The purified α subunits of G(o) and G(i) from bovine brain require βγ for association with phospholipid vesicles

The purified G-proteins from bovine brain were examined for potential solubility in the absence of detergent. The isolated α(o) and α(i) subunits migrated through sucrose with rates consistent with the existence of monomeric species either in the presence or the absence of cholate. The βγ subunits or holo-G-proteins aggregated extensively if cholate was absent. Al³⁺, Mg²⁺, and F^- prevented the aggregation of α(o) and α(i) caused by the addition of βγ and could also prevent the aggregation of α(s) when G(s) was examined at higher temperature. The association of subunits with phospholipid vesicles was examined. Whereas βγ associated totally with phospholipid vesicles, purified α(o) showed little interaction. α(o) did bind to vesicles containing βγ (βγ vesicles) in a saturable fashion that indicated a stoichiometric association between the subunits. Treatment with guanosine 5'-(3-O-thio)triphosphate could partially dissociate α(o) that was bound to βγ vesicles. These data suggest that βγ may be an anchor for association of α subunits with membranes and that regulation by these proteins may not be limited to the plasma membrane. This possibility and its implications are discussed. The reversible association of α(o) to βγ vesicles may provide a very sensitive system for the study of the interactions between these subunits.