Scar, a WASp-related protein, activates nucleation of actin filaments by the Arp2/3 complex

Abstract

The Arp2͞3 complex, a stable assembly of two actin-related proteins (Arp2 and Arp3) with five other subunits, caps the pointed end of actin filaments and nucleates actin polymerization with low efficiency. WASp and Scar are two similar proteins that bind the p21 subunit of the Arp2͞3 complex, but their effect on the nucleation activity of the complex was not known. We report that full-length, recombinant human Scar protein, as well as N-terminally truncated Scar proteins, enhance nucleation by the Arp2͞3 complex. By themselves, these proteins either have no effect or inhibit actin polymerization.

Spatial and temporal regulation of actin polymerization for cytoskeleton formation through Arp2/3 complex and WASP/WAVE proteins

“…Addition of pre-formed filaments to the Arp2/3 complex and VCA -mediated actin polymerization reaction mixture further enhanced actin polymerization [Machesky et al, 1999]. This observation can be explained by both the side branching and end branching models.…”

“…Further, the actin polymerization mediated by Arp2/3 complex and WASP family proteins proceeds in an auto-catalytic fashion, by which the new filaments by Arp2/3 and WASP family proteins activate other Arp2/3 complex molecules, then contribute to the development of the branched filaments . Such autocatalysis is also suggested by the finding that addition of pre-formed filaments to actin polymerization mixture containing Arp2/3 and WASP family members enhances the polymerization [Machesky et al, 1999]. The observations of filament structures and kinetic analyses of polymerization have led to the proposal of two models of branched filament formation: the side branching model, in which branch occurs at the side of filaments, and the barbed end branching model, in which branch occurs at the barbed ends of filaments Pantaloni et al, 2000;Pollard et al, 2000].…”

“…An abundant protein complex, actin-related protein (Arp) 2/3 complex, which functions as a nucleation core has been identified [Machesky et al, 1994]. The nucleation core activity of the Arp2/3 complex is tightly regulated by members of Wiskott-Aldrich syndrome family protein (WASP, N-WASP, and WAVE proteins; WAVE is also called SCAR), which enable rapid polymerization of actin for promotion of changes in the actin cytoskeleton [Machesky and Insall, 1998;Machesky et al, 1999;Rohatgi et al, 1999]. However, de novo nucleation and subsequent actin polymerization are not adequate for the formation of integrated actin structures soon after initiation of polymerization, because newly generated filaments are not connected to pre-existing filaments (Fig.…”

“…2A) activates the Arp2/3 complex Pantaloni et al, 2000;Rohatgi et al, 1999], suggesting that the VCA domain is a constitutive activator of the Arp2/3 complex at least in vitro. The V domain associates directly with actin monomers ], and the CA region interacts with the Arp2/3 complex, forming a nucleation core for actin polymerization [Machesky et al, 1999;Rohatgi et al, 1999]. A mutation in the V domain that disrupts the association with actin causes the loss of Arp2/3 complex activation both in vitro and in vivo Yamaguchi et al, 2000], indicating that activation of Arp2/3 complex requires the actin monomer binding activity of the V domain.…”

Reversal of Aging‐Induced Increases in Aortic Stiffness by Targeting Cytoskeletal Protein‐Protein Interfaces

“…The CA domain of N‐WASP is a subset of the C‐terminal VCA domain by which N‐WASP activates the Arp 2/3 complex, inducing actin polymerization at branches from the side of parent actin filaments 36, 37. The CA domain binds to G‐actin, and because it lacks the V domain, acts as a dominant negative inhibitor of N‐WASP‐mediated actin polymerization in airway smooth muscle,36 but its effects in vascular smooth muscle are unknown.…”