Leukemia-Associated Rho Guanine Nucleotide Exchange Factor Promotes Gαq-Coupled Activation of RhoA

Leukemia-associated Rho guanine-nucleotide exchange factor (LARG) belongs to the subfamily of Dbl homology RhoGEF proteins (including p115 RhoGEF and PDZ-RhoGEF) that possess amino-terminal regulator of G protein signaling (RGS) boxes also found within GTPase-accelerating proteins (GAPs) for heterotrimeric G protein subunits. p115 RhoGEF stimulates the intrinsic GTP hydrolysis activity of G12/13 subunits and acts as an effector for G13-coupled receptors by linking receptor activation to RhoA activation. The presence of RGS box and Dbl homology domains within LARG suggests this protein may also function as a GAP toward specific G subunits and couple G activation to RhoA-mediating signaling pathways. Unlike the RGS box of p115 RhoGEF, the RGS box of LARG interacts not only with G12 and G13 but also with Gq. In cellular coimmunoprecipitation studies, the LARG RGS box formed stable complexes with the transition state mimetic forms of Gq, G12, and G13. Expression of the LARG RGS box diminished the transforming activity of oncogenic G protein-coupled receptors (Mas, G2A, and m1-muscarinic cholinergic) coupled to Gq and G13. Activated Gq, as well as G12 and G13, cooperated with LARG and caused synergistic activation of RhoA, suggesting that all three G subunits stimulate LARG-mediated activation of RhoA. Our findings suggest that the RhoA exchange factor LARG, unlike the related p115 RhoGEF and PDZ-RhoGEF proteins, can serve as an effector for Gq-coupled receptors, mediating their functional linkage to RhoA-dependent signaling pathways. Four classes of heterotrimeric G alpha proteins, Gs, Gi, Gq, and G12, couple heptahelical G protein-coupled receptors (GPCRs) to effectors to relay extracellular signals into eukaryotic cells (14). Each of these heterotrimeric G proteins is composed of an , , and subunit. The GDP-bound hetero