Serine 363 Is Required for Nociceptin/Orphanin FQ Opioid Receptor (NOPR) Desensitization,Internalization, and Arrestin Signaling

We determined the role of carboxyl-terminal regulation of NOPR (nociceptin, orphanin FQ receptor) signaling and function. We mutated C-terminal serine and threonine residues and examined their role in NOPR trafficking, homologous desensitization, and arrestin-dependent MAPK signaling. The NOPR agonist, nociceptin, caused robust NOPR-YFP receptor internalization, peaking at 30 min. Mutation of serine 337, 346, and 351, had no effect on NOPR internalization. However, mutation of C-terminal threonine 362, serine 363, and threonine 365 blocked nociceptin-induced internalization of NOPR. Furthermore, point mutation of only Ser-363 was sufficient to block NOPR internalization. Homologous desensitization of NOPR-mediated calcium channel blockade and inhibition of cAMP were also shown to require Ser-363. Additionally, NOPR internalization was absent when GRK3, and Arrestin3 were knocked down using siRNA, but not when GRK2 and Arrestin2 were knocked down. We also found that nociceptin-induced NOPR-mediated JNK but not ERK signaling requires Ser-363, GRK3, and Arrestin3. Dominant-positive Arrestin3 but not Arrestin2 was sufficient to rescue NOPR-S363A internalization and JNK signaling. These findings suggest that NOPR function may be regulated by GRK3 phosphorylation of Ser-363 and Arrestin3 and further demonstrates the complex nature of G-protein-dependent and -independent signaling in opioid receptors.