B-296. Control of Effector Secretion by the P. aeruginosa Type III Secretion System

P-C. Lee, C. Stopford, A. Rietsch;
Case Western Reserve Univ., Cleveland, OH.

Pseudomonas aeruginosa is a common nosocomial pathogen that uses a wide variety of virulence factors to cause disease. One of the principal virulence factors in acute infections is the type III secretion system (T3SS), which allows protein toxins (effectors) to be directly injected into targeted host cells. This direct injection of secreted effectors into eukaryotic cells (translocation) requires the action of three translocator proteins, the needle-tip protein PcrV, as well as the two pore-forming proteins PopB and PopD. Effector secretion via the T3SS is normally inhibited. Upon cell-contact, however, this block is relieved. We recently provided evidence that translocator proteins are secreted before effector proteins and that cell-contact results in a switch in secretion specificity resulting in the export of effectors. Effector secretion is controlled by two protein complexes that are both thought to be cytoplasmic, the PcrG/PcrV complex, as well as the PopN complex. Removal of any of their constituent proteins results in de-regulated effector secretion. In particular, data will be presented that all components of the PopN complex are required for this regulation, an observation that had been cast into doubt. The mechanism by which these protein complexes control effector secretion is unclear. Interestingly, however, PcrV and PopN belong to two different secretion classes: translocators and effectors, respectively. Here we present data that the amino-terminus of PcrV is required to both target the protein for export, but also to help maintain calcium-control of effector secretion. We have also constructed a strain expressing a mutant of the T3SS component PscU, which fails to undergo cleavage at a conserved motif. We find that, as in Yersinia, this strain is unable to secrete translocator proteins, while still allowing secretion of effectors. This mutant will be used to test the hypothesis that the PcrG/PcrV complex controls effector secretion by disallowing effectors access to the T3SS via the ‘translocator’ route, whereas the PopN complex controls access via the acceptor site for effector proteins.