Scar1 and the related Wiskott-Aldrich syndrome protein, WASP, regulate theactin cytoskeleton through the Arp2/3 complex

Background: The actin-related proteins Arp2 and Arp3 are part of a seven-pr otein complex which is localized in the lamellipodia of a variety of cell t ypes, and in actin-rich spots of unknown function. The Arp2/3 complex enhan ces actin nucleation and causes branching and crosslinking of actin filamen ts in vitro; in vivo it is thought to drive the formation of lamellipodia a nd to be a control center for actin-based motility. The Wiskott-Aldrich syn drome protein, WASP, is an adaptor protein implicated in the transmission o f signals from tyrosine kinase receptors and small GTPases to the actin cyt oskeleton. Scar1 is a member of a new family of proteins related to WASP, a nd it may also have a role in regulating the actin cytoskeleton. Scar1 is t he human homologue of Dictyostelium Scar1, which is thought to connect G-pr otein-coupled receptors to the actin cytoskeleton. The mammalian Scar famil y contains at least four members. We have examined the relationships betwee n WASP, Scar1, and the Arp2/3 complex. Results: We have identified WASP and its relative Scar1 as proteins that in teract with the Arp2/3 complex. We have used deletion analysis to show that both WASP and Scar1 interact with the p21 subunit of the Arp2/3 complex th rough their carboxyl termini. Overexpression of carboxy-terminal fragments of Scar1 or WASP in cells caused a disruption in the localization of the Ar p2/3 complex and, concomitantly, induced a complete loss of lamellipodia an d actin spots. The induction of lamellipodia by platelet-derived growth fac tor was also suppressed by overexpression of the fragment of Scar1 that bin ds to the Arp2/3 complex. Conclusions: We have identified a conserved sequence domain in proteins of the WASP family that binds to the Arp2/3 complex. Overexpression of this do main in cells disrupts the localization of the Arp2/3 complex and inhibits lamellipodia formation. Our data suggest that WASP-related proteins may reg ulate the actin cytoskeleton through the Arp2/3 complex.