Unipolar localization and ATPase activity of IcsA, a Shigella flexneri protein involved in intracellular movement

Shigella flexneri uses elements of the host cell cytoskeleton to move within cells and from cell to cell. IcsA, an S. flexneri protein involved in this movement, was purified and studied in vitro. IcsA bound the radiolabelled ATP analog 3'(2')-O-(4-benzoyl)benzoyl-ATP and hydrolyzed ATP. In addition, the surface localization of IcsA on both extracellular and intracellular shigellae was unipolar. Further, in HeLa cells infected with shigellae, IcsA antiserum labelled the actin tail throughout its length, thereby suggesting that IcsA interacts with elements within the tail. Localization of IcsA within the tail at a distance from the bacterium would require its secretion; we demonstrate here that in vitro IcsA is secreted into the culture supernatant in a cleaved form.