Crystal structure of Rac1 bound to its effector phospholipase C-β2

Although diverse signaling cascades require the coordinated regulation of heterotrimeric G proteins and small GTPases, these connections remain poorly understood. We present the crystal structure of the GTPase Rac1 bound to phospholipase C-beta 2 (PLC-beta 2), a classic effector of heterotrimeric G proteins. Rac1 engages the pleckstrin-homology (PH) domain of PLC-beta 2 to optimize its orientation for substrate membranes. G beta gamma also engages the PH domain to activate PLC-beta 2, and these two activation events are compatible, leading to additive stimulation of phospholipase activity. In contrast to PLC-delta, the PH domain of PLC-beta 2 cannot bind phosphoinositides, eliminating this mode of regulation. The structure of the Rac1-PLC-beta 2 complex reveals determinants that dictate selectivity of PLC-beta isozymes for Rac GTPases over other Rho-family GTPases, and substitutions within PLC-beta 2 abrogate its stimulation by Rac1 but not by G beta gamma, allowing for functional dissection of this integral signaling node.