Actin assembly factor; C-terminal WCA domain activates Arp2/3 complex-mediated nucleation of branched actin filaments, polyproline domain able to nucleate actin filaments in absence of Arp2/3; contains 3 binding sites for monomeric actin which overlap with SH3 binding sites; binding between SH3 and actin is competitive; mutants are defective in endocytosis, bud site selection, cytokinesis; human homolog WAS (Wiskott-Aldrich Syndrome) implicated in severe immunodeficiency