SseBCD proteins are secreted by the type III secretion system of Salmonella pathogenicity island 2 and function as a translocon

The type III secretion system encoded by Salmonella pathogenicity island 2 (SPI2) is required for systemic infections and intracellular accumulation o f Salmonella enterica. This system is induced by intracellular Salmonella a nd subsequently transfers effector proteins into the host cell. Growth cond itions either inducing expression of the type III secretion system or the s ecretion of substrate proteins were defined. Here we report the identificat ion of a set of substrate proteins consisting of SseB, SseC, and SseD that are secreted by the SPI2 system in vitro. Secretion was observed if bacteri al cells were exposed to acidic pH after growth in minimal medium with limi tation of Mg2+ or phosphate. SseB, -C, and -D were isolated in a fraction d etached from the bacterial cell surface by mechanical shearing, indicating that these proteins are predominantly assembled into complexes on the bacte rial cell surface. The three proteins were required for the translocation o f SPI2 effector proteins SspH1 and SspH2 into infected host cells. Thus, Ss eB, SseC, and SseD function as the translocon for effector proteins by intr acellular Salmonella.