 Arrestins block GPCR coupling to G proteins in two ways.First, arrestin binding to the cytoplasmic face of the receptor occludes the binding site for heterotrimeric G-protein, preventing its activation (desensitization).Second, arrestin links the receptor to elements of the internalization machinery, clathrin and clathrin adaptor AP2, which promotes receptor internalization via coated pits and subsequent transport to internal compartments, called endosomes.