Functional reconstitution of a highly purified mu-opioid receptor protein with purified G proteins in liposomes.

A mu-opioid receptor protein (mu-ORP) purified to homogeneity from bovine striatal membranes has been functionally reconstituted in liposomes with highly purified heterotrimeric guanine nucleotide regulatory proteins (G proteins). A mixture of bovine brain G proteins, predominantly GoA, was used for most of the experiments, but some experiments were performed with individual pure G proteins, GoA, GoB, Gi1, and Gi2. Low Km GTPase was stimulated up to 150% by mu-opioid receptor agonists when both mu-ORP and a G protein (either the brain G protein mixture or a single heterotrimeric G protein) were present in the liposomes. Stimulation by a selective mu-agonist was concentration dependent and was reversed by the antagonist (-)-naloxone, but not by its inactive enantiomer, (+)-naloxone. The mu selectivity of mu-ORP was demonstrated by the inability of delta and kappa agonists to stimulate GTPase in this system. High-affinity mu-agonist binding was also restored by reconstitution with the brain G protein mixture and with each of the four pure Gi and G(o) proteins studied. The binding of mu agonists is sensitive to inhibition by GTP gamma S and by sodium.