Agonist-induced Desensitization of the μ Opioid Receptor Is Determined by Threonine 394 Preceded by Acidic Amino Acids in the COOH-terminal Tail

To identify the structural determinants necessary for μ opioid receptor desensitization, we serially ablated potential phosphorylation sites in the carboxyl tail of the receptor and examined their effects on [d-Ala2,N-Me-Phe4,Gly-ol5]enkephalin (DAMGO)-induced desensitization. First, we replaced Thr394with alanine (T394A) and stably expressed this mutant receptor in Chinese hamster ovary cells. The T394A receptor did not desensitize after 1 h of treatment with DAMGO, indicating that Thr394 is required for agonist-induced early desensitization. To test whether Thr394 was the only residue necessary, we investigated the importance of 7 potential phosphorylation sites between residues 363 and 383, which were all replaced by alanines with the Thr394 maintained. This mutant (AT) showed partial loss of desensitization (30%), which was attributable to the Ala mutation at Thr383, since complete desensitization was achieved by restoring Thr383 (ATT). These results suggest that Thr394 is the primary recognition site for G protein-coupled receptor kinases, but Thr383 is also required for complete agonist-induced desensitization. The specificity of Thr394 as the primary initiation site appears to be dependent on the preceding acidic amino acid stretch, because in a mutant in which glutamic acid residues at 388, 391, and 393 were replaced by glutamines (EQ), agonist-induced desensitization was completely abolished, identical to the T394A mutant.