Listeria monocytogenes: Actin-based motility and the ActA protein

The interaction of microbial pathogens with mammalian cells is of scientific interest for two reasons. First, many of the world\u27s most devastating diseases are caused by microbes. By gaining a better understanding of the molecular events contributing to the infectious process, we improve our ability to combat disease. Second, by investigating the mechanisms of host/pathogen interaction, we gain insight into innate cellular functions of the mammalian host. Listeria monocytogenes is a food-borne, facultative, Gram-positive, intracytosolic bacterial pathogen that is infectious in humans and many other mammals. The infectious life cycle of L. monocytogenes is characterized by its ability to enter and grow inside the cytosol of eukaryotic cells. A second feature of cellular infection is the bacteria\u27s interaction with and subversion of the eukaryotic actin cytoskeleton. Host actin accumulation along the bacterial surface is directed by a bacterial surface protein, ActA. Polymerization of actin into filaments generates a directional force, which the bacteria use to spread from cell to cell and propagate infection. We here demonstrate that the ActA protein is essential for bacterial interaction with host actin, and evaluate the role of ActA in mediating the virulence of L. monocytogenes in tissue culture and animal models of infection. We further show that the ActA protein is capable of mediating all the features of the actin-based motility of L. monocytogenes, in in vitro systems lacking all other listerial components. We next dissect ActA function by mutational analysis, and describe four separable functions encoded in the ActA protein all of which contribute to bacterial motility. Lastly, we describe the phosphorylation of the ActA protein during intracellular infection, and address the role of phosphorylation to ActA function and bacterial virulence