Proposed 4:2:1 Nck/N-WASp/Arp2/3 mechanism leading to actin polymerization. In our proposed mechanism, two Nck molecules bind each N-WASp; one Nck molecule binds directly to the proline-rich region of N-WASp, whereas a second Nck molecule binds the proline-rich region of WIP, which in turn binds the WH1 domain of N-WASp. Two activated N-WASp molecules then bind and activate Arp2/3, resulting in actin filament nucleation and polymerization. Although not drawn precisely to scale, the molecular mechanism presented here is intended to demonstrate the spatial arrangement of all proteins near the membrane in our experimental system. B, basic region; GBD, GTPase binding domain; WBD, WASp binding domain.