Structural and functional insight into the interaction of Clostridioides difficile toxin B and FZD.

The G protein-coupled receptors of the Frizzled (FZD) family, in particular FZD, are receptors that are exploited by Clostridioides difficile toxin B (TcdB), the major virulence factor responsible for pathogenesis associated with Clostridioides difficile infection. We employ a live-cell assay examining the affinity between full-length FZDs and TcdB. Moreover, we present cryoelectron microscopy structures of TcdB alone and in complex with full-length FZD, which reveal that large structural rearrangements of the combined repetitive polypeptide domain are required for interaction with FZDs and other TcdB receptors, constituting a first step for receptor recognition. Furthermore, we show that bezlotoxumab, an FDA-approved monoclonal antibody to treat Clostridioides difficile infection, favors the apo-TcdB structure and thus disrupts binding with FZD. The dynamic transition between the two conformations of TcdB also governs the stability of the pore-forming region. Thus, our work provides structural and functional insight into how conformational dynamics of TcdB determine receptor binding.