The making of a gradient: IcsA (VirG) polarity in Shigella flexneri

The generation and maintenance of subcellular organization in bacteria is c ritical for many cell processes and properties, including growth, structura l integrity and, in pathogens, virulence. Here, we investigate the mechanis ms by which the virulence protein IcsA (VirG) is distributed on the bacteri al surface to promote efficient transmission of the bacterium Shigella flex neri from one host cell to another. The outer membrane protein IcsA recruit s host factors that result in actin filament nucleation and, when concentra ted at one bacterial pole, promote unidirectional actin-based motility of t he pathogen. We show here that the focused polar gradient of IcsA is genera ted by its delivery exclusively to one pole followed by lateral diffusion t hrough the outer membrane. The resulting gradient can be modified by alteri ng the composition of the outer membrane either genetically or pharmacologi cally. The gradient can be reshaped further by the action of the protease I csP (SopA), whose activity we show to be near uniform on the bacterial surf ace. Further, we report polar delivery of IcsA in Escherichia coli and Yers inia pseudotuberculosis, suggesting that the mechanism for polar delivery o f some outer membrane proteins is conserved across species and that the vir ulence function of IcsA capitalizes on a more global mechanism for subcellu lar organization.