MOLECULAR HETEROGENEITY OF THE BETA-GAMMA-SUBUNITS OF GTP-BINDING PROTEINS IN BOVINE BRAIN MEMBRANES

The guanine nucleotide-binding proteins (G proteins) are heterotrimers composed of α-,β-, and γ-subunits, and each of the constituent subunits has been reported to exhibit a molecular heterogeneity. The β- and γ-subunits form a functional unit that does not separate under physiological conditions and interact with various α-subunits that appear to mainly regulate specific effectors. We thus purified the βγ-complex of G proteins from bovine brain membranes and found that there were chromatographically multiple forms of βγ-subunits which could be reassociated with various α-subunits. The major findings observed with the purified proteins were summarized as follows. (a) The constituent βγ-subunits in the brain membrane G proteins appeared to be divided into two groups in their elution profiles from a hydrophobic column. (b) Each of the two groups contained at least five different components of βγ-subunits upon analyzing by a high-resolution, anion-exchange column. (c) Distribution of the heterogenous βγ-subunits was not identical among various trimeric G proteins such as Gi, G0, and Gs. (d) The heterogenous βγ-components were able to interact with a specific α-subunit resulting in the αβγ-trimer that served as the substrate of pertussis toxin-catalyzed ADP-ribosylation. (e) However, the apparent abilities of some βγ-subunits to support the toxin-induced modification were significantly different in a special comparison between the two βγ-groups that were eluted from the hydrophobic column. These results indicated that there were multiple forms of βγ-subunits associating with the specific α-subunit of a trimeric G protein and that some of those had different affinities for various α-subunits in terms of their tight associations. A possible role of the heterogeneity in βγ-subunits is also discussed in terms of G protein-mediated signal transductions. © 1992.