An acidic motif within the third intracellular loop of the alpha2C2 adrenergic receptor is required for agonist-promoted phosphorylation and desensitization.

The alpha 2C2 adrenergic receptor contains a highly acidic stretch of amino acids (EDEAEEEEEEEEEEEE) within the third intracellular loop. To investigate the role of this region, we utilized site-directed mutagenesis to delete these 16 amino acids as well as to substitute them with glutamine, thereby conserving size but not charge. The wild-type and mutated alpha 2C2 receptors were permanently expressed in CHO cells. Neither substitution nor deletion of this region affected receptor expression, agonist or antagonist binding affinities, guanine nucleotide-sensitive formation of the high-affinity agonist-receptor-G protein complex, or functional coupling of the receptor to Gi. We considered that since alpha 2C2 agonist-promoted desensitization is due to phosphorylation by the beta-adrenergic receptor kinase (or a related kinase), that this region may be important for establishing the acidic mileau required by this kinase. Therefore, the consequences of 30 min of agonist preexposure on subsequent alpha 2C2-mediated inhibition of adenylyl cyclase and on high-affinity agonist binding were determined for the wild-type and these two mutants. The wild-type alpha 2C2 receptor underwent approximately 52% functional desensitization and a approximately 40% loss of high-affinity binding after such exposure. In contrast, deletion and substitution of this acidic stretch of amino acids ablated desensitization as assessed by both approaches. These results correlated with those obtained in whole cell phosphorylation experiments. Cells expressing each receptor were incubated with [32P]orthophosphate and exposed to agonist, and receptors were purified by immunoprecipitation. The deletion and the substitution mutant receptors underwent agonist-promoted phosphorylation at levels of only 44 +/- 5% and 50 +/- 15%, respectively, relative to wild-type alpha 2C2.(ABSTRACT TRUNCATED AT 250 WORDS)