Step-by-Step Maturation Mechanism of Binary Toxin Pore Revealed by Cryo-EM Analysis

Membrane pore-forming proteins (PFPs) form ring-shaped membrane-translocating oligomers on membranes, contributing to infection, immunity, and cell death functions. Binary toxins produced by some bacteria consist of an enzymatic component that acts as a toxin and a membrane-binding component that forms a pore that delivers the enzymatic component into target cells. Cryo-electron microscopy (cryo-EM) has advanced our understanding of these translocation mechanisms by revealing several binary toxin complexes’ structures. However, the mechanisms underlying the initial pore formation remain unclear. We determined the structures of several oligomeric forms of the membrane-binding component, Ib, of the Iota toxin from Clostridium perfringens at various stages of pore formation. Structural comparisons revealed that the symmetrically arranged soluble oligomer (prepore) became a transmembrane oligomer (pore). These findings enhance our understanding of the PFP mechanisms and provide a structural basis for developing nanodevices using membrane pores.