The N terminus of type III secretion needle protein YscF from Yersinia pestis functions to modulate innate immune responses.

The type III secretion system is employed by many pathogens, including the genera Yersinia, Shigella, Pseudomonas, and Salmonella, to deliver effector proteins into eukaryotic cells. The injectisome needle is formed by the polymerization of a single protein, e.g., YscF (Yersinia pestis), PscF (Pseudomonas aeruginosa), PrgI (Salmonella enterica SPI-1), SsaG (Salmonella enterica SPI-2), or MxiH (Shigella flexneri). In this study, we demonstrated that the N termini of some needle proteins, particularly the N terminus of YscF from Yersinia pestis, influences host immune responses. The N termini of several needle proteins were truncated and tested for the ability to induce inflammatory responses in a human monocytic cell line (THP-1 cells). Truncated needle proteins induced proinflammatory cytokines to different magnitudes than the corresponding wild-type proteins, except SsaG. Notably, N-terminally truncated YscF induced significantly higher activation of NF-κB and/or AP-1 and higher induction of proinflammatory cytokines, suggesting that a function of the N terminus of YscF is interference with host sensing of YscF, consistent with Y. pestis pathogenesis. To directly test the ability of the N terminus of YscF to suppress cytokine induction, a YscF-SsaG chimera with 15 N-terminal amino acids from YscF added to SsaG was constructed. The chimeric YscF-SsaG induced lower levels of cytokines than wild-type SsaG. However, the addition of 15 random amino acids to SsaG had no effect on NF-κB/AP-1 activation. These results suggest that the N terminus of YscF can function to decrease cytokine induction, perhaps contributing to a favorable immune environment leading to survival of Y. pestis within the eukaryotic host.