Guanine-nucleotide binding proteins (G proteins) are involved in transmitting signals from cell surface receptors to downstream pathways. The G12/13 class of G proteins includes Ga12 and Ga13, which diverged from a single ancestral G12/13 protein during evolution. These proteins drive proliferation and migration in certain cell types, and also regulate cytoskeleton rearrangements and other cellular events. In order to identify key signaling determinants that evolved in the G12/13 class, we engineered mutants in which class-distinctive residues in Ga13 were substituted for residues conserved at the same structural position in the non-G12/13 classes of Ga proteins. These mutants were transfected into human kidney cells, and serum response element (SRE) activation assays identified several substitutions in Ga12 that hindered SRE activation. Surprisingly, the corresponding mutations in Ga13 showed no significant reduction in this serum response. Furthermore, we engineered chimeras in which Ga12 and Ga13 incorporated regions from the fruit fly G12/13 homolog, Concertina, and found a C-terminal region for which substitution of Concertina sequence abolished Ga12 signaling through SRE yet was fully tolerated in Ga13. These findings suggest Ga12 and Ga13 utilize distinct structural mechanisms for engaging the pathway leading to SRE-mediated transcription.
