Establishment of unipolar localization of IcsA in Shigella flexneri 2a is not dependent on virulence plasmid determinants

Unipolar localization of IcsA on the surface of Shigella flexneri is requir ed for efficient formation of actin tails and protrusions in infected eucar yotic cells. Lipopolysaccharide (LPS) mutations have been demonstrated to a ffect either the establishment or the maintenance of IcsA in a unipolar loc ation, although the mechanism is unknown. In order to analyze the contribut ion of virulence plasmid determinants on the unipolar localization of IcsA, we examined the localization of IcsA expressed from a cloned plasmid copy in two different genetic backgrounds. The localization of IcsA was first ex amined in a virulence plasmid-cured derivative of the wild-type S. flexneri 2a isolate 2457T. This approach examined the contribution of virulence pla smid-borne factors, including the previously identified virulence plasmid-b orne protease that is responsible for cleaving IcsA in the outer membrane a nd releasing the 95-kDa secreted form from the cell surface. IcsA localizat ion in a related but nonpathogenic Escherichia coil strain expressing LPS o f the O8 serotype was also examined. IcsA surface presentation in both of t hese genetic backgrounds continued to be unipolar, demonstrating that virul ence plasmid-borne determinants are not responsible for unipolar localizati on of IcsA. The unipolar localization of IcsA in the E. coli background sug gests that a common pathway that allows IcsA to be spatially restricted to one pole on the bacterial cell surface exists in Shigella and E. coli.