SscA is required for the secretion of SseC. (A) Proteins isolated from the cytoplasm and those secreted into the culture medium by wt and an ∆sscA mutant were probed by Western blot for the translocon components SseB, SseC and SseD. All proteins were detected in the cytoplasmic fraction from both strains. Wild type cells secreted each of the translocator apparatus proteins, however, SseC was undetectable in the secreted fraction from ∆sscA with no affect on SseB or SseD. Anti-DnaK antibody was used as a control to verify the absence of cytoplasmic protein in the secreted protein fractions. (B) Complementation of ∆sscA modestly restores SseC secretion. Whole cell lysates and secreted protein fractions from wild type, ∆sscA, and ∆sscA transformed with a plasmid encoding sscA were probed for SseC by Western blot. SseC was detected in the secreted fraction from complemented ∆sscA, albeit to lower levels than that seen from wild type cells. Secretion experiments were performed three times with similar results.