Activation of Leukemia-associated RhoGEF by Gα13 with Significant Conformational Rearrangements in the Interface

The transient protein-protein interactions induced by guanine nucleotide-dependent conformational changes of G proteins play central roles in G protein-coupled receptor-mediated signaling systems. Leukemia-associated RhoGEF (LARG), a guanine nucleotide exchange factor for Rho, contains an RGS homology (RH) domain and Dbl homology/pleckstrin homology (DH/PH) domains and acts both as a GTPase-activating protein (GAP) and an effector for G alpha(13). However, the molecular mechanism of LARG activation upon G alpha(13) binding is not yet well understood. In this study, we analyzed the G alpha(13)-LARG interaction using cellular and biochemical methods, including a surface plasmon resonance (SPR) analysis. The results obtained using various LARG fragments demonstrated that active G alpha(13) interacts with LARG through the RH domain, DH/PH domains, and C-terminal region. However, an alanine substitution at the RH domain contact position in G alpha(13) resulted in a large decrease in affinity. Thermodynamic analysis revealed that binding of G alpha(13) proceeds with a large negative heat capacity change (Delta Cp degrees), accompanied by a positive entropy change (Delta S degrees). These results likely indicate that the binding of G alpha(13) with the RH domain triggers conformational rearrangements between G alpha(13) and LARG burying an exposed hydrophobic surface to create a large complementary interface, which facilitates complex formation through both GAP and effector interfaces, and activates the RhoGEF. We propose that LARG activation