SseA acts as the chaperone for the SseB component of the Salmonella Pathogenicity Island 2 translocon

The Salmonella Pathogenicity Island 2 (SPI2) encodes a type III secretion system (TTSS) shown to be critical for adaptation to the intracellular environment within both phagocytic and epithelial cell types. Within SPI2, the Effector region encodes several exported proteins that comprise the SPI2 translocon (SseB, C, D). SseA is the first protein encoded within the Effector region but remains an unclassified factor that is essential for SPI2 function. In the present study, we determined that SseA shares several features with TTSS chaperones: it is small (12.5 kDa), located directly upstream of a TTSS export target (SseB), and contains an amphipathic, C-terminal alpha-helix. Construction and analysis of a DeltasseA mutant demonstrated that the total amount of SseB is significantly reduced and SPI2 export of SseB to the bacterial surface is prevented. SseB accumulation and export were restored when SseA was provided in trans. Loss of SseA does not cause a generalized defect in SPI2 secretory function as export of SseC, encoded downstream of SseB, still occurs in the DeltasseA strain. Quantitative PCR indicates that the loss of SseB in DeltasseA does not occur at the transcriptional level. Co-purification studies demonstrate that SseA directly binds to SseB. Collectively, these results demonstrate that SseA functions as a TTSS chaperone for the SPI2 translocon component, SseB.