From N-WASP to WAVE: key molecules for regulation of cortical actin organization.

We first isolated N-WASP as one of the proteins bound to Ash/Grb2 SH3 domain. This protein has a VCA region (verplorin-like, cofilin-like, acidic region) at the C-terminus, which binds to G-actin and Arp2/3 complex, and several functional domains at the N-terminus, such as WHD (WASP homology domain) and GBD/CRIB domain. N-WASP activates Arp2/3 complex-dependent actin polymerization through the VCA region, leading to filopodium formation. Next, we found WAVE1, WAVE2 and WAVE3. All these proteins have also VCA regions at C-terminal areas and induce membrane ruffle formation. To clarify the different roles of WAVE1 and WAVE2, we established WAVE1- and WAVE2-deficient mouse embryonic fibroblasts (MEFs), because these two WAVEs are expressed in MEF. When wild-type MEFs are stimulated randomly by PDGF, two types of ruffles, peripheral and dorsal, are formed. However, dorsal ruffle formation does not occurin WAVE1-deficient MEFs. In contrast, peripheral ruffle formation is diminished in WAVE2-deficient MEFs. On the other hand, in MEFs migrating towards a chemoattractant gradient, only peripheral ruffles (lamellipodia) are formed. In this migration, WAVE1-deficient MEFs still could form lamellipodia but WAVE2-deficient MEFs could not. All these data show that WAVE2 but not WAVE1 is essential for lamellipodium formation and directed migration.