We have made significant progress in the structural elucidation of macro-molecular bacterial protein secretion systems, including the type III (T3SS)and type VII (T7SS) secretion systems. Such complexes are common and essential to the pathogenicity of both Gram-negative, enteropathogenic Escherichia coli, Salmonella, Shigella, Bordetella, Chlamydia, and Pseudomonas, and Gram-positive, Staphylococcus aureus, Clostridium difficile, and Mycobacterium tuberculosis pathogens.

The T3SS adopts a needle-like structure that allows the specific and direct transport of bacterial virulent proteins across three membrane bilayers delivering them from the cytoplasm to human host cells. This way effector proteins are able to mediate a wide range of pathogenic effects resulting from manipulation and repurposing of host cytoskeletal proteins, signalling and replication machinery. The high level of specificity between secretion systems and its effector proteins make these multicomponent systems excellent targets for the design of novel antimicrobials and vaccines that can potentially focus and incapacitate pathogenic bacteria with minimal effects on “good bacteria” already residing within 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 use a combination of customized cryo-EM, cryo-ET, X-ray crystallography, NMR, mass spectroscopy, Rosetta-based molecular modeling, and cellular microbiology approaches to study this massive multi-membrane spanning macro-complex. 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.


Cryo-EM structural snapshots capture the transitions associated with needle assembly and secretin gating of the Type III injectisome needle complex. Here, the assembly of the internalized helical needle is shown, which ultimately projects from the bacteria to the host for injection of virulence effectors. - Hu et al. (2019), Nature Microbiology.

Selected Publications