UCSF

Actin filament networks assemble on cellular membranes in response to signals that locally activate neural Wiskott-Aldrich-syndrome protein (N-WASP) and the Arp2/3 complex. An inactive conformation of N-WASP is stabilized by intramolecular contacts between the GTPase binding domain (GBD) and the C helix of the verprolin-homology, connector-helix, acidic motif (VCA) segment. Multiple SH3 domain-containing adapter proteins can bind and possibly activate N-WASP, but it remains unclear how such binding events relieve autoinhibition to unmask the VCA segment and activate the Arp2/3 complex. Here, we have used purified components to reconstitute a signaling cascade driven by membrane-localized Src homology 3 (SH3) adapters and N-WASP, resulting in the assembly of dynamic actin networks. Among six SH3 adapters tested, Nck was the most potent activator of N-WASP-driven actin assembly. We identify within Nck a previously unrecognized activation motif in a linker between the first two SH3 domains. This linker sequence, reminiscent of bacterial virulence factors, directly engages the N-WASP GBD and competes with VCA binding. Our results suggest that animals, like pathogenic bacteria, have evolved peptide motifs that allosterically activate N-WASP, leading to localized actin nucleation on cellular membranes.