GβγActivatesPIP2 Hydrolysis by Recruiting and OrientingPLCβon the Membrane Surface

PLCβscatalyze the hydrolysis ofPIP2 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.PLCβsare regulated by GPCRs through direct interaction withGα_qandGβγ. This study addresses the mechanism by whichGβγactivatesPLCβ3. We show thatPLCβ3 functions as a slow Michaelis-Menten enzyme (k_cat~2sec⁻¹,K_M~0.43mol%) on membrane surfaces. Its partition coefficient (K_x~2.9 * 10⁴) is such that only a small quantity ofPLCβ3 exists in the membrane in the absence ofGβγ. WhenGβγis present, equilibrium binding (K_eq~0.009mol%) increasesPLCβ3 in the membrane, increasingV_maxin proportion. Atomic structures on membrane vesicle surfaces show that twoGβγanchorPLCβ3 with its catalytic site oriented toward the membrane surface. This principle of activation explains rapid stimulated catalysis with low background catalysis.