Bacterial Type IV secretions systems (T4SSs) are large macromolecular complexes that transport effector proteins and genetic material across bacterial membranes. The T4SSs are clinically relevant as several pathogenic bacteria employ the T4SS to cause infections in plants, animals, and humans. Furthermore, T4SS-mediated horizontal gene transfer plays a key role in the proliferation of antibiotic resistance genes. Although widely studied, the molecular/structural basis of effector delivery through the T4SS is unknown.
Here we investigated the mechanism of T4SS-mediated effector delivery in the model organism Legionella pneumophilia, a clinically relevant intracellular bacterial parasite. We utilised the unrivalled power of Cryo-Electron Tomography and Sub-Tomogram averaging to visualise the T4SS in situ during an infection. Our in-situ averages of the T4SS during an infection shows several large-scale structural rearrangements. The most striking of these changes occur in the outer membrane core complex (OMCC), which undergoes a 45o shift downwards toward the inner membrane. The downward movement of the OMCC is coupled to protein density alteration at the central interface of the OMCC and the outer membrane. This structural insight provides one of the first insights into a mechanistic understanding of structural reorganisation necessary for effector protein secretion through the Legionella pneumophilia T4SS.