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Nat Microbiol. 2017 Dec;2(12):1592-1599. doi: 10.1038/s41564-017-0033-2. Epub 2017 Oct 9.

Evolution of host adaptation in the Salmonella typhoid toxin.

Author information

1
Department of Microbial Pathogenesis, Yale University School of Medicine, New Haven, CT, 06536, USA.
2
Glycobiology Research and Training Center, Departments of Medicine, Pathology and Cellular and Molecular Medicine, University of California San Diego, La Jolla, CA, 92093, USA.
3
GlycoMimetics, Inc., 9708 Medical Center Drive, Rockville, MD, 20850, USA.
4
Department of Chemistry, University of California Davis, Davis, CA, 95616, USA.
5
Department of Microbial Pathogenesis, Yale University School of Medicine, New Haven, CT, 06536, USA. jorge.galan@yale.edu.

Abstract

The evolution of virulence traits is central for the emergence or re-emergence of microbial pathogens and for their adaptation to a specific host 1-5 . Typhoid toxin is an essential virulence factor of the human-adapted bacterial pathogen Salmonella Typhi 6,7 , the cause of typhoid fever in humans 8-12 . 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) sialoglycans 6,13 prominently present in humans 14 . Here, we examine the functional and structural relationship between typhoid toxin and ArtAB, an evolutionarily related AB5 toxin encoded by the broad-host Salmonella Typhimurium 15 . 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 mammals 14 . 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.

PMID:
28993610
PMCID:
PMC5705260
DOI:
10.1038/s41564-017-0033-2
[Indexed for MEDLINE]
Free PMC Article

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