WASP family verprolin-homologous proteins (WAVEs) function downstream of RAC1 and are involved in activation of the ARP2/3 complex. The resulting actin polymerization mediates the projection of the plasma membrane in lamellipodia and membrane ruffles. WAVEs exist as a pentameric hetero-complex called WAVE Regulatory Complex (WRC). The WRC consists of a WAVE family protein (WASF1, WASF2 or WASF3 - commonly known as WAVE1, WAVE2 or WAVE3), ABI (Abelson-interacting protein), NCKAP1 (NAP1, p125NAP1), CYFIP1 (SRA1) or the closely related CYFIP2 (PIR121), and BRK1 (HSPC300, BRICK). Of the three structurally conserved WAVEs in mammals, the importance of WAVE2 in activation of the ARP2/3 complex and the consequent formation of branched actin filaments is best established. WAVEs in the WRC are intrinsically inactive and are stimulated by RAC1 GTPase and phosphatidylinositols (PIP3). The C-terminal VCA domain of WAVE2 (and likely WAVE1 and WAVE3) which can bind both the ARP2/3 complex and actin monomers (G-actin) is masked in the inactive state. After PIP3 binds to the polybasic region of WAVE2 (and likely WAVE1 and WAVE3) and RAC1:GTP binds to the CYFIP1 (or CYFIP2) subunit of the WRC, allosteric changes most likely occur which allow WAVEs to interact with the ARP2/3 complex. The interactions between WAVEs and RAC1 are indirect. BAIAP2/IRSp53, an insulin receptor substrate, acts as a linker, binding both activated RAC1 and the proline-rich region of WAVE2 (and likely WAVE1 and WAVE3) and forming a trimolecular complex. CYFIP1 (or CYFIP2) in the WAVE regulatory complex binds directly to RAC1:GTP and links it to WAVE2 (and likely WAVE1 and WAVE3) (Derivery et al. 2009, Yamazaki et al. 2006, Takenawa & Suetsugu 2007, Chen et al. 2010, Pollard 2007, Lebensohn & Kirschner 2009).