GRK phosphorylation specifically prepares the activated receptor for arrestin binding.
Arrestins block GPCR coupling to G proteins via two mechanisms. First, arrestin binding to the cytoplasmic tip of the receptor occludes the binding site for the heterotrimeric G-protein, preventing its activation (desensitization). Second, arrestins link the receptor to elements of the internalization machinery, clathrin and clathrin adaptor AP2, which promotes receptor internalization via coated pits and subsequent transport to an internal compartment, called endosome. Subsequently, the receptor could be either targeted to degradation compartments (lysosomes) or recycled back to the plasma membrane where it can signal again. The strength of arrestin-receptor interaction plays a role in this choice: tighter complexes tend to increase the probability of receptor degradation, whereas more transient complexes favor recycling, although this "rule" is far from absolute.