Phosphorylation sites of KOPR (κ opioid receptor) following treatment with the selective agonist U50,488H {(−)(trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidiny)cyclo-hexyl]benzeneacetamide} were identified after affinity purification, SDS/PAGE, in-gel digestion with Glu-C and HPLC–MS/MS. Single- and double-phosphorylated peptides were identified containing phosphorylated Ser356, Thr357, Thr363 and Ser369 in the C-terminal domain. Antibodies were generated against three phosphopeptides containing pSer356/pThr357, pThr363 and pSer369 respectively, and affinity-purified antibodies were found to be highly specific for phospho-KOPR. U50,488H markedly enhanced staining of the KOPR by pThr363-, pSer369- and pSer356/pThr357-specific antibodies in immunoblotting, which was blocked by the selective KOPR antagonist norbinaltorphimine. Ser369 phosphorylation affected Thr363 phosphorylation and vice versa, and Thr363 or Ser369 phosphorylation was important for Ser356/Thr357 phosphorylation, revealing a phosphorylation hierarchy. U50,488H, but not etorphine, promoted robust KOPR internalization, although both were full agonists. U50,488H induced higher degrees of phosphorylation than etorphine at Ser356/Thr357, Thr363 and Ser369 as determined by immunoblotting. Using SILAC (stable isotope labelling by amino acids in cell culture) and HPLC–MS/MS, we found that, compared with control (C), U50,488H (U) and etorphine (E) KOPR promoted single phosphorylation primarily at Thr363 and Ser369 with U/E ratios of 2.5 and 2 respectively. Both induced double phosphorylation at Thr363+Ser369 and Thr357+Ser369 with U/E ratios of 3.3 and 3.4 respectively. Only U50,488H induced triple phosphorylation at Ser356+Thr357+Ser369. An unphosphorylated KOPR-(354–372) fragment containing all of the phosphorylation sites was detected with a C/E/U ratio of 1/0.7/0.4, indicating that ∼60% and ∼30% of the mouse KOPR are phosphorylated following U50,488H and etorphine respectively. Thus KOPR internalization requires receptor phosphorylation above a certain threshold, and higher-order KOPR phosphorylation may be disproportionally important.

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