Roles of Receptor Phosphorylation and Rab Proteins in G Protein-Coupled Receptor Function and Trafficking

The G protein-coupled receptors form the most abundant family of membrane proteins and are crucial physiologic players in the homeostatic equilibrium, which we define as health. They also participate in the pathogenesis of many diseases and are frequent targets of therapeutic intervention. Considering their importance, it is not surprising that different mechanisms regulate their function, including desensitization, resensitization, internalization, recycling to the plasma membrane, and degradation. These processes are modulated in a highly coordinated and specific way by protein kinases and phosphatases, ubiquitin ligases, protein adaptors, interaction with multifunctional complexes, molecular motors, phospholipid metabolism, and membrane distribution. This review describes significant advances in the study of the regulation of these receptors by phosphorylation and endosomal traffic (where signaling can take place); we revisited the bar code hypothesis and include two additional observations: 1) that different phosphorylation patterns seem to be associated with internalization and endosome sorting for recycling or degradation, and 2) that, surprisingly, phosphorylation of some G protein-coupled receptors appears to be required for proper receptor insertion into the plasma membrane. SIGNIFICANCE STATEMENT: G protein-coupled receptor phosphorylation is an early event in desensitization/signaling switching, endosomal traffic, and internalization. These events seem crucial for receptor responsiveness, cellular localization, and fate (recycling/degradation) with important pharmacological/therapeutic implications. Phosphorylation sites vary depending on the cells in which they are expressed and on the stimulus that leads to such covalent modification. Surprisingly, evidence suggests that phosphorylation also seems to be required for proper insertion into the plasma membrane for some receptors.