What initiates actin polymerization?

A major breakthrough in analysing the regulation of actin polymerization has been the ability to initiate the process in vitro using purified proteins and a combination of biochemical fractionation and informed guesswork. Different laboratories have used either the bacteria Shigella and Listeria, the GTPase Cdc42, or micelles of the phospholipid phosphatidylinositol (4,5) bisphosphate (PIP2) to initiate actin polymerization in vitro. Amazingly, the cellular components required for each of these initiators to induce actin polymerization are mostly identical (Figure 1). In all cases, the Arp2/3 complex is the key central component that somehow brings together actin monomers to form a new nucleus for actin polymerization, but the precise mechanisms whereby each initiator recruits the Arp2/3 complex varies. The Arp2/3 complex can be activated in vitro by binding to the Wiskott-Aldrich syndrome protein WASP/N-WASP via the carboxy-terminal acidic region of these proteins (Tom Pollard, Salk Institute). Cdc42 can bind to full-length N-WASP and stimulate its ability to activate the Arp2/3 complex, thereby inducing actin polymerization. Shigella recruits N-WASP via its IcsA protein, whereas Listeria bypasses the requirement for WASP and binds to and activates the Arp2/3 complex directly through an acidic region of the Listeria ActA protein (Matt Welch, University of California Berkeley).Using a biochemical fractionation approach, Rajat Rohatgi and Marc Kirschner (Harvard Medical School) reported that Cdc42- and PIP2-induced actin polymerization in vitro requires a complex of proteins that includes N-WASP, the Arp2/3 complex, and an as yet unidentified 140kDa Cdc42-interacting protein. Polymerization can be initiated by adding purified N-WASP or the carboxy-terminal WA region of N-WASP to the other components (Figure 2). Interestingly, the amino-terminal WH1 domain of N-WASP,which has been reported to bind to PIP2, is not required for PIP2-induced actin polymerization. This im