Integration of multiple signals through cooperative regulation of the N-WASP-Arp2/3 complex

The protein N-WASP [a homolog to the Wiskott-Aldrich syndrome protein (WASP )I regulates actin polymerization by stimulating the actin-nucleating activ ity of the actin-related protein 2/3 (Arp2/3) complex. N-WASP is tightly re gulated by multiple signals: Only costimulation by Cdc42 and phosphatidylin ositol (4,5)-bisphosphate (PIP2) yields potent polymerization. We found tha t regulation requires N-WASP's constitutively active output domain (VCA) an d two regulatory domains: a Cdc42-binding domain and a previously undescrib ed PIP2-binding domain. In the absence of stimuli, the regulatory modules t ogether hold the VCA-Arp2/3 complex in an inactive "closed" conformation. I n this state, both the Cdc42- and PIP2-binding sites are masked. Binding of either input destabilizes the closed state and enhances binding of the oth er input. This cooperative activation mechanism shows how combinations of s imple binding domains can be used to integrate and amplify coincident signa ls.