Gα q Directly Activates p63RhoGEF and Trio via a Conserved Extension of the Dbl Homology-associated Pleckstrin Homology Domain

The coordinated cross-talk from heterotrimeric G proteins to Rho GTPases is essential during a variety of physiological processes. Emerging data suggest that members of the Gα12/13 and Gαq/11 families of heterotrimeric G proteins signal downstream to RhoA via distinct pathways. Although studies have elucidated mechanisms governing Gα12/13-mediated RhoA activation, proteins that functionally couple Gαq/11 to RhoA activation have remained elusive. Recently, the Dbl-family guanine nucleotide exchange factor (GEF) p63RhoGEF/GEFT has been described as a novel mediator of Gαq/11 signaling to RhoA based on its ability to synergize with Gαq/11 resulting in enhanced RhoA signaling in cells. We have used biochemical/biophysical approaches with purified protein components to better understand the mechanism by which activated Gαq directly engages and stimulates p63RhoGEF. Basally, p63RhoGEF is autoinhibited by the Dbl homology (DH)-associated pleckstrin homology (PH) domain; activated Gαq relieves this autoinhibition by interacting with a highly conserved C-terminal extension of the PH domain. This unique extension is conserved in the related Dbl-family members Trio and Kalirin and we show that the C-terminal Rho-specific DH-PH cassette of Trio is similarly activated by Gαq.