Different functional forms of G-protein βγ-subunits, βγ-I and βγ-II, in bovine brain

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Heterotrimeric GTP binding regulatory proteins (G proteins) are involved in the signal transduction process in cells. We have previously demonstrated that G protein (Gi/o) in bovine brain contains two subspecies of the βγ-subunit, βγ-I and βγ-II, with distinct γ subunits, i.e., γ-I and γ-II, but identical β-subunit. We found that γ-I agreed with the γ-subunit reported elsewhere, while γ-II was a novel γ-subunit. In the present study we separated the fractions containing G-protein isoforms, Go∗, Gi1, Go and Gi2, with a Mono Q column and they were subjected to 15% polyacrylamide gel electrophoresis. All isoforms were shown to possess both γ subunits, γ-I and γ-II. The molar ratio of the two γ-subunit isoforms was one to one, based on the relative intensity of Coomassie blue-stained bands. Differences in biological activity between G proteins composed of βγ-I and βγ-II were investigated. The amount of GTPγS bound to the α-subunit was larger in αβγ-I than in αβγ-II. The pertussis toxin-catalyzed ADP-ribosylation on α-subunit was enhanced by either βγ-subunit subspecies, but the effect was larger with βγ-I than with βγ-II. From gel filtration with a Sephacryl S 300, it appeared that all α- and either βγ-subunit, i.e., βγ-I or βγ-II, formed a trimer complex. These findings suggest the possible existence of two different functional forms in each G-protein isoform depending on the βγ-subunit subspecies.