Abstract

Activation of the A₃ adenosine receptor (A₃AR) contributes to the cardioprotective, bronchoconstrictive, and hypotensive effects of adenosine. Agonist occupation of the A₃AR results in a rapid desensitization of receptor function, which is associated with the phosphorylation of the receptor protein by one or more members of the G protein-coupled receptor kinase family of protein kinases. Although we demonstrated previously that phosphorylation of the C-terminal 14 amino acids of the rat A₃AR is crucial for rapid desensitization to occur, the identity of the critical phosphorylation sites has remained unknown. Here, we demonstrate that the simultaneous mutation of Thr³⁰⁷, Thr³¹⁸, and Thr³¹⁹ to Ala residues dramatically reduces agonist-stimulated phosphorylation and rapid desensitization of the rat A₃AR. Individual mutation of each residue demonstrated that Thr³¹⁸ and Thr³¹⁹ are the major sites of phosphorylation. Phosphorylation at Thr³¹⁸ appeared to be necessary to observe phosphorylation at Thr³¹⁹, but not vice versa. However, the replacement of Thr³¹⁸ with a glutamate residue demonstrated that the simple addition of negative charge at position 318 was not sufficient to rescue phosphorylation at position 319. In addition, the mutation of two predicted palmitoylation-site cysteine residues proximal to the regulatory domain resulted in the appearance of an agonist-independent basal phosphorylation. Therefore, G protein-coupled receptor kinase-mediated phosphorylation of the C-terminal tail of the A₃AR in situ appears to follow a sequential mechanism, perhaps involving receptor depalmitoylation, with phosphorylation at Thr³¹⁸ being particularly important.