The acidic regions of WASp and N-WASP can synergize with CDC42Hs and Rac1 to induce filopodia and lamellipodia.

The acidic (A) region of WASp family proteins is thought to represent a high-affinity binding site for Arp2/3 complex without activating properties. Here we show that GST-fused WASp-A and N-WASP-A, but not a WASP-A/W500S mutant, several truncated WASp-A constructs or WAVE1-A can pull down Arp2/3 complex from cell lysates. Significantly, WASp-A and N-WASP-A synergistically trigger formation of filopodia or lamellipodia when coinjected with sub-effective concentrations of V12CDC42Hs or V12Rac1, respectively, into macrophages. The ability of WASp family A region constructs to induce this effect is closely correlated with their ability to bind Arp2/3 complex in vitro. These results imply that (i) Arp2/3 complex is critically involved in filopodia and lamellipodia formation in macrophages and (ii) acidic regions of WASp and N-WASP are not simply binding sites for Arp2/3 complex but can prime it for RhoGTPase-triggered signals leading to actin nucleation.