Role for the C-terminus in agonist-induced mu opioid receptor phosphorylation and desensitization

Determining which domains and amino acid residues of the mu opioid receptor are phosphorylated is critical for understanding the mechanism of mu opioi d receptor phosphorylation. The role of the C-terminus of the receptor was investigated by examining the C-terminally truncated or point-mutated mu op ioid receptors in receptor phosphorylation and desensitization. Both wild-t ype and mutated receptors were stably expressed in Chinese hamster ovary (C HO) cells. The receptor expression was confirmed by receptor radioligand bi nding and immunoblotting. After exposure to 5 mu M of DAMGO, phosphorylatio n of the C-terminally truncated receptor and the mutant receptor T394A was reduced to 30 and 10% of that of the wild-type receptor, respectively. Muta tion effects on agonist-induced desensitization were studied using adenylyl cyclase inhibition assays. The C-terminally truncated receptor and mutant receptor T394A both showed complete loss of DAMGO-induced desensitization, while the mutant T/S-7A receptor only lost part of its ability to desensiti ze. Taken together, these results suggest that the C-terminus of the mu opi oid receptor participates in receptor phosphorylation and desensitization w ith threonine 394, a crucial residue for both features. DAMGO-induced mu op ioid receptor phosphorylation and desensitization are associated and appear to involve both the mu opioid receptor C-terminus and other domains of the receptor.