Role and structural mechanism of WASP-triggered conformational changes in branched actin filament nucleation by Arp2/3 complex

Abstract

Significance Assembly of actin filaments is tightly regulated to orchestrate basic cellular processes such as motility, division, and differentiation. To control when and where actin filaments assemble, cells rely on actin filament nucleators including the Arp2/3 (Actin-related proteins 2/3) complex, a seven-subunit protein assembly that nucleates branched actin filaments to create dendritic actin networks. Activity of the Arp2/3 complex is controlled by WASP (Wiskott–Aldrich syndrome protein) proteins, which bind directly to it to activate nucleation. To understand how WASP proteins activate the complex, we used chemical cross-linking to engineer an Arp2/3 complex that is locked into an “on” state without WASP. In addition, we used biochemical experiments to determine how WASP proteins stimulate the on state. These results have important implications for understanding how cells control the actin cytoskeleton.