Requirement for exported proteins in secretion through the invasion-associated type III system of Salmonella typhimurium

The inv and spa loci of Salmonella typhimurium encode a type III protein secretion system which is essential for the ability of this microorganism to gain access to cultured epithelial cells, These loci are located at centisome 63 in the Salmonella chromosome. We have carried out a functional analysis of several genes of these loci and have found that two exported proteins encoded in this region, InvJ and SpaO, are required for secretion through the invasion-associated type III secretion system, These findings suggest the existence of a hierarchy in the export process, since mutations in other targets of this secretory system have no effect on protein secretion. We have also shown that the spaO, spaP, spaQ, and spaR genes are required for protein secretion and for the ability of S. typhimurium to gain access to cultured epithelial cells. In addition, we investigated the ability of an invJ S. typhimurium mutant strain to present the SipB protein to the bacterial surface and demonstrated that, in contrast to Spa32, its putative Shigella homolog, InvJ is not involved in the surface presentation of the Sip proteins.