A Polybasic Motif Allows N-WASP to Act as a Sensor of PIP2 Density
Phosphatidylinositol 4,5-bisphosphate (PIP2) 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 PIP2 in a multivalent, cooperative manner. As a result, PIP2 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 PIP2 (Hill coefficient nH = ~20). We show that the sharpness of the PIP2 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 PIP2 levels yet leave it poised for activation upon subtle, signaling-induced perturbations in PIP2 distribution.