Characterization of functional domains of the Shigella actin assembly protein IcsA
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Shigella flexneri is a gram-negative facultative bacterium that causes bacillary dysentery. The bacterium induces its entry into the colonic epithelial cell and is confined to a vacuole from which it rapidly escapes. Once in the cytosol, Shigella elaborates a tail of actin filaments, which it uses to move within the cytosol and spread into adjacent cells. The outer membrane protein IcsA is the sole factor required for Shigella to polymerize actin and move in the cytosol of cells. Localized at the old pole of the bacillus, IcsA is anchored in the outer membrane via its carboxy-terminal third (the beta domain), thereby allowing the amino-terminal two-thirds (the alpha domain) to be exposed on the surface of the bacterium.;IcsA shares significant sequence similarity with the R. rickettsii outer membrane protein A (OmpA), which has been implicated in the actin-based motility of this pathogen. Deletion of the region of similarity in IcsA, amino acids 446 to 506, resulted in loss of actin recruitment and motility. However, replacement of IcsA amino acids 446 to 506 with the rickettsial region of similarity did not restore actin recruitment, indicating that the rickettsial OmpA domain is not sufficient for actin recruitment. Further experiments suggest that IcsA amino acids 446 to 506 are not directly involved in interacting with host cell factors that mediate actin-based motility. This region likely plays a role in the maintenance of the native conformation of IcsA.;Recent data indicate that IcsA is targeted directly to the old pole. To gain insight into the targeting process, I determined whether the alpha domain of IcsA was sufficient to target the Aequorea victoria green fluorescent protein (GFP) to the old pole in the bacterial cytoplasm. Shigella strains harboring these constructs displayed GFP at the old pole in the bacterial cytoplasm, indicating that the alpha domain is sufficient to direct GFP to the old pole. Two regions within the a domain were found to be independently required for localization of GFP in the cytoplasm: amino acids 1 to 104 and amino acids 506 to 730.