A Polybasic Motif Allows N-WASP to Act as a Sensor of PIP 2 Density

Abstract

Phosphatidylinositol 4,5-bisphosphate (PIP 2) activates the actin regulatory protein N-WASP by binding to a short polybasic region involved in N-WASP autoinhibition. Here, we show that unlike canonical lipid binding modules, such as PH domains, this polybasic motif binds PIP 2 in a multivalent, cooperative manner. As a result, PIP 2 activation of N-WASP-mediated actin polymerization in vitro and in extracts is ultrasensitive: above a certain threshold, N-WASP responds in a switch-like manner to a small increase in the density of PIP 2 (Hill coefficient n H = ∼20). We show that the sharpness of the PIP 2 activation threshold can be tuned by varying the length of the polybasic motif. This sharp activation threshold may help suppress N-WASP activation by quiescent PIP 2 levels yet leave it poised for activation upon subtle, signaling-induced perturbations in PIP 2 distribution.