Complete structures of the YenTc holotoxin prepore and pore reveal the evolutionary basis for chitinase incorporation into ABC toxins.

ABC toxins are toxin-translocating, pore-forming proteins found in a wide range of insecticidal bacteria and some mammalian pathogens. The Yersinia entomopahaga toxin complex (YenTc) belongs to a distinct subclass of ABC toxins, defined by a divergent molecular architecture. Structural details that define their mechanism of action remain to be elucidated. Here we determine structures of the YenTc holotoxin assembly in both prepore and pore-forming configurations using cryo-EM in conjunction with Alphafold2-assisted structural modelling of flexible domains. We define the structural mechanism via which enzymatically-active chitinase subunits are incorporated, and show using phylogenetic analyses that this subclass-defining feature has evolved relatively recently. Our structures point to the existence of distinct conformational states in YenTc, which may distinguish it from other structurally-characterised ABC toxins, or represent states on a shared mechanistic trajectory. Thus, our findings enhance our understanding of the structural diversity that defines distinct ABC toxin subclasses.