Translocational unfolding in clostridial binary iota toxin complex

Protein translocation across the membrane is critical for microbial pathogenesis and various cellular functions. Bacterial binary toxins such as anthrax toxin are composed of enzyme components and a translocation channel, which catalyses substrate unfolding and translocation. Here we report the structures of the clostridial binary toxin (iota toxin) translocation channel Ib-pore and its complex with ADP-ribosyltransferase Ia. The Ib-pore structure at atomic resolution provides a similar structural framework as observed for the catalytic Φ-clamp of the anthrax protective antigen pore. However, the Ia-bound Ib-pore structure showed a unique binding mode of Ia: one Ia binds to the Ib-pore, and the Ia N-terminal domain interacts with Ib via two other Ib-pore bottlenecks with multiple weak interactions. Furthermore, Ib-binding induces Ia N-terminal α-helix tilting and partial unfolding, whereupon the unfolded N-terminus continues to the Φ-clamp gate. This study reveals the novel mechanism of N-terminal unfolding, which is crucial for protein translocation.