RECONSTITUTION IN LIPOSOMES OF A MU-OPIOID BINDING-PROTEIN PURIFIED TO HOMOGENEITY FROM BOVINE STRIATAL MEMBRANES

An opioid binding protein (OBP) purified to homogeneity from bovine st riatal membranes was reconstituted into liposomes. For most experiment s a CHAPS extract of bovine striatum, devoid of opioid binding, served as source of G-proteins and lipids. Liposomes were formed by precipit ation of a mixture of OBP and CHAPS extract with polyethylene glycol-6 000 (PEG). Reconstituted OBP bound mu agonist ligands stereospecifical ly and with high affinity, similar to that of membrane-bound mu-recept ors. The binding was highly selective for mu-ligands, as compared to d elta and kappa-ligands and was completely inhibited by GTP gamma S. Si milar results were obtained in reconstitution experiments with purifie d G-proteins. Stimulation of low Km GTPase by mu-agonists was observed . These results indicate that recoupling of purified receptor with G p rotein has taken place in this system and confirm that OBP is a mu bin ding protein.