Blackwell Publishing Journal Backfiles 1879-2005
Although palmitoylation of the β2-adrenergic receptor (β2AR), as well as its phosphorylation by the cyclic AMP-dependant protein kinase (PKA) and the β-adrenergic receptor kinase (βARK), 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 (Ser345,346) of the β2AR plays in its rapid agonist-promoted phosphorylation and desensitization. Mutation of this site (Ala345,346β2AR) significantly reduced the rate and extent of the rapid desensitization promoted by sustained treatment with the agonist isoproterenol. The direct contribution of Ser345,346 in desensitization was then studied by mutating all other putative PKA and βARK phosphorylation sites (Ala261,262βARK−β2AR). 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, Ala261,262βARK−β2AR did not undergo rapid desensitization upon agonist treatment, indicating that phosphorylation of Ser345,346 alone is not sufficient to promote desensitization. Taken with the observation that mutation of either Ser345,346 or of the βARK phosphorylation sites prevented both the hyper-phosphorylation and constitutive desensitization of a palmitoylation-less mutant (Gly341β2AR), 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 Gly341β2AR by the catalytic subunit of PKA facilitated further phosphorylation of the receptor by purified βARK. Our study therefore allows us to propose a coordinated mechanism by which sequential depalmitoylation, and phosphorylation by PKA and βARK lead to the functional uncoupling and desensitization of the β2AR.
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