Serine 329 of the mu-opioid receptor interacts differently with agonists

To investigate the effect of the hydrophilic Ser amino acid in position 329 of the human mu-opioid receptor (hMORwt) on the potency of various agonists, we mutated this residue to Ala (hMORS329A). Taking advantage of the functional coupling of the opioid receptor with the heteromultimeric G-protein-coupled inwardly rectifying potassium channel (GIRK1/GIRK2), either the wild-type hMOR or the mutated receptor (hMORS329A) was functionally coexpressed with GIRK1 and GIRK2 channels together with a regulator of G-protein signaling (RGS4) in Xenopus laevis oocytes. The two-microelectrode voltage-clamp technique was used to measure the opioid receptor activated GIRK1/GIRK2 channel responses. The potency of the peptide agonist [D-Ala(2),N-MePhe(4),Gly(5)-ol]-enkephalin (DAMGO) decreased as measured via hMORS329A, whereas the potency of nonpeptide agonists like morphine, fentanyl, and beta-hydroxyfentanyl (R004333) increased via the mutated receptor. Our results are indicative for the existence of hydrophilic interactions between Ser(329) and DAMGO, thereby decreasing the potency of DAMGO via the mutated receptor, whereas hydrophobic interactions between the mutated receptor and the N-phenylethyl of morphine and fentanyl can explain the increased potency. We conclude that the hydroxyl group of Ser(329) is not involved in the formation of a hydrogen bond with the beta-hydroxy group of fentanyl and that mutation of this residue to alanine caused dual effects depending on the nature of the ligand.