Bovine Brain GO Isoforms Have Distinct γ Subunit Compositions (∗)

The γ subunit composition of the major bovine brain GO and Gi proteins (GOA, GOB, GOC, Gi1, and Gi2) was characterized using antibodies against specific γ isoforms. Each of the purified G protein heterotrimers contained a heterogeneous population of γ subunits, and the profiles of the γ subunits found with Gi1, Gi2, and GOA were similar. In contrast, each GO isoform had a distinct pattern of associated γ subunits. These differences were surprising given that all three αO isoforms are thought to share a common amino-terminal sequence important for the binding of βγ dimers and that the αOA and αi1 proteins may come from the same αO mRNA. The free αOA and αi1 subunits had unique elution behaviors during MonoQ chromatography, compatible with differences in their post-translational processing. These results indicate that both the α and γ subunit compositions of heterotrimers define the structure of an intact G protein. Furthermore, the exact subunit composition of G protein heterotrimers may depend upon regulated expression of different subunit isoforms or upon cellular processing of α subunits.