They are constituted by an amino acid chain whose amino terminus is located in the extracellular portion of the cell and the carboxyl terminus is in the cytoplasm, the peptide chain crosses the plasma membrane in seven instances Figure 1.
Están constituidos por una cadena de aminoácidos cuyo extremo amino-terminal se localiza en la porción extracelular de la célula y el extremo carboxilo en el citoplasma; la cadena atraviesa la membrana plasmática en siete ocasiones Figura 1.