Bovine Brain GO Isoforms Have Distinct γ Subunit Compositions

Abstract

The gamma subunit composition of the major bovine brain Go and Gi proteins (GOA, GOB, GOC, Gi1, and Gi2) was characterized using antibodies against specific gamma isoforms. Each of the purified G protein heterotrimers contained a heterogeneous population of gamma subunits, and the profiles of the gamma subunits found with Gi1, Gi2, and GOA were similar. In contrast, each GO isoform had a distinct pattern of associated gamma subunits. These differences were surprising given that all three alpha O isoforms are thought to share a common amino-terminal sequence important for the binding of beta gamma dimers and that the alpha OA 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 differences in their post-translational processing. These results indicate that both the alpha and gamma 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 alpha subunits.

Characterization of the Major Bovine Brain Go α Isoforms

“…An ␣ oC -like protein has been observed by a large number of groups (21)(22)(23)(27)(28)(29)(30). Recently, we have found that it is a relatively abundant protein in brain, constituting about a third of the G o of cerebral cortex.…”

“…Three of the isoforms, ␣ oA , ␣ oB , and ␣ oC , are purified associated with ␤␥ dimers as G oA , G oB , and G oC , while ␣ oD , an inconsistently observed ␣ o isoform, is not. Immunological evidence and peptide mapping suggest that ␣ oB and ␣ oD are protein products of the ␣ o2 mRNA, whereas ␣ oA and ␣ oC are translated from the ␣ o1 mRNA (23)(24)(25)(26). The functional significance of these different ␣ o isoforms is not yet clear, but the G oA , G oB , and G oC heterotrimers each contain a different assortment of ␤␥ dimers (23).…”

“…Based upon protein characterization, however, there are at least four ␣ o isoforms in bovine brain, ␣ oA , ␣ oB , ␣ oC , and ␣ oD (21)(22)(23). Three of the isoforms, ␣ oA , ␣ oB , and ␣ oC , are purified associated with ␤␥ dimers as G oA , G oB , and G oC , while ␣ oD , an inconsistently observed ␣ o isoform, is not.…”

“…Immunological evidence and peptide mapping suggest that ␣ oB and ␣ oD are protein products of the ␣ o2 mRNA, whereas ␣ oA and ␣ oC are translated from the ␣ o1 mRNA (23-26). The functional significance of these different ␣ o isoforms is not yet clear, but the G oA , G oB , and G oC heterotrimers each contain a different assortment of ␤␥ dimers (23).An ␣ oC -like protein has been observed by a large number of groups (21)(22)(23)(27)(28)(29)(30). Recently, we have found that it is a relatively abundant protein in brain, constituting about a third of the G o of cerebral cortex.…”

“…Recently, we have found that it is a relatively abundant protein in brain, constituting about a third of the G o of cerebral cortex. 1 Several studies have tried to identify the way in which ␣ oC differs from its immunologically related ␣ oA protein (23)(24)(25)(26) but without success. One suggestion is that ␣ oC differs from ␣ oA at the N terminus and that it might be an unmyristoylated form of the protein (29,30).…”