IpgD, a protein secreted by the type III secretion machinery of Shigella flexneri, is chaperoned by IpgE and implicated in entry focus formation

Invasion of epithelial cells by Shigella flexneri involves entry and interc ellular dissemination. Entry of bacteria into non-phagocytic cells requires the IpaA-D proteins that are secreted by the Mxi-Spa type III secretion ma chinery. Type III secretion systems are found in several Gram-negative path ogens and serve to inject bacterial effector proteins directly into the cyt oplasm of host cells. In this study, we have analysed the IpgD protein of S . flexneri, the gene of which is located on the virulence plasmid at the 5' end of the mxi-spa locus. We have shown that IpgD (i) is stored in the bac terial cytoplasm in association with a specific chaperone, IpgE; (ii) is se creted by the Mxi-Spa type III secretion system in amounts similar to those of the IpaA-D proteins; (iii) is associated with IpaA in the extracellular medium; and (iv) is involved in the modulation of the host cell response a fter contact of the bacterium with epithelial cells. This suggests that Ipg D is an effector that might be injected into host cells to manipulate cellu lar processes during infection.