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.
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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.).
Electronic supplementary material
Supplementary Figures 1–29, Supplementary Table 6.
ArtB and PltB-binding to a customized sialoglycan microarray.
Analysis of fine ligand specificity of ArtB and PltB.
Binding of ArtB andmutant derivative to a customized sialoglycan microarray.
Analysis of the Neu5Acα6Gal/GalNAc-binding specificity of ArtB mutants.
Analysis of fine ligand specificity of ArtB and its mutants.
About this article
Nature Communications (2019)
Nature Microbiology (2018)