Agonist-induced endocytosis and receptor phosphorylation mediate resensitization of dopamine D₂ receptors

The regulatory mechanisms and functional roles of agonist-induced internalization of G proteincoupled receptors (GPCRs) were analyzed using mutant dopamine D₂ receptors (D₂Rs) in which all possible GPCR kinase (GRK) phosphorylation sites were mutated or the affinity for β-arrestins was altered. Agonist-induced internalization of D₂Rs involved a phosphorylation-dependent component, which was mediated by serine/threonine (S/T) residues in the second loop and T225 in the third loop, and a phosphorylation-independent component. GRK2-mediated enhancement of the internalization and inhibition of D₂R signaling did not involve receptor phosphorylation, and only the former required the enzymatic activity of GRK2. The phosphorylation-deficient mutant (D₂R-intracellular loop 2/3) recycled more slowly and showed more agonist-induced desensitization than did the wild-type D₂R, suggesting that receptor phosphorylation mediates the recycling of the internalized receptors and enhances receptor resensitization. Blockade of the agonist-induced internalization of D ₂R-intracellular loop 2/3 provoked desensitization as in wild-type D₂R, suggesting that certain cellular processes other than receptor dephosphorylation occurring within the endocytic vesicle are responsible for the resensitization of D₂R. When dissociation between D₂R and β-arrestin was inhibited or when the expression of cellular β-arrestins was decreased, agonist-induced desensitization of D ₂R did not occur, suggesting that dissociation from β-arrestin is the main cellular process required for resensitization of D2R and is achieved through agonist-induced internalization. These results indicate that, in the regulation of some GPCRs, phosphorylation-independent association with β-arrestin plays a major role in agonistinduced desensitization.