BOVINE BRAIN G(O) ISOFORMS HAVE DISTINCT GAMMA-SUBUNIT COMPOSITIONS

The gamma subunit composition of the major bovine brain G(O) and G(i) proteins (G(OA), G(OB), G(OC), G(i1), and G(i2)) was characterized usi ng antibodies against specific gamma isoforms. Each of the purified Gr protein heterotrimers contained a heterogeneous population of gamma s ubunits, and the profiles of the gamma subunits found with G(i1), G(i2 ), and G, were similar. In contrast, each G(O) isoform had a distinct pattern of associated gamma subunits, These differences were surprisin g given that all three alpha(O) isoforms are thought to share a common amino-terminal sequence important for the binding of beta gamma dimer s and that the alpha(CO) and alpha(OC) proteins may come from the same alpha(O1) mRNA. The free alpha(OA) and alpha(OC) subunits had unique elution behaviors during MonoQ chromatography, compatible with differe nces in their post-translational processing, These results indicate th at both the alpha and gamma subunit compositions of heterotrimers defi ne the structure of an intact G protein, Furthermore, the exact subuni t composition of G protein heterotrimers may depend upon regulated exp ression of different subunit isoforms or upon cellular processing of a lpha subunits.