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Some bacteria express a binary toxin translocation system, consisting of an enzymatic subunit and translocation pore, that delivers enzymes into host cells through endocytosis. The most clinically important bacterium with such a system is Clostridioides difficile (formerly Clostridium). The CDTa and CDTb proteins from its system represent important therapeutic targets. CDTb has been proposed to be a di-heptamer, but its physiological heptameric structure has not yet been reported. Here, we report the cryo-EM structure of CDTa bound to the CDTb-pore, which reveals that CDTa binding induces partial unfolding and tilting of the first CDTa α-helix. In the CDTb-pore, an NSS-loop exists in ‘in’ and ‘out’ conformations, suggesting its involvement in substrate translocation. Finally, 3D variability analysis revealed CDTa movements from a folded to an unfolded state. These dynamic structural information provide insights into drug design against hypervirulent C. difficile strains.

Besides two large cytotoxins (TcdA and TcdB), certain Clostridioides difficile strains also produce a binary toxin, called C. difficile toxin (CDT) composed of an enzymatic subunit involved in actin ADP-ribosylation (CDTa) and translocation pore (CDTb) that delivers CDTa into host cells through receptor-mediated endocytosis. CDTb is proposed to be a di-heptamer, but its physiological heptameric structure has not been reported to date. Here, we report the CDTa-bound CDTb-pore (heptamer) as a physiological complexes using cryo-EM. The high-resolution structure of the CDTa-bound CDTb-pore at 2.56-Å resolution revealed that CDTa binding to CDTb-pore induces partial unfolding and tilting of the first CDTa a-helix, and the translocation. In the CDTb-pore, the NSS-loop exists in “in” and “out” conformations, suggesting their involvement in substrate translocation through formation of weak, non-specific interactions. This structural information provides insights into drug design against hypervirulent C. difficile strains.

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