 Many actin @-@ related molecules create a free barbed end for polymerization by <unk> or severing pre @-@ existing filaments and using these as actin nucleation cores . However , the Arp2 / 3 complex stimulates actin polymerization by creating a new nucleation core . Actin nucleation is an initial step in the formation of an actin filament . The nucleation core activity of Arp2 / 3 is activated by members of the <unk> @-@ Aldrich syndrome family protein ( WASP , N @-@ WASP , WAVE , and <unk> proteins ) . The V domain of a WASP protein interacts with actin monomers while the CA region associates with the Arp2 / 3 complex to create a nucleation core . However , de novo nucleation followed by polymerization is not sufficient to form integrated actin networks , since these newly synthesized polymers would not be associated with pre @-@ existing filaments . Thus , the Arp2 / 3 complex binds to pre @-@ existing filaments so that the new filaments can grow on the old ones and form a functional actin cytoskeleton . Capping proteins limit actin polymerization to the region activated by the Arp2 / 3 complex , and the elongated filament ends are recapped to prevent <unk> and thus conserve the actin filament . 
