A 14-amino acid region of the G protein gamma subunit is sufficient to confer selectivity of gamma binding to the beta subunit.

Heterotrimeric guanine nucleotide-binding proteins are important signaling molecules composed of an alpha, beta, and gamma subunit. The beta subunits must form dimers with gamma subunits to function. Several subtypes of beta and gamma have been identified, but not all combinations of beta and gamma subtypes can form dimers. For example, the gamma 2 subunit can form dimers with beta 1 and beta 2, but gamma 1 forms dimers only with beta 1, not with beta 2. Selective dimerization may play a role in the regulation of beta gamma dimer-mediated signal transduction. In order to identify the region of gamma responsible for selective dimer formation, a series of gamma 1/gamma 2 chimeras was constructed, transcribed, and translated in vitro. The ability of these gamma chimeras to form dimers with beta 1 and beta 2 was assayed by trypsin protection and chemical cross-linking. When amino acids 36-49 of gamma 1 were substituted for 33-46 of gamma 2, the chimera behaved like gamma 1 and dimerized only with beta 1; the reciprocal chimera, in which 14 residues from gamma 2 were substituted for the corresponding amino acids of gamma 1, behaved like gamma 2 and interacted with both beta 1 and beta 2. This 14-amino acid region was sufficient for gamma 1 to discriminate between the beta subunits. All gamma chimeras were functional because they were able to interact with beta 1, which is capable of forming dimers with both gamma 1 and gamma 2. All dimers of chimeric gamma subunits plus beta 1 were able to interact with purified alpha o subunit, indicating that beta gamma dimers containing chimeric gamma molecules were capable of interacting with an appropriate third molecule. This lays the foundation for using these gamma chimeras to study selective dimer interactions with various effectors and receptors.