Disruption of IcsP, the major Shigella protease that cleaves IcsA, accelerates actin-based motility

Shigella pathogenesis involves bacterial invasion of colonic epithelial cells and movement of bacteria through the cytoplasm and into adjacent cells by means of actin-based motility. The Shigella protein ---IcsA (VirG) is unipolar on the bacterial surface and is both necessary and sufficient for actin-based motility. --IcsA is inserted into the outer membrane as a 120-kDa polypeptide that is subsequently slowly cleaved, thereby releasing the 95-kDa amino-terminal portion into the culture supernatant. --IcsP, the major Shigella protease that cleaves ---IcsA, was identified and cloned. It has significant sequence similarity to the E. coli serine proteases, OmpP and OmpT. Disruption of ---icsP in serotype 2a S. flexneri leads to a marked reduction in ---IcsA cleavage, increased amounts of --IcsA associated with the bacterium and altered distribution of ---IcsA on the bacterial surface. The --icsP mutant displays significantly increased rates of actin-based motility, with a mean speed 27% faster than the wild-type strain; moreover, a significantly greater percentage of the ---icsP mutant moves in the cytoplasm. Yet, plaque formation on epithelial monolayers by the mutant was not altered detectably. These data suggest that ---rcsA, and not a host protein, is limiting in the rate of actin-based motility of wild-type serotype 2a S. flexneri.