G Protein independent phosphorylation and internalization of the δ-opioid receptor

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Agonist activation of the δ-opioid receptor leads to internalization via Gβγ recruitment of G protein coupled receptor kinase-2, which phosphorylates the receptor at several sites, including Ser363, allowing β-arrestin binding and localization to clathrin coated pits. Using human embryonic kidney cells expressing a δ-opioid receptor we tested the hypothesis that prevention of receptor coupling to G protein by treatment with pertussis toxin (PTX) will block these processes. PTX treatment did not reduce phosphorylation of δ-opioid receptor Ser363 in response to the agonist [D-Pen2, D-Pen5]enkephalin, or recruitment of β-arrestin 2-green fluorescent protein to the membrane and only slowed, but did not prevent, [D-Pen2, D-Pen5]enkephalin-induced internalization. Similarly, PTX treatment only partially prevented the ability of the δ-opioid peptide agonists deltorphin II and [Met5]enkephalin and the non-peptide agonist BW373U86 to induce receptor internalization. No internalization was seen with morphine, oxymorphindole or the putative δ1-opioid agonist TAN-67 in the presence or absence of PTX, even though TAN-67 showed a strong activation of G protein, as measured by guanosine-5′-O-(3-[35S]thio)triphosphate binding. The ability of an agonist to stimulate phosphorylation at Ser363 was predictive of its capacity to induce internalization. The results suggest a role for G protein in δ-opioid receptor internalization, but show that alternative G protein independent pathways exist.

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