Recruitment of the Arp2/3 complex by Listeria, Shigella, Cdc42 andPIP2 micelles. The Listeria membrane protein ActA directly binds to theArp2/3 complex to stimulate actin polymerization in order for it to move from the cytoplasm of cells. Shigella can also propel itself through host cells using actin polymerization, but does not recruit the Arp2/3 complex directly. Instead, the Shigella membrane protein IcsA binds to N-WASP, which in turn recruits the Arp2/3 complex through its carboxy-terminal region. IcsA thus mimics Cdc42, which can also bind to N-WASP and thereby stimulate Arp2/3 complex-mediated actin polymerization. Phospholipid micelles containing PIP2 can also stimulate actin polymerization in vitro, and appear to require Cdc42, N-WASP, and the Arp2/3 complex for this process, although the link between the micelles and Cdc42 is not yet clear.