INTERACTION BETWEEN G-PROTEIN BETA-SUBUNIT AND GAMMA-SUBUNIT TYPES IS SELECTIVE

Signal-transducing guanine nucleotide-binding proteins (G proteins) are made up of three subunits, alpha, beta, and gamma. Each of these subunits comprises a family of proteins. The rules for association between members of one family with members of another to form a multimer are not known; it is not clear whether associations are specific or nonspecific. Other than transducin (G(t)), the G protein in rod photoreceptors, most purified G proteins contain more than one subtype of beta or gamma-subunits. The G(t) alpha-subunit is associated only with beta-1 and gamma-1. It is not known whether this specificity is due to the differential expression of these subunit types in a cell type or due to intrinsically different affinities between different beta and gamma-subunit types. We have used a transfected cell assay system to examine the association of the beta-1, beta-2, and beta-3 proteins with the gamma-1, and gamma-2 proteins. Results show that gamma-1 does not associate with beta-2 and that beta-3 does not associate with gamma-1 or gamma-2. Differences in affinities between types of G protein subunits will impose restrictions on the formation of certain heterotrimers and determine which G protein will be active in a cell. A chimeric molecule of beta-1 and beta-2 was used to broadly map the regions on these subunits that determine specificity of association.