Salmonella pathogenicity island 2-encoded proteins SseC and SseD are essential for virulence and are substrates of the type III secretion system

Survival of Salmonella enterica serovar Typhimurium within host phagocytic cells is a critical step in establishing systemic infection in mice. Genes within Salmonella pathogenicity island 2 (SPI-2) encode a type III secretion system that is required for establishment of systemic infection. Several proteins encoded by SPI-2 have homology to type III secreted proteins from enteropathogenic Escherichia coli and Yersinia and, based on that homology, are predicted to be secreted through the SPI-2, type III secretion system. We have investigated the roles of two of these proteins, SseC and SseD, We demonstrate here that the SseD protein is required for systemic Salmonella infection of the mouse, and we confirmed the virulence requirement for the SseC protein. Experiments were performed, using cellular fractionation and immunoblotting, to identify the subcellular location of the SseC and SseD proteins. Both proteins were found to localize predominantly to the bacterial cell membrane. In addition, our work revealed that SseC and SseD are exposed to the extracellular environment and are loosely associated with the bacterial membrane, Furthermore, localization of SseC and SseD to the bacterial membrane was found to require a functional SPI-2 type III secretion system, Collectively, these results indicate that the SseC and SseD proteins are secreted by the SPI-2 type III secretion system to the bacterial membrane in order to perform their virulence functions.