In work on other novel targets, our laboratory has made significant progress in the structural elucidation of proteins and subcomplexes that compose bacterial protein secretion systems, including the type III secretion apparatus (T3SS), which is common and essential to the pathogenicity of many Gram-negative pathogens, such as enteropathogenic Escherichia coli, Salmonella, Shigella, Bordetella, Chlamydia, and Pseudomonas.

The needle-like T3SS allows specific and direct injection of bacterial virulence proteins into the cytoplasm of human host cells, where they mediate a wide range of pathogenic effects by manipulating host cytoskeletal proteins, signaling machinery, and other core cellular processes. The injected virulence proteins differ from bacterial pathogen to pathogen, but the T3SS apparatus is highly conserved and thus presents an excellent target for the design of antimicrobials and vaccines that disable only pathogenic bacteria, leaving “good bacteria” to continue to colonize in the patient. The T3SS is composed of approximately two dozen proteins that create an oligomerized set of membrane-spanning rings and connecting hollow filaments reaching from the bacterial cytoplasm to the host cytoplasm.

We developed a combination of customized x-ray crystallography, NMR, electron microscopy, mass spectroscopy, Rosetta-based molecular modeling, and cellular microbiology approaches to study this massive multimembrane-spanning assembly. This effort has contributed major insights to the current high-resolution models of the T3SS needle complex and mode of action, including the cytoplasmic ATPase and inner- and outer-membrane rings, which provide a foundation for all subsequent T3SS assembly and the extended, hollow translocation filaments/pore-forming complex, which together allow delivery of T3SS bacterial virulence effectors directly into the host cytoplasm.