP? The answer is that these channels, when open, are highly permeable to Ca2+, which acts as an intracellular signal in the postsynaptic cell, triggering a cascade of changes that are responsible for LTP. Thus, LTP is prevented when Ca2+ levels are held artificially low in the postsynaptic cell by injecting the Ca2+ chelator EGTA into it, and LTP can be induced by artificially raising intracellular Ca2+ levels in the cell. Among the long-term changes that increase the sensitivity of the postsynaptic cell to glutamate is the insertion of new AMPA receptors into the plasma membrane (Figure 11–44). In some forms of LTP, changes occur in the presynaptic cell as well, so that it releases more glutamate than normal when it is activated subsequently.


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Cell_Biology_Alberts. If synapses were capable only of LTP they would quickly become saturated, and thus be of limited value as an information-storage device. In fact, they also exhibit long-term depression (LTD), with the long-term effect of reducing the number of AMPA receptors in the post-synaptic membrane. This feat is accomplished by degrading AMPA receptors after their selective endocytosis. Surprisingly, LTD also requires NMDA receptor activation and a rise in Ca2+. How does Ca2+ trigger opposite effects at the same synapse? It turns out that this bidirectional control of synaptic strength depends on the magnitude of the rise in Ca2+: high Ca2+ levels activate protein kinases and LTP, whereas modest Ca2+ levels activate protein phosphatases and LTD.


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Cell_Biology_Alberts. There is evidence that NMDA receptors have an important role in synaptic plasticity and learning in other parts of the brain, as well as in the hippocampus. Moreover, they have a crucial role in adjusting the anatomical pattern of synaptic connections in the light of experience during the development of the nervous system. Thus, neurotransmitters released at synapses, besides relaying transient electrical signals, can also alter concentrations of intracellular mediators that bring about lasting changes in the efficacy of synaptic transmission. However, it is still uncertain how these changes endure for weeks, months, or a lifetime in the face of the normal turnover of cell constituents.


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Cell_Biology_Alberts. Ion channels form aqueous pores across the lipid bilayer and allow inorganic ions of appropriate size and charge to cross the membrane down their electrochemical gradients at rates about 1000 times greater than those achieved by any known transporter. The channels are “gated” and usually open transiently in response to a specific perturbation in the membrane, such as a change in membrane potential (voltage-gated channels), or the binding of a neurotransmitter to the channel (transmitter-gated channels). K+-selective leak channels have an important role in determining the resting membrane potential across the plasma membrane in most animal cells. increased Ca2+ in the cytosol induces postsynaptic cell to insert new AMPA receptors in the plasma membrane, increasing the cell's sensitivity to glutamate


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Cell_Biology_Alberts. increased Ca2+ in the cytosol induces postsynaptic cell to insert new AMPA receptors in the plasma membrane, increasing the cell's sensitivity to glutamate Figure 11–44 The signaling events in longterm potentiation. Although not shown, transmission-enhancing changes can also occur in the presynaptic nerve terminals in LTP, which may be induced by retrograde signals from the postsynaptic cell. Voltage-gated cation channels are responsible for the amplification and propaga-WHAT WE DON’T KNOW tion of action potentials in electrically excitable cells, such as neurons and skeletal muscle cells. Transmitter-gated ion channels convert chemical signals to electrical How do individual neurons establish signals at chemical synapses. Excitatory neurotransmitters, such as acetylcholine and maintain their characteristic and glutamate, open transmitter-gated cation channels and thereby depolarize the intrinsic firing properties?


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Cell_Biology_Alberts. postsynaptic membrane toward the threshold level for firing an action potential. Inhibitory neurotransmitters, such as GABA and glycine, open transmitter-gated


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Cell_Biology_Alberts. Even organisms with very simple Cl– or K+ channels and thereby suppress firing by keeping the postsynaptic mem-nervous systems have dozens of brane polarized. A subclass of glutamate-gated ion channels, called NMDA-recep-different K+ channels. Why is it tor channels, is highly permeable to Ca2+, which can trigger the long-term changes important to have so many? in synapse efficacy (synaptic plasticity) such as LTP and LTD that are thought to be involved in some forms of learning and memory. • Why do cells that are not electrically Ion channels work together in complex ways to control the behavior of electri-active contain voltage-gated ion cally excitable cells. A typical neuron, for example, receives thousands of excitatory channels? and inhibitory inputs, which combine by spatial and temporal summation to produce a combined postsynaptic potential (PSP) at the initial segment of its axon. The • How are memories stored for so magnitude of the PSP is translated into the rate of firing


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Cell_Biology_Alberts. summation to produce a combined postsynaptic potential (PSP) at the initial segment of its axon. The • How are memories stored for so magnitude of the PSP is translated into the rate of firing of action potentials by a many years in the human brain? mixture of cation channels in the initial segment membrane.


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Cell_Biology_Alberts. Which statements are true? explain why or why not. 11–1 Transport by transporters can be either active or b. Does the linked action of these two pumps cause passive, whereas transport by channels is always passive. imbalances in either the K+ concentration or the membrane potential? Why or why not? 11–2 Transporters saturate at high concentrations of 11–7 Microvilli increase the surface area of intestinal the transported molecule when all their binding sites are cells, providing more efficient absorption of nutrients.


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Cell_Biology_Alberts. occupied; channels, on the other hand, do not bind the Microvilli are shown in profile and cross section in Figure ions they transport and thus the flux of ions through a Q11–1. From the dimensions given in the figure, estimate channel does not saturate. the increase in surface area that microvilli provide (for 11–3 The membrane potential arises from movements the portion of the plasma membrane in contact with the of charge that leave ion concentrations practically unaf-lumen of the gut) relative to the corresponding surface of a fected, causing only a very slight discrepancy in the num-cell with a “flat” plasma membrane. ber of positive and negative ions on the two sides of the membrane. Discuss the following problems. 11–4 Order Ca2+, CO2, ethanol, glucose, RNA, and H2O according to their ability to diffuse through a lipid bilayer, beginning with the one that crosses the bilayer most readily. Explain your order.


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Cell_Biology_Alberts. 11–4 Order Ca2+, CO2, ethanol, glucose, RNA, and H2O according to their ability to diffuse through a lipid bilayer, beginning with the one that crosses the bilayer most readily. Explain your order. 11–5 How is it possible for some molecules to be at 1 µm 0.1 µm equilibrium across a biological membrane and yet not be at the same concentration on both sides?


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Cell_Biology_Alberts. Figure Q11–1 Microvilli of intestinal epithelial cells in profile and cross section (Problem 11–7). (Left panel, from Rippel Electron Microscope 11–6 Ion transporters are “linked” together—not physi-Facility, Dartmouth College; right panel, from David Burgess.) cally, but as a consequence of their actions. For example, cells can raise their intracellular pH, when it becomes too acidic, by exchanging external Na+ for internal H+, using 11–8 According to Newton’s laws of motion, an ion a Na+–H+ antiporter. The change in internal Na+ is then exposed to an electric field in a vacuum would experience redressed using the Na+-K+ pump. a constant acceleration from the electric driving force, just a. Can these two transporters, operating together, as a falling body in a vacuum constantly accelerates due to normalize both the H+ and the Na+ concentrations inside gravity. In water, however, an ion moves at constant velocthe cell? ity in an electric field. Why do you suppose that is? 21.4 nm


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Cell_Biology_Alberts. 21.4 nm Figure Q11–2 A “ball” tethered by a “chain” to a voltage-gated K+ channel (Problem 11–9). 11–9 In a subset of voltage-gated K+ channels, the N-terminus of each subunit acts like a tethered ball that occludes the cytoplasmic end of the pore soon after it opens, thereby inactivating the channel. This “ball-andchain” model for the rapid inactivation of voltage-gated K+ channels has been elegantly supported for the shaker K+ channel from Drosophila melanogaster. (The shaker K+ channel in Drosophila is named after a mutant form that causes excitable behavior—even anesthetized flies keep twitching.) Deletion of the N-terminal amino acids from the normal shaker channel gives rise to a channel that opens in response to membrane depolarization, but stays open instead of rapidly closing as the normal channel does. A peptide (MAAVAGLYGLGEDRQHRKKQ) that corresponds to the deleted N-terminus can inactivate the open channel at 100 µM.


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Cell_Biology_Alberts. Is the concentration of free peptide (100 µM) that is required to inactivate the defective K+ channel anywhere near the local concentration of the tethered ball on a normal channel? Assume that the tethered ball can explore a hemisphere [volume = (2/3)πr3] with a radius of 21.4 nm, which is the length of the polypeptide “chain” (Figure Q11–2). Calculate the concentration for one ball in this hemisphere. How does that value compare with the concentration of free peptide needed to inactivate the channel? 11–10 The giant axon of the squid (Figure Q11–3) occupies a unique position in the history of our understanding of cell membrane potentials and nerve action. When an electrode is stuck into an intact giant axon, the membrane potential registers –70 mV. When the axon, suspended in a bath of seawater, is stimulated to conduct a nerve impulse, the membrane potential changes transiently from –70 mV to +40 mV.


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Cell_Biology_Alberts. Table Q11–1 Ionic composition of seawater and of the cytosol in the squid giant axon (Problem 11–10). For univalent ions and at 20°C (293 K), the Nernst equation reduces to where Co and Ci are the concentrations outside and inside, respectively. Using this equation, calculate the potential across the resting membrane (1) assuming that it is due solely to K+ and (2) assuming that it is due solely to Na+. (The Na+ and K+ concentrations in the axon cytosol and in seawater are given in Table Q11–1.) Which calculation is closer to the measured resting potential? Which calculation is closer to the measured action potential? Explain why these assumptions approximate the measured resting and action potentials. 11–11 Acetylcholine-gated cation channels at the neuromuscular junction open in response to acetylcholine released by the nerve terminal and allow Na+ ions to enter the muscle cell, which causes membrane depolarization and ultimately leads to muscle contraction. a.


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Cell_Biology_Alberts. a. Patch-clamp measurements show that young rat muscles have cation channels that respond to acetylcholine (Figure Q11–4). How many kinds of channel are there? How can you tell? b. For each kind of channel, calculate the number of ions that enter in one millisecond. (One ampere is a current of one coulomb per second; one pA equals 10–12 ampere. An ion with a single charge such as Na+ carries a charge of 1.6 × 10–19 coulomb.) Figure Q11–4 Patch-clamp measurements of acetylcholine-gated cation channels in young rat muscle (Problem 11–11). Figure Q11–3 The squid Loligo (Problem 11–10). This squid is about 15 cm in length. Engel A & Gaub HE (2008) Structure and mechanics of membrane proteins. Annu. Rev. Biochem. 77, 127–148. Hille B (2001) Ionic Channels of Excitable Membranes, 3rd ed. Sunderland, MA: Sinauer. Stein WD (2014) Channels, Carriers, and Pumps: An Introduction to Membrane Transport, 2nd ed. San Diego, CA: Academic Press.


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Cell_Biology_Alberts. Stein WD (2014) Channels, Carriers, and Pumps: An Introduction to Membrane Transport, 2nd ed. San Diego, CA: Academic Press. Vinothkumar KR & Henderson R (2010) Structures of membrane proteins. Q. Rev. Biophys. 43, 65–158. Principles of Membrane Transport Al-Awqati Q (1999) One hundred years of membrane permeability: does Overton still rule? Nat. Cell Biol. 1, E201–E202. Forrest LR & Sansom MS (2000) Membrane simulations: bigger and better? Curr. Opin. Struct. Biol. 10, 174–181. Gouaux E & MacKinnon R (2005) Principles of selective ion transport in channels and pumps. Science 310, 1461–1465. Mitchell P (1977) Vectorial chemiosmotic processes. Annu. Rev. Biochem. 46, 996–1005. Tanford C (1983) Mechanism of free energy coupling in active transport. Annu. Rev. Biochem. 52, 379–409. Almers W & Stirling C (1984) Distribution of transport proteins over animal cell membranes. J. Membr. Biol. 77, 169–186.


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Cell_Biology_Alberts. Almers W & Stirling C (1984) Distribution of transport proteins over animal cell membranes. J. Membr. Biol. 77, 169–186. Baldwin SA & Henderson PJ (1989) Homologies between sugar transporters from eukaryotes and prokaryotes. Annu. Rev. Physiol. 51, 459–471. Doige CA & Ames GF (1993) ATP-dependent transport systems in bacteria and humans: relevance to cystic fibrosis and multidrug resistance. Annu. Rev. Microbiol. 47, 291–319. Forrest LR & Rudnick G (2009) The rocking bundle: a mechanism for ion-coupled solute flux by symmetrical transporters. Physiology 24, 377–386. Gadsby DC (2009) Ion channels versus ion pumps: the principal difference, in principle. Nat. Rev. Mol. Cell Biol. 10, 344–352. Higgins CF (2007) Multiple molecular mechanisms for multidrug resistance transporters. Nature 446, 749–757. Kaback HR, Sahin-Tóth M & Weinglass AB (2001) The kamikaze approach to membrane transport. Nat. Rev. Mol. Cell Biol. 2, 610–620.


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Cell_Biology_Alberts. Kaback HR, Sahin-Tóth M & Weinglass AB (2001) The kamikaze approach to membrane transport. Nat. Rev. Mol. Cell Biol. 2, 610–620. Kühlbrandt W (2004) Biology, structure and mechanism of P-type ATPases. Nat. Rev. Mol. Cell Biol. 5, 282–295. Lodish HF (1986) Anion-exchange and glucose transport proteins: structure, function, and distribution. Harvey Lect. 82, 19–46. Møller JV, Olesen C, Winther AML & Nissen P (2010) The sarcoplasmic Ca2+-ATPase: design of a perfect chemi-osmotic pump. Q. Rev. Biophys. 43, 501–566. Perez C, Koshy C, Yildiz O & Ziegler C (2012) Alternating-access mechanism in conformationally asymmetric trimers of the betaine transporter BetP. Nature 490, 126–130. Rees D, Johnson E & Lewinson O (2009) ABC transporters: the power to change. Nat. Rev. Mol. Cell Biol. 10, 218–227. – Romero MF & Boron WF (1999) Electrogenic Na+/HCO3 cotransporters: cloning and physiology. Annu. Rev. Physiol. 61, 699–723.


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Cell_Biology_Alberts. Romero MF & Boron WF (1999) Electrogenic Na+/HCO3 cotransporters: cloning and physiology. Annu. Rev. Physiol. 61, 699–723. Rudnick G (2011) Cytoplasmic permeation pathway of neurotransmitter transporters. Biochemistry 50, 7462−7475. Saier MH Jr (2000) Vectorial metabolism and the evolution of transport systems. J. Bacteriol. 182, 5029–5035. Stein WD (2002) Cell volume homeostasis: ionic and nonionic mechanisms. The sodium pump in the emergence of animal cells. Int. Rev. Cytol. 215, 231–258. Toyoshima C (2009) How Ca2+-ATPase pumps ions across the sarcoplasmic reticulum membrane. Biochim. Biophys. Acta 1793, 941–946. Yamashita A, Singh SK, Kawate T et al. (2005) Crystal structure of a bacterial homologue of Na+/Cl–-dependent neurotransmitter transporters. Nature 437, 215–223. Channels and the electrical Properties of Membranes Armstrong C (1998) The vision of the pore. Science 280, 56–57.


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Cell_Biology_Alberts. Channels and the electrical Properties of Membranes Armstrong C (1998) The vision of the pore. Science 280, 56–57. Arnadóttir J & Chalfie M (2010) Eukaryotic mechanosensitive channels. Annu. Rev. Biophys. 39, 111–137. Bezanilla F (2008) How membrane proteins sense voltage. Nat. Rev. Mol. Cell Biol. 9, 323–332. Catterall WA (2010) Ion channel voltage sensors: structure, function, and pathophysiology. Neuron 67, 915–928. Davis GW (2006) Homeostatic control of neural activity: from phenomenology to molecular design. Annu. Rev. Neurosci. 29, 307–323. Greengard P (2001) The neurobiology of slow synaptic transmission. Science 294, 1024–1030. Hodgkin AL & Huxley AF (1952) A quantitative description of membrane current and its application to conduction and excitation in nerve. J. Physiol. 117, 500–544. Hodgkin AL & Huxley AF (1952) Currents carried by sodium and potassium ions through the membrane of the giant axon of Loligo. J. Physiol. 116, 449–472.


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Cell_Biology_Alberts. Hodgkin AL & Huxley AF (1952) Currents carried by sodium and potassium ions through the membrane of the giant axon of Loligo. J. Physiol. 116, 449–472. Jessell TM & Kandel ER (1993) Synaptic transmission: a bidirectional and self-modifiable form of cell–cell communication. Cell 72(Suppl), 1–30. Julius D (2013) TRP channels and pain. Annu. Rev. Cell Dev. Biol. 29, 355–384. Katz B (1966) Nerve, Muscle and Synapse. New York: McGraw-Hill. King LS, Kozono D & Agre P (2004) From structure to disease: the evolving tale of aquaporin biology. Nat. Rev. Mol. Cell Biol. 5, 687–698. Liao M, Cao E, Julius D & Cheng Y (2014) Single particle electron cryo-microscopy of a mammalian ion channel. Curr. Opin. Struct. Biol. 27, 1–7. MacKinnon R (2003) Potassium channels. FEBS Lett. 555, 62–65. Miesenböck G (2011) Optogenetic control of cells and circuits. Annu. Rev. Cell Dev. Biol. 27, 731–758. Moss SJ & Smart TG (2001) Constructing inhibitory synapses. Nat. Rev. Neurosci. 2, 240–250.


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Cell_Biology_Alberts. Miesenböck G (2011) Optogenetic control of cells and circuits. Annu. Rev. Cell Dev. Biol. 27, 731–758. Moss SJ & Smart TG (2001) Constructing inhibitory synapses. Nat. Rev. Neurosci. 2, 240–250. Neher E & Sakmann B (1992) The patch clamp technique. Sci. Am. 266, 44–51. Nicholls JG, Fuchs PA, Martin AR & Wallace BG (2000) From Neuron to Brain, 4th ed. Sunderland, MA: Sinauer. Numa S (1987) A molecular view of neurotransmitter receptors and ionic channels. Harvey Lect. 83, 121–165. Payandeh J, Scheuer T, Zheng N & Catterall WA (2011) The crystal structure of a voltage-gated sodium channel. Nature 475, 353–358. Scannevin RH & Huganir RL (2000) Postsynaptic organization and regulation of excitatory synapses. Nat. Rev. Neurosci. 1, 133–141. Snyder SH (1996) Drugs and the Brain. New York: WH Freeman/ Scientific American Books. Sobolevsky AI, Rosconi MP & Gouaux E (2009) X-ray structure, symmetry and mechanism of an AMPA-subtype glutamate receptor. Nature 462, 745–756.


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Cell_Biology_Alberts. Sobolevsky AI, Rosconi MP & Gouaux E (2009) X-ray structure, symmetry and mechanism of an AMPA-subtype glutamate receptor. Nature 462, 745–756. Stevens CF (2004) Presynaptic function. Curr. Opin. Neurobiol. 14, 341–345. Verkman AS (2013) Aquaporins. Curr. Biol. 23, R52–R55. Unlike a bacterium, which generally consists of a single intracellular compartment surrounded by a plasma membrane, a eukaryotic cell is elaborately subdivided into functionally distinct, membrane-enclosed compartments. Each compartment, or organelle, contains its own characteristic set of enzymes and other specialized molecules, and complex distribution systems transport specific products from one compartment to another. To understand the eukaryotic cell, it is essential to know how the cell creates and maintains these compartments, what occurs in each of them, and how molecules move between them.


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Cell_Biology_Alberts. Proteins confer upon each compartment its characteristic structural and functional properties. They catalyze the reactions that occur there and selectively transport small molecules into and out of the compartment. For membrane-enclosed organelles in the cytoplasm, proteins also serve as organelle-specific surface markers that direct new deliveries of proteins and lipids to the appropriate organelle. An animal cell contains about 10 billion (1010) protein molecules of perhaps 10,000 kinds, and the synthesis of almost all of them begins in the cytosol, the space of the cytoplasm outside the membrane-enclosed organelles. Each newly synthesized protein is then delivered specifically to the organelle that requires it. The intracellular transport of proteins is the central theme of both this chapter and the next. By tracing the protein traffic from one compartment to another, one can begin to make sense of the otherwise bewildering maze of intracellular membranes.


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Cell_Biology_Alberts. The ComparTmenTalizaTion of Cells In this brief overview of the compartments of the cell and the relationships between them, we organize the organelles conceptually into a small number of discrete families, discuss how proteins are directed to specific organelles, and explain how proteins cross organelle membranes. all eukaryotic Cells have the same Basic set of membrane-enclosed organelles


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Cell_Biology_Alberts. Many vital biochemical processes take place in membranes or on their surfaces. Membrane-bound enzymes, for example, catalyze lipid metabolism; and oxidative phosphorylation and photosynthesis both require a membrane to couple the transport of H+ to the synthesis of ATP. In addition to providing increased membrane area to host biochemical reactions, intracellular membrane systems form enclosed compartments that are separate from the cytosol, thus creating functionally specialized aqueous spaces within the cell. In these spaces, subsets of molecules (proteins, reactants, ions) are concentrated to optimize the biochemical reactions in which they participate. Because the lipid bilayer of cell membranes is impermeable to most hydrophilic molecules, the membrane of an organelle must contain membrane transport proteins to import and export specific metabolites. Each organelle membrane must also have a mechanism for importing, and incorporating into the organelle, the specific proteins that


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Cell_Biology_Alberts. transport proteins to import and export specific metabolites. Each organelle membrane must also have a mechanism for importing, and incorporating into the organelle, the specific proteins that make the organelle unique.


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Cell_Biology_Alberts. The ComparTmenTalizaTion of Cells The TransporT of moleCUles BeTWeen The nUCleUs anD The CYTosol The TransporT of proTeins inTo miToChonDria anD ChloroplasTs The enDoplasmiC reTiCUlUm 642 Chapter 12: intracellular Compartments and protein sorting endoplasmic reticulum with membrane-bound polyribosomes Figure 12–1 illustrates the major intracellular compartments common to eukaryotic cells. The nucleus contains the genome (aside from mitochondrial and chloroplast DNA), and it is the principal site of DNA and RNA synthesis. The surrounding cytoplasm consists of the cytosol and the cytoplasmic organelles suspended in it. The cytosol constitutes a little more than half the total volume of the cell, and it is the main site of protein synthesis and degradation. It also performs most of the cell’s intermediary metabolism—that is, the many reactions that degrade some small molecules and synthesize others to provide the building blocks for macromolecules (discussed in Chapter 2).


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Cell_Biology_Alberts. About half the total area of membrane in a eukaryotic cell encloses the labyrinthine spaces of the endoplasmic reticulum (ER). The rough ER has many ribosomes bound to its cytosolic surface. Ribosomes are organelles that are not membrane-enclosed; they synthesize both soluble and integral membrane proteins, most of which are destined either for secretion to the cell exterior or for other organelles. We shall see that, whereas proteins are transported into other membrane-enclosed organelles only after their synthesis is complete, they are transported into the ER as they are synthesized. This explains why the ER membrane is unique in having ribosomes tethered to it. The ER also produces most of the lipid for the rest of the cell and functions as a store for Ca2+ ions. Regions of the ER that lack bound ribosomes are called smooth ER. The ER sends many of its proteins and lipids to the Golgi apparatus, which often consists of organized stacks of disc-like compartments called Golgi


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Cell_Biology_Alberts. ER that lack bound ribosomes are called smooth ER. The ER sends many of its proteins and lipids to the Golgi apparatus, which often consists of organized stacks of disc-like compartments called Golgi cisternae. The Golgi apparatus receives lipids and proteins from the ER and dispatches them to various destinations, usually covalently modifying them en route.


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Cell_Biology_Alberts. Mitochondria and chloroplasts generate most of the ATP that cells use to drive reactions requiring an input of free energy; chloroplasts are a specialized version of plastids (present in plants, algae, and some protozoa), which can also have other functions, such as the storage of food or pigment molecules. Lysosomes contain digestive enzymes that degrade defunct intracellular organelles, as well as macromolecules and particles taken in from outside the cell by endocytosis. On the way to lysosomes, endocytosed material must first pass through a series of organelles called endosomes. Finally, peroxisomes are small vesicular compartments that contain enzymes used in various oxidative reactions. In general, each membrane-enclosed organelle performs the same set of basic functions in all cell types. But to serve the specialized functions of cells, these organelles vary in abundance and can have additional properties that differ from cell type to cell type.


Cell_Biology_Alberts

Cell_Biology_Alberts. On average, the membrane-enclosed compartments together occupy nearly half the volume of a cell (Table 12–1), and a large amount of intracellular membrane is required to make them. In liver and pancreatic cells, for example, the Figure 12–1 The major intracellular compartments of an animal cell. The cytosol (gray), endoplasmic reticulum, Golgi apparatus, nucleus, mitochondrion, endosome, lysosome, and peroxisome are distinct compartments isolated from the rest of the cell by at least one selectively permeable membrane (see movie 9.2). endoplasmic reticulum has a total membrane surface area that is, respectively, 25 times and 12 times that of the plasma membrane (Table 12–2). The membrane-enclosed organelles are packed tightly in the cytoplasm, and, in terms of area and mass, the plasma membrane is only a minor membrane in most eukaryotic cells (Figure 12–2).


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Cell_Biology_Alberts. The abundance and shape of membrane-enclosed organelles are regulated to meet the needs of the cell. This is particularly apparent in cells that are highly specialized and therefore disproportionately rely on specific organelles. Plasma cells, for example, which secrete their own weight every day in antibody molecules into the bloodstream, contain vastly amplified amounts of rough ER, which is found in large, flat sheets. Cells that specialize in lipid synthesis also expand their ER, but in this case the organelle forms a network of convoluted tubules. Moreover, membrane-enclosed organelles are often found in characteristic positions in the cytoplasm. In most cells, for example, the Golgi apparatus is located close to the nucleus, whereas the network of ER tubules extends from the nucleus throughout the entire cytosol. These characteristic distributions depend on interactions of the organelles with the cytoskeleton. The localization of both the ER and the Golgi apparatus, for


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Cell_Biology_Alberts. nucleus throughout the entire cytosol. These characteristic distributions depend on interactions of the organelles with the cytoskeleton. The localization of both the ER and the Golgi apparatus, for instance, depends on an intact microtubule array; if the microtubules are experimentally depolymerized with a drug, the Golgi apparatus fragments and disperses throughout the cell, and the ER network collapses toward the cell center (discussed in Chapter 16). The size, shape, composition, and location are all important and regulated features of these organelles that ultimately contribute to the organelle’s function.


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Cell_Biology_Alberts. evolutionary origins may help explain the Topological relationships of organelles


Cell_Biology_Alberts

Cell_Biology_Alberts. To understand the relationships between the compartments of the cell, it is helpful to consider how they might have evolved. The precursors of the first eukaryotic cells are thought to have been relatively simple cells that—like most bacterial and archaeal cells—have a plasma membrane but no internal membranes. The plasma membrane in such cells provides all membrane-dependent functions, including the pumping of ions, ATP synthesis, protein secretion, and lipid synthesis. Typical present-day eukaryotic cells are 10–30 times larger in linear dimension and 1000–10,000 times greater in volume than a typical bacterium such as E. coli. The profusion of internal membranes can be regarded, in part, as an adaptation to this increase in size: the eukaryotic cell has a much smaller ratio of surface area to volume, and its plasma membrane therefore presumably has too small an area to sustain the many vital functions that membranes perform. The extensive internal membrane systems of a eukaryotic


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Cell_Biology_Alberts. area to volume, and its plasma membrane therefore presumably has too small an area to sustain the many vital functions that membranes perform. The extensive internal membrane systems of a eukaryotic cell alleviate this problem.


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Cell_Biology_Alberts. The evolution of internal membranes evidently went hand-in-hand with the specialization of membrane function. A hypothetical scheme for how the first eukaryotic cells, with a nucleus and ER, might have evolved by the invagination and pinching off of the plasma membrane of an ancestral cell is illustrated in Figure 12–3.This process would create membrane-enclosed organelles with an interior or lumen that is topologically equivalent to the exterior of the cell. We shall see that this topological relationship holds for all of the organelles involved in the secretory and endocytic pathways, including the ER, Golgi apparatus, endosomes, lysosomes, and peroxisomes. We can therefore think of all of these organelles as members of the same topologically equivalent compartment. As we discuss in detail in the next chapter, their interiors communicate extensively with one another and with the outside of the cell via transport vesicles, which bud off from one organelle and fuse with another


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Cell_Biology_Alberts. in detail in the next chapter, their interiors communicate extensively with one another and with the outside of the cell via transport vesicles, which bud off from one organelle and fuse with another (Figure 12–4).


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Cell_Biology_Alberts. As described in Chapter 14, mitochondria and plastids differ from the other membrane-enclosed organelles because they contain their own genomes. The nature of these genomes, and the close resemblance of the proteins in these organelles to those in some present-day bacteria, strongly suggest that mitochondria and plastids evolved from bacteria that were engulfed by other cells with which they initially lived in symbiosis (see Figures 1–29 and 1–31): the inner membrane of mitochondria and plastids presumably corresponds to the original plasma membrane of the bacterium, while the lumen of these organelles evolved from the bacterial cytosol. Like the bacteria from which they were derived, both mitochondria and plastids are enclosed by a double membrane and they remain isolated from the extensive vesicular traffic that connects the interiors of most of the other membrane-enclosed organelles to each other and to the outside of the cell.


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Cell_Biology_Alberts. Figure 12–2 An electron micrograph of part of a liver cell seen in cross section. examples of most of the major intracellular organelles are indicated. (Courtesy of Daniel s. friend.) ARCHAEAL AND BACTERIAL) WALL IN ANCIENT GREATLY INCREASES ANAEROBIC ARCHAEON HORIZONTAL GENE FACILITATES HORIZONTAL TRANSFERS, SPEEDING GENE TRANSFERS EVOLUTIONARY PROCESSES The evolutionary schemes just described group the intracellular compartments in eukaryotic cells into four distinct families: (1) the nucleus and the cytosol, which communicate with each other through nuclear pore complexes and are thus topologically continuous (although functionally distinct); (2) all organelles that function in the secretory and endocytic pathways—including the ER, Golgi apparatus, endosomes, and lysosomes, the numerous classes of transport intermediates such as transport vesicles that move between them, and peroxisomes; (3) the mitochondria; and (4) the plastids (in plants only).


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Cell_Biology_Alberts. The synthesis of all proteins begins on ribosomes in the cytosol, except for the few that are synthesized on the ribosomes of mitochondria and plastids. Their subsequent fate depends on their amino acid sequence, which can contain sorting signals that direct their delivery to locations outside the cytosol or to organelle surfaces. Some proteins do not have a sorting signal and consequently remain in the cytosol as permanent residents. Many others, however, have specific sorting signals that direct their transport from the cytosol into the nucleus, the ER, mitochondria, plastids, or peroxisomes; sorting signals can also direct the transport of proteins from the ER to other destinations in the cell.


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Cell_Biology_Alberts. Figure 12–3 One suggested pathway for the evolution of the eukaryotic cell and its internal membranes as discussed in Chapter 1, there is evidence that the nuclear genome of a eukaryotic cell evolved from an ancient archeaon. for example, clear homologs of actin, tubulin, histones, and the nuclear Dna replication system are found in archaea, but not in bacteria. Thus, it is now thought that the first eukaryotic cells arose when an ancient anaerobic archaeon joined forces with an aerobic bacterium roughly 1.6 billion years ago. as indicated, the nuclear envelope may have originated from an invagination of the plasma membrane of this ancient archaeon—an invagination that protected its chromosome while still allowing access of the Dna to the cytosol (as required for Dna to direct protein synthesis). This envelope may have later pinched off completely from the plasma membrane, so as to produce a separate nuclear compartment surrounded by a double membrane. Because this double membrane is


Cell_Biology_Alberts

Cell_Biology_Alberts. This envelope may have later pinched off completely from the plasma membrane, so as to produce a separate nuclear compartment surrounded by a double membrane. Because this double membrane is penetrated by nuclear pore complexes, the nuclear compartment is topologically equivalent to the cytosol. in contrast, the lumen of the er is continuous with the space between the inner and outer nuclear membranes, and it is topologically equivalent to the extracellular space (see figure 12–4). (adapted from J. martijn and


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Cell_Biology_Alberts. T.J.G. ettema, Biochem. Soc. Trans. 41: 451–457, 2013.) To understand the general principles by which sorting signals operate, it is important to distinguish three fundamentally different ways by which proteins move from one compartment to another. These three mechanisms are described below, and the transport steps at which they operate are outlined in Figure 12–5. We discuss the first two mechanisms (gated transport and transmembrane transport) in this chapter, and the third (vesicular transport, green arrows in Figure 12–5) in Chapter 13. 1. In gated transport, proteins and RNA molecules move between the cytosol and the nucleus through nuclear pore complexes in the nuclear envelope. The nuclear pore complexes function as selective gates that support the active transport of specific macromolecules and macromolecular assemblies between the two topologically equivalent spaces, although they also allow free diffusion of smaller molecules. 2.


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Cell_Biology_Alberts. 2. In protein translocation, transmembrane protein translocators directly transport specific proteins across a membrane from the cytosol into a space that is topologically distinct. The transported protein molecule usually must unfold to snake through the translocator. The initial transport of selected proteins from the cytosol into the ER lumen or mitochondria, for example, occurs in this way. Integral membrane proteins often use the same translocators but translocate only partially across the membrane, so that the protein becomes embedded in the lipid bilayer. 3.


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Cell_Biology_Alberts. 3. In vesicular transport, membrane-enclosed transport intermediates— which may be small, spherical transport vesicles or larger, irregularly shaped organelle fragments—ferry proteins from one topologically equivalent compartment to another. The transport vesicles and fragments become loaded with a cargo of molecules derived from the lumen of one compartment as they bud and pinch off from its membrane; they discharge their cargo into a second compartment by fusing with the membrane enclosing that compartment (Figure 12–6). The transfer of soluble proteins from the ER to the Golgi apparatus, for example, occurs in this way. Because the


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Cell_Biology_Alberts. Figure 12–5 A simplified “roadmap” of protein traffic within a eukaryotic cell. proteins can move from one compartment to another by gated transport (red), protein translocation (blue), or vesicular transport (green). The sorting signals that direct a given protein’s movement through the system, and thereby determine its eventual location in the cell, are contained in each protein’s amino acid sequence. The journey begins with the synthesis of a protein on a ribosome in the cytosol and, for many proteins, terminates when the protein reaches its final destination. other proteins shuttle back and forth between the nucleus and cytosol. at each intermediate station (boxes), a decision is made as to whether the protein is to be retained in that compartment or transported further. a sorting signal may direct either retention in or exit from a compartment.


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Cell_Biology_Alberts. We shall refer to this figure often as a guide in this chapter and the next, highlighting in color the particular pathway being discussed.


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Cell_Biology_Alberts. Figure 12–4 Topologically equivalent compartments in the secretory and endocytic pathways in a eukaryotic cell. Compartments are said to be topologically equivalent if they can communicate with one another, in the sense that molecules can get from one to the other without having to cross a membrane. Topologically equivalent spaces are shown in red. (a) molecules can be carried from one compartment to another topologically equivalent compartment by vesicles that bud from one and fuse with the other. (B) in principle, cycles of membrane budding and fusion permit the lumen of any of the organelles shown to communicate with any other and with the cell exterior by means of transport vesicles. Blue arrows indicate the extensive outbound and inbound vesicular traffic (discussed in Chapter 13). some organelles, most notably mitochondria and (in plant cells) plastids, do not take part in this communication and are isolated from the vesicular traffic between organelles shown here.


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Cell_Biology_Alberts. = gated transport = transmembrane transport = vesicular transport KEY: Figure 12–6 Vesicle budding and fusion during vesicular transport. Transport vesicles bud from one compartment (donor) and fuse with another topologically equivalent (target) compartment. in the process, soluble components (red dots) are transferred from lumen to lumen. note that membrane is also transferred and that the original orientation of both proteins and lipids in the donor compartment membrane is preserved in the target compartment membrane. Thus, membrane proteins retain their asymmetric orientation, with the same domains always facing the cytosol. transported proteins do not cross a membrane, vesicular transport can move proteins only between compartments that are topologically equiva lent (see Figure 12–4).


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Cell_Biology_Alberts. transported proteins do not cross a membrane, vesicular transport can move proteins only between compartments that are topologically equiva lent (see Figure 12–4). Each mode of protein transfer is usually guided by sorting signals in the transported protein, which are recognized by complementary sorting receptors. If a large protein is to be imported into the nucleus, for example, it must possess a sorting signal that receptor proteins recognize to guide it through the nuclear pore complex. If a protein is to be transferred directly across a membrane, it must possess a sorting signal that the translocator recognizes. Likewise, if a protein is to be loaded into a certain type of vesicle or retained in certain organelles, a complementary receptor in the appropriate membrane must recognize its sorting signal. signal sequences and sorting receptors Direct proteins to the Correct Cell address


Cell_Biology_Alberts

Cell_Biology_Alberts. signal sequences and sorting receptors Direct proteins to the Correct Cell address Most protein sorting signals involved in transmembrane transport reside in a stretch of amino acid sequence, typically 15–60 residues long. Such signal sequences are often found at the N-terminus of the polypeptide chain, and in many cases specialized signal peptidases remove the signal sequence from the finished protein once the sorting process is complete. Signal sequences can also be internal stretches of amino acids, which remain part of the protein. Such signals are used in gated transport into the nucleus. Sorting signals can also be composed of multiple internal amino acid sequences that form a specific three-dimensional arrangement of atoms on the protein’s surface; such signal patches are sometimes used for nuclear import and in vesicular transport.


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Cell_Biology_Alberts. Each signal sequence specifies a particular destination in the cell. Proteins destined for initial transfer to the ER usually have a signal sequence at their Nterminus that characteristically includes a sequence composed of about 5–10 hydrophobic amino acids. Many of these proteins will in turn pass from the ER to the Golgi apparatus, but those with a specific signal sequence of four amino acids at their C-terminus are recognized as ER residents and are returned to the ER. Proteins destined for mitochondria have signal sequences of yet another type, in which positively charged amino acids alternate with hydrophobic ones. Finally, many proteins destined for peroxisomes have a signal sequence of three characteristic amino acids at their C-terminus.


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Cell_Biology_Alberts. Table 12–3 presents some specific signal sequences. Experiments in which the peptide is transferred from one protein to another by genetic engineering techniques have demonstrated the importance of each of these signal sequences for protein targeting. Placing the N-terminal ER signal sequence at the beginning of a cytosolic protein, for example, redirects the protein to the ER; removing or mutating the signal sequence of an ER protein causes its retention in the cytosol. Signal sequences are therefore both necessary and sufficient for protein targeting. Even though their amino acid sequences can vary greatly, the signal sequences of proteins having the same destination are functionally interchangeable; physical properties, such as hydrophobicity, often seem to be more important in the signal-recognition process than the exact amino acid sequence.


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Cell_Biology_Alberts. Signal sequences are recognized by complementary sorting receptors that guide proteins to their appropriate destination, where the receptors unload their cargo. The receptors function catalytically: after completing one round of targeting, they return to their point of origin to be reused. Most sorting receptors budding vesicle with contents selected for recognize classes of proteins rather than an individual protein species. They can therefore be viewed as public transportation systems, dedicated to delivering numerous different components to their correct location in the cell. most organelles Cannot Be Constructed De Novo: They require information in the organelle itself


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Cell_Biology_Alberts. When a cell reproduces by division, it has to duplicate its organelles, in addition to its chromosomes. In general, cells do this by incorporating new molecules into the existing organelles, thereby enlarging them; the enlarged organelles then divide and are distributed to the two daughter cells. Thus, each daughter cell inherits a complete set of specialized cell membranes from its mother. This inheritance is essential because a cell could not make such membranes from scratch. If the ER were completely removed from a cell, for example, how could the cell reconstruct it? As we discuss later, the membrane proteins that define the ER and perform many of its functions are themselves products of the ER. A new ER could not be made without an existing ER or, at least, a membrane that specifically contains the protein translocators required to import selected proteins into the ER from the cytosol (including the ER-specific translocators themselves). The same is true for mitochondria and


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Cell_Biology_Alberts. contains the protein translocators required to import selected proteins into the ER from the cytosol (including the ER-specific translocators themselves). The same is true for mitochondria and plastids.


Cell_Biology_Alberts

Cell_Biology_Alberts. Thus, it seems that the information required to construct an organelle does not reside exclusively in the DNA that specifies the organelle’s proteins. Information in the form of at least one distinct protein that preexists in the organelle membrane is also required, and this information is passed from parent cell to daughter cells in the form of the organelle itself. Presumably, such information is essential for the propagation of the cell’s compartmental organization, just as the information in DNA is essential for the propagation of the cell’s nucleotide and amino acid sequences.


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Cell_Biology_Alberts. As we discuss in more detail in Chapter 13, however, the ER buds off a constant stream of transport vesicles that incorporate only a subset of ER proteins and therefore have a composition different from the ER itself. Similarly, the plasma membrane constantly buds off various types of specialized endocytic vesicles. Thus, some organelles can form from other organelles and do not have to be inherited at cell division. Eukaryotic cells contain intracellular membrane-enclosed organelles that make up nearly half the cell’s total volume. The main ones present in all eukaryotic cells are the endoplasmic reticulum, Golgi apparatus, nucleus, mitochondria, lysosomes, endosomes, and peroxisomes; plant cells also contain plastids such as chloroplasts. These organelles contain distinct sets of proteins, which mediate each organelle’s unique function.


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Cell_Biology_Alberts. Each newly synthesized organelle protein must find its way from a ribosome in the cytosol, where the protein is made, to the organelle where it functions. It does so by following a specific pathway, guided by sorting signals in its amino acid sequence that function as either signal sequences or signal patches. Sorting signals are recognized by complementary sorting receptors, which deliver the protein to the appropriate target organelle. Proteins that function in the cytosol do not contain sorting signals and therefore remain there after they are synthesized. During cell division, organelles such as the ER and mitochondria are distributed to each daughter cell. These organelles contain information that is required for their construction, and so they cannot be made de novo. The TransporT of moleCUles BeTWeen The nUCleUs anD The CYTosol The nuclear envelope encloses the DNA and defines the nuclear compartment.


Cell_Biology_Alberts

Cell_Biology_Alberts. This envelope consists of two concentric membranes, which are penetrated by nuclear pore complexes (Figure 12–7). Although the inner and outer nuclear membranes are continuous, they maintain distinct protein compositions. The inner nuclear membrane contains proteins that act as binding sites for chromosomes and for the nuclear lamina, a protein meshwork that provides structural support for the nuclear envelope; the lamina also acts as an anchoring site for chromosomes and the cytoplasmic cytoskeleton (via protein complexes that span the nuclear envelope). The inner membrane is surrounded by the outer nuclear membrane, which is continuous with the membrane of the ER. Like the ER membrane (discussed later), the outer nuclear membrane is studded with ribosomes engaged in protein synthesis. The proteins made on these ribosomes are transported into the space between the inner and outer nuclear membranes (the perinuclear space), which is continuous with the ER lumen (see Figure 12–7).


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Cell_Biology_Alberts. Bidirectional traffic occurs continuously between the cytosol and the nucleus. The many proteins that function in the nucleus—including histones, DNA polymerases, RNA polymerases, transcriptional regulators, and RNA-processing proteins—are selectively imported into the nuclear compartment from the cytosol, where they are made. At the same time, almost all RNAs—including mRNAs, rRNAs, tRNAs, miRNAs, and snRNAs—are synthesized in the nuclear compartment and then exported to the cytosol. Like the import process, the export process is selective; mRNAs, for example, are exported only after they have been properly modified by RNA-processing reactions in the nucleus. In some cases, the transport process is complex. Ribosomal proteins, for instance, are made in the cytosol and imported into the nucleus, where they assemble with newly made ribosomal RNA into particles. The particles are then exported to the cytosol, where they assemble into ribosomes. Each of these steps requires selective


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Cell_Biology_Alberts. nucleus, where they assemble with newly made ribosomal RNA into particles. The particles are then exported to the cytosol, where they assemble into ribosomes. Each of these steps requires selective transport across the nuclear envelope.


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Cell_Biology_Alberts. nuclear pore Complexes perforate the nuclear envelope


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Cell_Biology_Alberts. Large and elaborate nuclear pore complexes (NPCs) perforate the nuclear envelope in all eukaryotes. Each NPC is composed of a set of approximately 30 different proteins, or nucleoporins. Reflecting the high degree of internal symmetry, each nucleoporin is present in multiple copies, resulting in 500–1000 protein molecules in the fully assembled NPC, with an estimated mass of 66 million daltons in yeast and 125 million daltons in vertebrates (Figure 12–8). Most nucleoporins are composed of repetitive protein domains of only a few different types, which have evolved through extensive gene duplication. Some of the scaffold nucleoporins (see Figure 12–8) are structurally related to vesicle coat protein complexes, such as clathrin and COPII coatomer (discussed in Chapter 13), which shape transport vesicles; and one protein is used as a common building block in both NPCs and vesicle coats. These similarities suggest a common evolutionary origin for NPCs and vesicle coats: they may derive


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Cell_Biology_Alberts. vesicles; and one protein is used as a common building block in both NPCs and vesicle coats. These similarities suggest a common evolutionary origin for NPCs and vesicle coats: they may derive from an early membrane-bending protein module that helped shape the elaborate membrane systems of eukaryotic cells, and in present-day cells stabilize the sharp membrane bends required to form a nuclear pore.


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Cell_Biology_Alberts. The nuclear envelope of a typical mammalian cell contains 3000–4000 NPCs, although that number varies widely, from a few hundred in glial cells to almost 20,000 in Purkinje neurons. The total traffic that passes through each NPC is enormous: each NPC can transport up to 1000 macromolecules per second and can transport in both directions at the same time. How it coordinates the bidirectional flow of macromolecules to avoid congestion and head-on collisions is not known.


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Cell_Biology_Alberts. Each NPC contains aqueous passages, through which small water-soluble molecules can diffuse passively. Researchers have determined the effective size of these passages by injecting labeled water-soluble molecules of different sizes into the cytosol and then measuring their rate of diffusion into the nucleus. Small molecules (5000 daltons or less) diffuse in so fast that we can consider the nuclear envelope freely permeable to them. Large proteins, however, diffuse in much more slowly, and the larger a protein, the more slowly it passes through the NPC. Proteins larger than 60,000 daltons cannot enter by passive diffusion. This size cut-off to free diffusion is thought to result from the NPC structure (see Figure 12–8). The channel nucleoporins with extensive unstructured regions form a disordered tangle (much like a kelp bed in the ocean) that restricts the diffusion of large macromolecules while allowing smaller molecules to pass.


Cell_Biology_Alberts

Cell_Biology_Alberts. Because many cell proteins are too large to diffuse passively through the NPCs, the nuclear compartment and the cytosol can maintain different protein compositions. Mature cytosolic ribosomes, for example, are about 30 nm in diameter and thus cannot diffuse through the NPC, confining protein synthesis to the cytosol. But how does the nucleus export newly made ribosomal subunits or import large molecules, such as DNA polymerases and RNA polymerases, which have subunit molecular masses of 100,000–200,000 daltons? As we discuss next, these and most other transported protein and RNA molecules bind to specific receptor proteins that actively ferry large molecules through NPCs. Even small proteins like histones frequently use receptor-mediated mechanisms to cross the NPC, thereby increasing transport efficiency. nuclear localization signals Direct nuclear proteins to the nucleus


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Cell_Biology_Alberts. nuclear localization signals Direct nuclear proteins to the nucleus When proteins are experimentally extracted from the nucleus and reintroduced into the cytosol, even the very large ones reaccumulate efficiently in the nucleus. Sorting signals called nuclear localization signals (NLSs) are responsible for the selectivity of this active nuclear import process. The signals have been precisely defined by using recombinant DNA technology for numerous nuclear proteins, as well as for proteins that enter the nucleus only transiently (Figure 12–9). In many nuclear proteins, the signals consist of one or two short sequences that are rich in the positively charged amino acids lysine and arginine (see Table 12–3, p. 648), with the precise sequence varying for different proteins. Other nuclear proteins contain different signals, some of which are not yet characterized.