PhotorhabdusVirulence Cassettes: extracellular multi-protein needle complexes for delivery of small protein effectors into host cells

Photorhabdus is a highly effective insect pathogen and symbiont of insecticidal nematodes. To exert its potent insecticidal effects, it elaborates a myriad of toxins and small molecule effectors. Among these, the Photorhabdus Virulence Cassettes (PVCs) represent an elegant self-contained delivery mechanism for diverse protein toxins. Importantly, these self-contained nanosyringes overcome host cell membrane barriers, and act independently, at a distance from the bacteria itself. In this study, we demonstrate that Pnf, a PVC needle complex associated toxin, is a Rho-GTPase, which acts via deamidation and transglutamination to disrupt the cytoskeleton. TEM and Western blots have shown a physical association between Pnf and its cognate PVC delivery mechanism. We demonstrate that for Pnf to exert its effect, translocation across the cell membrane is absolutely essential. SIGNIFICANCE STATEMENT Here we provide an up to date analysis of the nano-scale syringe-like molecular devices that Photorhabdus use to manipulate invertebrate hosts, the PVC system. They are related to the Serratia Anti-Feeding Prophage and the Psuedoalteromonas MAC system. All these systems are in turn more distantly related to the well characterized Type VI secretion system currently receiving a great deal of attention. We demonstrate for the first time that the PVC nanosyringes are physically “loaded” with an effector protein payload before being freely released. The PVCs therefore represent bacterial molecular machines that are used as “long-range” protein delivery systems. This widespread class of toxin delivery system will likely prove of great significance in understanding many diverse bacteria/host interactions in future.