Cryo-EM structures of the YsaN ATPase reveal dodecameric and tetradecameric assemblies in the Yersinia enterocolitica T3SS

Bacterial pathogens deploy sophisticated nanomachines such as the Type III Secretion System (T3SS) to inject effector proteins directly into host cells, thereby subverting cellular defenses and promoting infection. Central to this machinery is a conserved homo-oligomeric ATPase that fuels substrate unfolding and translocation. Despite extensive structural and biochemical studies, the precise mechanism of T3SS ATPase oligomer assembly and regulation remains incompletely understood, events crucial for the overall functionality of the T3SS. Here, we report high-resolution cryo-electron microscopy (cryo-EM) structures of the Yersinia enterocolitica T3SS ATPase YsaN, captured in two previously uncharacterized oligomeric states, a dodecamer (12-mer) at 3.51 Å and a tetradecamer (14-mer) at 3.80 Å resolution. Both assemblies exhibit helical-like symmetry and conformational heterogeneity. A conserved longitudinal fissure interrupts the helical lattice and appears to mediate interconversion between the two states, while an N-terminal interface stabilized by inter-tier disulfide bonds and an extensive non-covalent network locks the assembly together. Molecular dynamics simulations reveal that ATP hydrolysis drives a global, correlated compaction upon transition to the ADP-bound state. Together, these structural, biochemical, and MD simulation data establish T3SS ATPases as dynamic, oligomerically plastic molecular motors and provide a mechanistic framework for chaperone-effector dissociation and substrate translocation.