Identification of a discrete region of the G protein gamma subunit conferring selectivity in beta gamma complex formation.

The identification of multiple G protein beta and gamma subunit subtypes suggests a potential diversity of beta gamma heterodimers, which may contribute to the specificity of signal transduction between receptors and effectors. The assembly of beta and gamma subtypes is selective. For example, gamma 1 can assemble with beta 1 but not with beta 2, whereas gamma 2 assembles with both beta isoforms. To identify the structural features of the beta and gamma subunits governing selectivity in heterodimer assembly, a series of nonisoprenylated chimeras of gamma 1 and gamma 2 was constructed, and their interaction with beta 1 and beta 2 was assessed by their ability to direct beta expression to the cytosol in cotransfected COS cells. All of the gamma 1/gamma 2 chimeras were capable of interacting with beta 1 as judged by the cotransfection assay. Chimeras containing gamma 2 sequence near the middle of the molecule between two conserved sequence motifs were capable of interacting as well with beta 2. Among 12 divergent residues in this region, it was found that replacement of three consecutive amino acids in gamma 1, Glu-Glu-Phe (residues 38-40), with the three corresponding amino acids of gamma 2, Ala-Asp-Leu (residues 35-37), conferred the ability to assemble with beta 2. The reciprocal chimera containing Glu-Glu-Phe in the context of gamma 2 failed to assemble with beta 2. The last residue of this triplet is occupied by a leucine in all known mammalian gamma subunits except gamma 1 and appears to be a key determinant of the ability of a gamma subunit to assemble with beta 2. This locus maps to a region of predicted alpha-helical structure in the gamma subunit, likely to represent a point of physical contact with the beta subunit.