Mechanisms for Reversible Regulation between G13 and Rho Exchange Factors

Abstract The heterotrimeric G proteins, G12 and G13, mediate signaling between G protein-coupled receptors and the monomeric GTPase, RhoA. One pathway for this modulation is direct stimulation by Gα13 of p115 RhoGEF, an exchange factor for RhoA. The GTPase activity of both Gα12 and Gα13 is increased by the N terminus of p115 Rho guanine nucleotide exchange factor (GEF). This region has weak homology to the RGS box sequence of the classic regulators of G protein signaling (RGS), which act as GTPase-activating proteins (GAP) for Gi and Gq. Here, the RGS region of p115 RhoGEF is shown to be distinctly different in that sequences flanking the predicted “RGS box” region are required for both stable expression and GAP activity. Deletions in the N terminus of the protein eliminate GAP activity but retain substantial binding to Gα13 and activation of RhoA exchange activity by Gα13. In contrast, GTRAP48, a homolog of p115 RhoGEF, bound to Gα13 but was not stimulated by the α subunit and had very poor GAP activity. Besides binding to the N-terminal RGS region, Gα13 also bound to a truncated protein consisting only of the Dbl homology (DH) and pleckstrin homology (PH) domains. However, Gα13 did not stimulate the exchange activity of this truncated protein. A chimeric protein, which contained the RGS region of GTRAP48 in place of the endogenous N terminus of p115 RhoGEF, was activated by Gα13. These results suggest a mechanism for activation of the nucleotide exchange activity of p115 RhoGEF that involves direct and coordinate interaction of Gα13 to both its RGS and DH domains.