A genome-wide screen identifies a Bordetella type III secretion effector and candidate effectors in other species

Bordetella bronchiseptica utilizes a type III secretion system (TTSS) for induction of non-apoptotic cytotoxicity in host cells and modulation of host immunity. The identity of Bordetella TTSS effectors, however, has remained elusive. Here we report a genome-wide screen for TTSS effectors based on shared biophysical and functional characteristics of class I chaperones and their frequent colocalization with TTSS effectors. When applied to B. bronchiseptica, the screen identified the first TTSS chaperone-effector locus, btcA-bteA, and we experimentally confirmed its function. Expression of bteA is co-ordinated with expression of TTSS apparatus genes, BteA is secreted through the TTSS of B. bronchiseptica, it is required for cytotoxicity towards mammalian cells, and it is highly conserved in the human-adapted subspecies B. pertussis and B. parapertussis. Transfection of bteA into epithlieal cells results in rapid cell death, indicating that BteA alone is sufficient to induce potent cytotoxicity. Finally, an in vitro interaction between BteA and BtcA was demonstrated. The search for TTSS chaperones and effectors was then expanded to other bacterial genomes, including mammalian and insect pathogens, where we identified a large number of novel candidate chaperones and effectors. Although the majority of putative effectors are proteins of unknown function, several have similarities to eukaryotic protein domains or previously identified effectors from other species.