The G12/13 class of heterotrimeric G proteins, composed of the α subunits Gα12 and Gα13, is implicated in a variety of receptor-mediated cellular responses that include cell proliferation and cytoskeletal rearrangements. Gα12 and Gα13 are well-characterized activators of serum response element (SRE) mediated transcription, a pathway important in cell growth signaling. Invertebrate homologs of the G12/13 class harbor ability to drive cytoskeletal changes, but their signaling in cell growth has not been studied. We found the Drosophila and Caenorhabditis elegans α subunits, Concertina (Cta) and Gpa-12 respectively, to lack ability to drive SRE signaling as mutationally activated forms expressed in cultured human cells. To identify determinants of growth signaling that evolved in Gα12 and Gα13, we assembled chimeras using regions of Cta and Gpa-12 in place of corresponding sections of the mammalian α subunits. The C-terminal regions of Gα12 and Gα13 were found to utilize distinct structural features for driving SRE-mediated transcription. Insertion of a 45 amino acid region of Cta fully disrupted Gα12 signaling to SRE, whereas the same Cta substitution had no effect on Gα13 signaling. We identified key amino acids in this region required for Gα12 signaling to SRE, along with Gα12 target proteins that may require these residues for binding. We also generated Gα12 and Gα13 chimeras using regions of Gpa-12, and found Gα13 harboring the Gpa-12 C-terminus to exhibit normal SRE signaling. However, insertion of Gpa-12 sequence adjacent to the switch III region fully disrupted Gα13 signaling to SRE. These findings reveal key growth signaling determinants in the G12/13 C-terminal regions, and indicate that Gα13 utilizes structural features distinct from Gα12 in driving SRE-mediated transcription.
