Redox regulated auto-processing controls delivery of an antibacterial cysteine peptidase toxin

Contact-dependent growth inhibition (CDI) is a mechanism of inter-bacterial competition mediated by CdiA effectors, which deliver polymorphic C-terminal toxins (CT) into neighboring competitors. StbD from Citrobacter rodentium DBS100 is an unusual CdiA-like protein that carries a C-terminal cysteine peptidase toxin. Crystallography reveals that StbD-CT is composed of an N-terminal cytoplasm-entry domain connected to a C39 family peptidase by a flexible linker. The entry domain hijacks membrane-embedded YajC for translocation into the target-cell cytosol where the peptidase inactivates type II topoisomerases. Intoxication leads to a loss of DNA super-helicity, impaired chromosome segregation and cell filamentation. In addition to cleaving topoisomerases, StbD-CT exhibits auto-proteolytic processing under reducing conditions, and this activity is required for target cell intoxication. We propose that StbD-CT remains tethered to the cell periphery via interactions with YajC after delivery. Auto-processing releases the peptidase, enabling the domain to penetrate into the cell interior where it cleaves nucleoid-associated topoisomerases. Together, these findings identify a proteolytic effector that deactivates type II topoisomerases and reveal a redox regulatory strategy that coordinates toxin activation with intercellular delivery.