Actin polymerization is induced by Arp2/3 protein complex at the surface of Listeria monocytogenes

The pathogenic bacterium Listeria monocytogenes is capable of directed movement within the cytoplasm of infected host cells. Propulsion is thought to be driven by actin polymerization at the Bacterial cell surface, and moving bacteria leave in their wake a tail of actin filaments. Determining the mechanism by which L. monocytogenes polymerizes actin may aid the understanding of how actin polymerization is controlled in the cell. Actin assembly by L. monocytogenes requires the Bacterial surface protein ActA and protein components present in host cell cytoplasm. We have purified an eight-polypeptide complex that possesses the properties of the host-cell actin polymerization factor. The pure complex is sufficient to initiate ActA-dependent actin polymerization at the surface of L. monocytogenes, and is required to mediate actin tail formation and motility. Two subunits of this protein complex are actin-related proteins (ARPs) belonging to the Arp2 and Arp3 subfamilies. The Arp3 subunit localizes to the surface of stationary bacteria and the tails of motile bacteria in tissue culture cells infected with L. monocytogenes; this is consistent with a role for the complex in promoting actin assembly in vivo. The activity and subunit composition of the Arp2/3 complex suggests that it forms a template that nucleates actin polymerization.