ENHANCED SECRETION THROUGH THE SHIGELLA-FLEXNERI MXI-SPA TRANSLOCON LEADS TO ASSEMBLY OF EXTRACELLULAR PROTEINS INTO MACROMOLECULAR STRUCTURES

Genes required for entry of Shigella flexneri into epithelial cells in vitro are clustered in two adjacent loci, one of which encodes secret ory proteins, the IpaA-D proteins, and the other their dedicated secre tion apparatus, the Mxi-Spa translocon. Ipa secretion, which is induce d upon contact of bacteria with epithelial cells, is prevented during growth in vitro. Here, we show that ipaB and ipaD mutations lead to en hanced secretion of a set of about 15 proteins. These extracellular pr oteins and some Ipas associate in organized structures consisting of e xtended sheets. Growth of the wild-type strain in the presence of Cong o red is shown to induce protein secretion through the Mxi-Spa translo con. Cultures grown to stationary phase in the presence of Congo red c ontain extracellular filaments whose composition and morphology are si milar to those produced by the hypersecreting ipaB and ipaD mutants.