Expression of the μ‐Opioid Receptor in CHO Cells: Ability of μ‐Opioid Ligands to Promote α‐Azidoanilido[³²P]GTP Labeling of Multiple G Protein α Subunits

Abstract: The identities of heterotrimeric G proteins that can interact with the μ‐opioid receptor were investigated by α‐azidoanilido[³²P]GTP labeling of α subunits in the presence of opioid agonists in Chinese hamster ovary (CHO)‐MORIVA3 cells, a CHO clone that stably expressed μ‐opioid receptor cDNA (MOR‐1). This clone expressed 1.01 × 10⁶μ‐opioid receptors per cell and had higher binding affinity and potency to inhibit adenylyl cyclase for the μ‐opioid‐selective ligands [d‐Ala²,N‐MePhe⁴,Gly‐ol]‐enkephalin and [N‐MePhe³,d‐Pro⁴]‐morphiceptin, relative to the δ‐selective opioid agonist [d‐Pen²,d‐Pen⁵]‐enkephalin or the κ‐selective opioid agonist U‐50,488H. μ‐Opioid ligands induced an increase in α‐azidoanilido[³²P]GTP photoaffinity labeling of four Gα subunits in this clone, three of which were identified as G_i3α, G_i2α, and G_o2α. The same pattern of simultaneous interaction of the μ‐opioid receptor with multiple Gα subunits was also observed in two other clones, one expressing about three times more and the other 10‐fold fewer receptors as those expressed in CHO‐MORIVA3 cells. The opioid‐induced increase of labeling of these G proteins was agonist specific, concentration dependent, and blocked by naloxone and by pretreatment of these cells with pertussis toxin. A greater agonist‐induced increase of α‐azidoanilido[³²P]GTP incorporation into G_i2α (160–280%) and G_o2α (110–220%) than for an unknown Gα (G_?α) (60%) or G_i3α (40%) was produced by three different μ‐opioid ligands tested. In addition, slight differences were also found between the ability of various μ‐opioid agonists to produce half‐maximal labeling (ED₅₀) of any given Gα subunit, with a rank order of G_i3α > G_o2α > G_i2α = G_?α. In any case, these results suggest that the activated μ‐opioid receptor couples to four distinct G protein α subunits simultaneously.