A novel PDZ domain containing guanine nucleotide exchange factor links heterotrimeric G proteins to Rho

Small GTP-binding proteins of the Rho family play a critical role in signal transduction. However, there is still very limited information on how they are activated by cell surface receptors. Here, we used a consensus sequenc e for Dbl domains of Rho guanine nucleotide exchange factors (GEFs) to sear ch DNA data bases, and identified a novel human GEF for Rho-related GTPases harboring structural features indicative of its possible regulatory mechan ism(s). This protein contained a tandem DH/PH domain closely related to tho se of Rho-specific GEFs, a PDZ domain, a proline-rich domain, and an area o f homology to Lsc, p115-RhoGEF, and a Drosophila RhoGEF that was termed Lsc -homology (LH) domain. This novel molecule, designated PDZ-RhoGEF, activate d biological and biochemical pathways specific for Rho, and activation of t hese pathways required an intact DH and PH domain. However, the PDZ domain was dispensable for these functions, and mutants lacking the LH domain were more active, suggesting a negative regulatory role for the LII domain. A s earch for additional molecules exhibiting an LH domain revealed a limited h omology with the catalytic region of a newly identified GTPase-activating p rotein for heterotrimeric G proteins, RGS14. This prompted us to investigat e whether PDZ-RhoGEF could interact with representative members of each G p rotein family We found that PDZ-RhoGEF was able to form, in vivo, stable co mplexes with two members of the G alpha(12) family, G alpha(12) and G alpha (13) and that this interaction was mediated by the LR domain. Furthermore, we obtained evidence to suggest that PDZ-Rho-GEF mediates the activation of Rho by G alpha(12) and G alpha(13). Together, these findings suggest the e xistence of a novel mechanism whereby the large family of cell surface rece ptors that transmit signals through heterotrimeric G proteins activate Rho- dependent pathways: by stimulating the activity of members of the G alpha(1 2) family which, in turn, activate an exchange factor acting on Rho.