The Arp2͞3 complex, a seven-subunit protein complex containing two actin-related proteins, Arp2 and Arp3, initiates formation of actin filament networks in response to intracellular signals. The molecular mechanism of filament nucleation, however, is not well understood. Arp2 and Arp3 are predicted to bind ATP via a highly conserved nucleotide-binding domain found in all members of the actin superfamily and to form a heterodimer than mimics a conventional actin dimer. We show here that adenosine nucleotides bind with micromolar affinity to both Arp2 and Arp3 and that hydrolyzable ATP is required for actin nucleation activity. Binding of N-WASP WA increases the affinity of both Arp2 and Arp3 for ATP but does not alter the stoichiometry of nucleotides bound in the presence of saturating concentrations of ATP. The Arp2͞3 complex bound to ADP or the nonhydrolyzable ATP analogue AMP-PNP cannot nucleate actin filaments, but addition of the phosphate analogue BeF 3 partially restores activity to the ADP-Arp2͞3 complex. Bound nucleotide also regulates the affinity of the Arp2͞3 complex for its upstream activators N-WASP and ActA. We propose that the active nucleating form of the Arp2͞3 complex is the ADP-P i intermediate in the ATPase cycle and that the ATPase activity of the Arp2͞3 complex controls both nucleation of new filaments and release of the Arp2͞3 complex from membraneassociated activators.T he actin cytoskeleton determines the shape, motility, and internal organization of eukaryotic cells. Many actin-based structures, especially those involved in membrane protrusion, are assembled by coordinated polymerization and crosslinking of new actin filaments from actin monomers into either orthogonal or parallel filament networks (1). In these structures, work is accomplished by the free energy of polymerization. Subsequent ATP hydrolysis by filamentous actin then allows the networks to be disassembled. The rapid and regulated assembly and disassembly of actin filament networks lies at the heart of many cellular processes that involve membrane protrusion such as cell locomotion, endocytosis, and phagocytosis (2).The Arp2͞3 complex plays a central role in the regulated assembly of actin-based structures. The Arp2͞3 complex nucleates formation of new actin filaments in response to upstream signaling events and simultaneously crosslinks them into orthogonal networks. Activation of Rho-family G proteins, including Rac and Cdc42, leads to dramatic reorganization of the actin cytoskeleton (3, 4). Rac and Cdc42 promote activation of members of the WASP family of proteins (5-7) that include WASP, N-WASP, and several isoforms of Scar (8). WASP family proteins, in turn, recruit and activate the Arp2͞3 complex (7, 9, 10). The Arp2͞3 complex nucleates formation of new actin filaments from the sides of older filaments, creating a dendritic network of crosslinked actin filaments in vitro (11,12) and in vivo (13).We proposed that activation of Arp2͞3 complex involves the two actin-related proteins, Arp2 and Arp3, forming a heterodim...