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Structural basis for recognition of frizzled proteins by Clostridium difficile toxin B

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Science  11 May 2018:
Vol. 360, Issue 6389, pp. 664-669
DOI: 10.1126/science.aar1999

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Toxic hijack of a cell signaling pathway

The pathogen Clostridium difficile colonizes the human colon when the normal microbiota is disrupted, often after antibiotic treatment. It is a leading cause of hospital-acquired diarrhea, especially among elderly patients. Chen et al. describe a 2.5-Å-resolution crystal structure that shows how a major virulence factor in C. difficile, toxin B (TcdB), binds to the G protein–coupled receptor Frizzled (FZD). This receptor activates the Wnt signaling pathway, which regulates homeostasis of the colonic epithelium. Surprisingly, TcdB uses a lipid cofactor to recognize FZD. This cofactor replaces a lipid normally associated with the Wnt ligand that binds FZD to activate signaling. Inhibiting the Wnt pathway likely plays a role in C. difficile pathology.

Science, this issue p. 664

Abstract

Clostridium difficile infection is the most common cause of antibiotic-associated diarrhea in developed countries. The major virulence factor, C. difficile toxin B (TcdB), targets colonic epithelia by binding to the frizzled (FZD) family of Wnt receptors, but how TcdB recognizes FZDs is unclear. Here, we present the crystal structure of a TcdB fragment in complex with the cysteine-rich domain of human FZD2 at 2.5-angstrom resolution, which reveals an endogenous FZD-bound fatty acid acting as a co-receptor for TcdB binding. This lipid occupies the binding site for Wnt-adducted palmitoleic acid in FZDs. TcdB binding locks the lipid in place, preventing Wnt from engaging FZDs and signaling. Our findings establish a central role of fatty acids in FZD-mediated TcdB pathogenesis and suggest strategies to modulate Wnt signaling.

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