title: Mutation of a conserved serine in TM4 of opioid receptors confers full agonistic properties to classical antagonists.
abstract: The involvement of a conserved serine (Ser196 at the mu-, Ser177 at the delta-, and Ser187 at the kappa-opioid receptor) in receptor activation is demonstrated by site-directed mutagenesis. It was initially observed during our functional screening of a mu/delta-opioid chimeric receptor, mu delta2, that classical opioid antagonists such as naloxone, naltrexone, naltriben, and H-Tyr-Tic[psi,CH2NH]Phe-Phe-OH (TIPPpsi; Tic = 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) could inhibit forskolin-stimulated adenylyl cyclase activity in CHO cells stably expressing the chimeric receptor. Antagonists also activated the G protein-coupled inward rectifying potassium channel (GIRK1) in Xenopus oocytes coexpressing the mu delta2 opioid receptor and the GIRK1 channel. By sequence analysis and back mutation, it was determined that the observed antagonist activity was due to the mutation of a conserved serine to leucine in the fourth transmembrane domain (S196L). The importance of this serine was further demonstrated by analogous mutations created in the mu-opioid receptor (MORS196L) and delta-opioid receptor (DORS177L), in which classical opioid antagonists could inhibit forskolin-stimulated adenylyl cyclase activity in CHO cells stably expressing either MORS196L or DORS177L. Again, antagonists could activate the GIRK1 channel coexpressed with either MORS196L or DORS177L in Xenopus oocytes. These data taken together suggest a crucial role for this serine residue in opioid receptor activation.
authors.affiliation: Department of Pharmacology, Medical School, University of Minnesota, Minneapolis, 55455, USA.
authors.forename: P A
authors.initials: PA
authors.lastname: Claude
authors.forename: D R
authors.initials: DR
authors.lastname: Wotta
authors.forename: X H
authors.initials: XH
authors.lastname: Zhang
authors.forename: P L
authors.initials: PL
authors.lastname: Prather
authors.forename: T M
authors.initials: TM
authors.lastname: McGinn
authors.forename: L J
authors.initials: LJ
authors.lastname: Erickson
authors.forename: H H
authors.initials: HH
authors.lastname: Loh
authors.forename: P Y
authors.initials: PY
authors.lastname: Law
mesh_terms: D000818:Animals; D001483:Base Sequence; D017124:Conserved Sequence; D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D007930:Leucine / Q000235:genetics; D008969:Molecular Sequence Data; D009154:Mutation; D009292:Narcotic Antagonists / Q000494:pharmacology*; D009838:Oligodeoxyribonucleotides; D015221:Potassium Channels / Q000187:drug effects; D024661:Potassium Channels, Inwardly Rectifying; D011957:Receptors, Opioid / Q000819:agonists* / Q000235:genetics; D011993:Recombinant Fusion Proteins / Q000819:agonists / Q000037:antagonists & inhibitors / Q000235:genetics; D012694:Serine / Q000235:genetics*; D014982:Xenopus laevis
chemical_list: D051676:G Protein-Coupled Inwardly-Rectifying Potassium Channels; D009292:Narcotic Antagonists; D009838:Oligodeoxyribonucleotides; D015221:Potassium Channels; D024661:Potassium Channels, Inwardly Rectifying; D011957:Receptors, Opioid; D011993:Recombinant Fusion Proteins; D012694:Serine; D007930:Leucine