Letter | Published:

Evolution of host adaptation in the Salmonella typhoid toxin

Abstract

The evolution of virulence traits is central for the emergence or re-emergence of microbial pathogens and for their adaptation to a specific host1,2,3,4,5. Typhoid toxin is an essential virulence factor of the human-adapted bacterial pathogen Salmonella Typhi6,7, the cause of typhoid fever in humans8,9,10,11,12. Typhoid toxin has a unique A2B5 architecture with two covalently linked enzymatic ‘A’ subunits, PltA and CdtB, associated with a homopentameric ‘B’ subunit made up of PltB, which has binding specificity for the N-acetylneuraminic acid (Neu5Ac) sialoglycans6,13 prominently present in humans14. Here, we examine the functional and structural relationship between typhoid toxin and ArtAB, an evolutionarily related AB5 toxin encoded by the broad-host Salmonella Typhimurium15. We find that ArtA and ArtB, homologues of PltA and PltB, can form a functional complex with the typhoid toxin CdtB subunit after substitution of a single amino acid in ArtA, while ArtB can form a functional complex with wild-type PltA and CdtB. We also found that, after addition of a single-terminal Cys residue, a CdtB homologue from cytolethal distending toxin can form a functional complex with ArtA and ArtB. In line with the broad host specificity of S. Typhimurium, we found that ArtB binds human glycans, terminated in N-acetylneuraminic acid, as well as glycans terminated in N-glycolylneuraminic acid (Neu5Gc), which are expressed in most other mammals14. The atomic structure of ArtB bound to its receptor shows the presence of an additional glycan-binding site, which broadens its binding specificity. Despite equivalent toxicity in vitro, we found that the ArtB/PltA/CdtB chimaeric toxin exhibits reduced lethality in an animal model, indicating that the host specialization of typhoid toxin has optimized its targeting mechanisms to the human host. This is a remarkable example of a toxin evolving to broaden its enzymatic activities and adapt to a specific host.

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A correction to this article is available online at https://doi.org/10.1038/s41564-017-0070-x.

Change history

  • 01 November 2017

    The original version of this Letter has been modified in the abstract and main text to better reflect the distribution of Neu5Ac sialoglycans in humans. Additionally, co-author Lingquan Deng’s present address has been further clarified.

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Acknowledgements

The authors thank J. Wang for suggestions and for providing help with structure refinement, and the Galan Laboratory for careful review of the manuscript. G.S. is supported in part by a Postdoctoral Fellowship from the EMBO (ALTF 172-2015). Crystal screening was conducted at the Yale Macromolecular X-ray Core Facility (1S10OD018007-01). This work was supported by National Institutes of Health grants AI079022 (to J.E.G.) and GM32373 (to A.V.).

Author information

X.G., G.S., L.D., A.V. and J.E.G. designed the research and analysed data. X.G., G.S. and L.D. performed the research. Y.N.-M., H.Y. and X.C. provided critical reagents. X.G. and J.E.G. wrote the manuscript with input from all the authors.

Competing interests

The authors declare no competing financial interests.

Correspondence to Jorge E. Galán.

Electronic supplementary material

  1. Supplementary Information

    Supplementary Figures 1–29, Supplementary Table 6.

  2. Life Sciences Reporting Summary

  3. Supplementary Table 1

    ArtB and PltB-binding to a customized sialoglycan microarray.

  4. Supplementary Table 2

    Analysis of fine ligand specificity of ArtB and PltB.

  5. Supplementary Table 3

    Binding of ArtB andmutant derivative to a customized sialoglycan microarray.

  6. Supplementary Table 4

    Analysis of the Neu5Acα6Gal/GalNAc-binding specificity of ArtB mutants.

  7. Supplementary Table 5

    Analysis of fine ligand specificity of ArtB and its mutants.

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Further reading

Fig. 1: The ArtAB toxin components can form a functional complex with typhoid toxin subunits.
Fig. 2: ArtB binds Neu5Ac- and Neu5Gc-terminated glycans.
Fig. 3: The atomic structure of ArtB bound to its receptor shows the presence of an additional glycan-binding site.
Fig. 4: The ArtB/PltA/CdtB chimaeric toxin exhibits reduced lethality in mice relative to typhoid toxin.