The palmitoylation state of the beta(2)-adrenergic receptor regulates the synergistic action of cyclic AMP-dependent protein kinase and beta-adrenergic receptor kinase involved in its phosphorylation and desensitization

Although palmitoylation of the beta (2)-adrenergic receptor (beta (2)AR), as well as its phosphorylation by the cyclic AMP-dependant protein kinase (PKA) and the p-adrenergic receptor kinase (beta ARK), are known to play important roles in agonist-promoted desensitization, their relative contribution and mutual regulatory influences are still poorly understood. In this study, we investigated the role that the carboxyl tail PKA site (Ser(345,346)) of the beta (2)AR plays in its rapid agonist-promoted phosphorylation and desensitization. Mutation of this site (Ala(345,346)beta (2)AR) significantly reduced the rate and extent of the rapid desensitization promoted by sustained treatment with the agonist isoproterenol, The direct contribution of Ser(345,346) in desensitization was then studied by mutating all other putative PKA and beta ARK phosphorylation sites (Ala(261,262)beta ARK-beta (2)AR). We found this mutant receptor to be phosphorylated upon receptor activation but not following direct activation of PKA, suggesting a role in receptor-specific (homologous) but not heterologous phosphorylation. However, despite its phosphorylated state, Ala(261,262)beta ARK(-)beta (2)AR did not undergo rapid desensitization upon agonist treatment, indicating that phosphorylation of Ser(345,346) alone is not sufficient to promote desensitization. Taken with the observation that mutation of either Ser(345,346) or of the beta ARK phosphorylation sites prevented both the hyperphosphorylation and constitutive desensitization of a palmitoylation-less mutant (Gly(341)beta (2)AR), Our data suggest a concerted/synergistic action of the two kinases that depends on the palmitoylation state of the receptor. Consistent with this notion, in vitro phosphorylation of Gly(341)beta (2)AR by the catalytic subunit of PKA facilitated further phosphorylation of the receptor by purified beta ARK. Our study therefore allows us to propose a coordinated mechanism by which sequential depalmitoylation, and phosphorylation by PKA and beta ARK lead to the functional uncoupling and desensitization of the beta (2)AR.