Identification of two targets of the type III protein secretion system encoded by the inv and spa loci of Salmonella typhimurium that have homology to the Shigella IpaD and IpaA proteins

An important virulence factor of Salmonella spp. is their ability to gain access to host cells. A type III secretion system encoded in the inv and spa loci of these organisms is essential for this phenotype. We have identified two proteins, SipA and SipD, whose secretion from the bacterial cells is dependent on this system. The genes encoding these proteins are located at centisome 63 on the S. typhimurium chromosome, immediately downstream of the previously identified sipB and sipC genes (K. Kaniga, S. Tucker, D. Trollinger, and J. E. Galán, J. Bacteriol. 177:3965-3971, 1995). Nucleotide sequence analysis of the genes encoding these proteins indicated that SipA and SipD have significant sequence similarity to the Shigella IpaA and IpaD proteins. A nonpolar null mutation in sipD rendered S. typhimurium severely deficient for entry into cultured epithelial cells. In addition, this mutant strain exhibited increased secretion of a selected group of proteins whose export is controlled by the inv- and spa-encoded translocon. In contrast, a nonpolar mutation in sipA did not result in an invasion defect or in a significant decreased in virulence in a mouse model of infection. In addition, we have found an open reading frame immediately downstream of SipA that encodes a predicted protein with significant similarity to a family of acyl carrier proteins.