PopB-PcrV Interactions are Essential for Pore Formation in the Pseudomonas aeruginosa Type III Secretion System Translocon

Abstract The type III secretion system (T3SS) is a syringe-like virulence factor which delivers bacterial proteins directly into the cytoplasm of host cells. An essential component of the system is the translocon, which creates a pore in the host cell membrane through which proteins are injected. In Pseudomonas aeruginosa, the translocation pore is formed by proteins PopB and PopD and attaches to the T3SS needle via the needle tip protein PcrV. The pore is multimeric, but the exact stoichiometry and structure of the pore are unknown. We took a genetic approach to map contact points within the system by taking advantage of the fact that the translocator proteins of Pseudomonas aeruginosa and the related Aeromonas hydrophila T3SS are incompatible and cannot be freely exchanged. We created chimeric versions of P. aeruginosa PopB and A. hydrophila AopB to intentionally disrupt and restore protein-protein interactions. We identified a chimeric B-translocator that specifically breaks an interaction with the needle tip protein and interferes with the formation of the translocation pore. Breaking the interaction did not disrupt membrane insertion, arguing that the needle tip protein chaperones formation of the translocation pore.