Gβγ Activates PIP2 Hydrolysis by Recruiting and Orienting PLCβ on the Membrane Surface

PLCβs catalyze the hydrolysis of PIP2 into IP3 and DAG. PIP2 regulates the activity of many membrane proteins, while IP3 and DAG lead to increased intracellular Ca2+ levels and activate PKC, respectively. PLCβs are regulated by GPCRs through direct interaction with Gαq and Gβγ. This study addresses the mechanism by which Gβγ activates PLCβ3. We show that PLCβ3 functions as a slow Michaelis-Menten enzyme (kcat~2 sec−1, KM~0.43 mol%) on membrane surfaces. Its partition coefficient (Kx~2.9 * 104) is such that only a small quantity of PLCβ3 exists in the membrane in the absence of Gβγ. When Gβγ is present, equilibrium binding (Keq~0.009 mol%) increases PLCβ3 in the membrane, increasing Vmax in proportion. Atomic structures on membrane vesicle surfaces show that two Gβγ anchor PLCβ3 with its catalytic site oriented toward the membrane surface. This principle of activation explains rapid stimulated catalysis with low background catalysis.