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The type III secretion system is employed by many pathogens, including the generaYersinia ,Shigella ,Pseudomonas , andSalmonella , 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 fromYersinia 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 withY. 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 ofY. pestis within the eukaryotic host.

The N Terminus of Type III Secretion Needle Protein YscF from Yersinia pestis Functions To Modulate Innate Immune Responses