Allelic variation S268P of the human mu-opioid receptor affects both desensitization and G protein coupling

The decrease in mu-opioid receptor activity after chronic agonist exposure (1 mu M [D-Ala(2),N-MePhe(4),Gly-ol(5)]-enkephalin) is largely due to kinas e-mediated phosphorylation of intracellular receptor domains. We have recen tly shown that the substitution of two putative Ca2+/calmodulin-dependent p rotein kinase II (CaMK II) phosphorylation sites, S261 and S266, by alanine s in the third intracellular loop of the rat mu-opioid receptor (rMOR1) con fers resistance to CaMK II-induced receptor desensitization. In the present study, we show that the injection of active CaMK II in Xenopus laevis oocy tes led to the desensitization of S261A but not S266A receptor mutant, indi cating that S266 is the primary CaMK II phosphorylation site of the rMOR1. For the corresponding phosphorylation site in the human mu-opioid receptor (hMOR), an allelic variation S268P has been recently identified. After expr ession in X. laevis oocytes and human embryonic kidney 293 cells, this huma n S268P receptor and a corresponding rat S266P receptor mutant revealed a l oss of CaMK II-induced receptor desensitization and a decreased G protein c oupling compared with the wild-type receptors. Our results suggest that ser ines 266 (rMOR1) and 268 (hMOR) play crucial role in receptor desensitizati on and signaling and that the allelic variation S268P results in a human re ceptor type with a weaker but persistent G protein coupling after agonist t reatment.