Characterization of the interaction of IpaB and IpaD, proteins required for entry of Shigella flexneri into epithelial cells, with a lipid membrane

Entry of Shigella flexneri into epithelial cells and lysis of the phagosome involve the IpaB, IpaC, acid IpaD proteins, which are secreted by type III secretion machinery. We report here the purification of IpaB and IpaD and the characterization of their lipid-binding properties as a function of pH. The interaction of IpaB with the membrane was quite independent of the pH whereas that of IpaD took place only at low pH. To support the data obtaine d with the purified proteins, we designed a system in which protein secreti on by live bacteria was induced in the presence of liposomes, thereby allow ing interaction of proteins with lipids directly after secretion and bypass ing any purification step. In these conditions, both IpaB and IpaC, as well as minor amounts of IpaA and IpgD, were associated with the membrane and t he ratio of IpaB to IpaC was modulated by the pH. The relevance of these re sults with respect to the dual roles of IpaB, IpaC and IpaD in induction of membrane ruffles and lysis of the endosomal membrane is discussed.