Abstract
Many Gram-negative pathogens utilize dedicated secretion systems to export virulence factors such as exotoxins and effectors1,2,3,4. Several exotoxins are synthesized as precursors containing amino-terminal Sec signal peptides and are exported through the inner-membrane-bound Sec machinery to the periplasm, followed by secretion across the outer membrane to the exterior using a type II secretion system (T2SS)3,5. Here, we report that thermostable direct haemolysin (TDH), an exotoxin of the food-borne pathogen Vibrio parahaemolyticus, can be exported through the type III secretion system (T3SS), which engages in one-step secretion of effectors4, despite possessing a Sec signal peptide and being mainly secreted via the T2SS. Although the precursor of TDH is targeted to the Sec pathway, a fraction of mature TDH was observed to re-enter the bacterial cytoplasm. The N terminus of mature TDH comprises a T3SS signal sequence, allowing it to be loaded into the T3SS. We also show that T3SS-delivered TDH as an effector contributes to intestinal fluid accumulation in a rabbit diarrhoeal model of V. parahaemolyticus infection. Thus, our results show that an unconventional export mechanism for a bacterial toxin via the T3SS in tandem with the Sec machinery facilitates the virulence trait of V. parahaemolyticus.
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Data availability
All data supporting the findings of this study are available upon reasonable request from the corresponding authors.
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Acknowledgements
We thank Y. Akiyama, H. Mori, Y. Hizukuri and E. Ishii (Kyoto University, Kyoto, Japan) for providing E. coli strains AD202, secY39, IT41 and IT42, and for critical discussions and reading of the manuscript. We also thank S. Miyoshi for the kind gift of the anti-VPP1 antibody and Y. Yamaichi for critical reading of the manuscript. This study was in part supported by Grants-in-Aid for Scientific Research from the Japan Society for the Promotion of Science, grant 17K08828 (to S.M.) and grant 17K08829 (to T.K.). This work was also supported by Grants-in-Aid from the Institute for Fermentation, Osaka, from the Naito Foundation, and from the Takeda Science Foundation, Japan (to T.K.).
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S.M., S.T., H.H., K.G. and T.K. performed the experiments. R.O. contributed to experimental design and assisted in performing the experiments. T.I. provided valuable input on the manuscript. S.M. and T.K. designed the study, constructed the bacterial strains, analysed results and wrote the paper. All authors reviewed the manuscript and discussed the results.
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Matsuda, S., Okada, R., Tandhavanant, S. et al. Export of a Vibrio parahaemolyticus toxin by the Sec and type III secretion machineries in tandem. Nat Microbiol 4, 781–788 (2019). https://doi.org/10.1038/s41564-019-0368-y
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DOI: https://doi.org/10.1038/s41564-019-0368-y
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