Functions as a type III secretion system (TTSS) chaperone, which is required for SseB and SseD accumulation and secretion. May have a direct role in secretion of SseB and SseD, or may facilitate their correct folding, for efficient secretion and function. Required for survival and replication within epithelial cells and macrophages. Binds to SseB and SseD. May associate with the inner membrane. Expression is regulated by the two-component regulatory system SsrA/SsrB. The N-terminal region is dispensable for binding and stabilizing SseB, but is essential for export of SseB to the surface of the bacterium. Mutant is severely attenuated in virulence.