As they spread from cell to cell, Shigella move towards the plasma membrane, and then push against it, causing protrusions that penetrate the adjacent cell. Host actin nucleation proteins play critical roles in both processes, in the former by polymerizing actin to form propulsive tails in the cell body, and in the latter, by assembling actin used in protrusion formation. Marcia B. Goldberg of the Massachusetts General Hospital and the Harvard Medical School, Boston, Mass., and colleagues illuminate protrusion formation, identifying a family of host actin nucleation proteins, called formins, which are involved in this process but are different from those involved in assembling the tails within the cell body. "Of particular interest for our work is the ability of formins to nucleate, elongate, and bundle actin filaments during protrusion formation," and the fact that Shigella uses two distinct host cell actin nucleation pathways during infection, says Goldberg. "While our paper is the first demonstration of bacterial manipulation of host formin proteins by bacteria, it was previously shown that vaccinia viruses inhibit formin activity during egress from infected cells," says Goldberg. "These findings highlight the formin family protein as a central molecular target for multiple pathogens," she says, adding that "Other intracellular pathogens are likely to use the formins during infection to enhance their survival or dissemination."

(J. E. Heindl, I. Saran, C.-R. Yi, C. F. Lesser, and M. B. Goldberg. 2010. Requirement for formin-induced actin polymerization during spread of Shigella flexneri. Infect. Immun. 78:193-203.)