Agonist-dependent phosphorylation of the α₂-adrenergic receptor by the β-adrenergic receptor kinase

Desensitization of the β-adrenergic receptor, a receptor which is coupled to the stimulation of adenylate cyclase, may be regulated via phosphorylation by a unique protein kinase. This recently discovered enzyme, known as the β-adrenergic receptor kinase, only phosphorylates the agonist-occupied form of the β-adrenergic receptor. To assess whether receptors coupled to the inhibition of adenylate cyclase might also be substrates, we examined the effects of β-adrenergic receptor kinase on the partially purified human platelet α₂-adrenergic receptor. Phosphorylation of the reconstituted α₂-adrenergic receptor was dependent on agonist occupancy and was completely blocked by co-incubation with α₂-antagonists. The time course of phosphorylation of the α₂-adrenergic receptor was virtually identical to that observed with the β-adrenergic receptor with maximum stoichiometries of 7-8 mol of phosphate/mol of receptor in each case. In contrast, the α₁-adrenergic receptor, which is coupled to stimulation of phosphatidylinositol hydrolysis, is not a substrate for the β-adrenergic receptor kinase. These results suggest that receptors coupled to either stimulation or inhibition of adenylate cyclase may be regulated by an agonist-dependent phosphorylation mediated by the β-adrenergic receptor kinase.