Pseudomonas aeruginosa is a dreadful opportunistic pathogen that can cause both acute and chronic infections in humans. This bacterium exhibits great abundance of virulence factors and has the ability to flourish in a wide range of environmental niches, partly due to its remarkable arsenal of protein secretion systems. The type VI secretion system (T6SS), is a contractile injection apparatus that translocates a spike loaded with various effectors directly into eukaryotic and prokaryotic target cells. P. aeruginosa can load either one of its three T6SSs with a variety of bullets by specific but different modes. This dissertation reveals work focussed on the molecular details of how effector delivery relies on the appropriate attachment onto the T6SS apparatus and spike.
Our work provides details on how the spike components of the T6SS can be manipulated to deliver effector proteins, which highlights an exceptional modularity for loading the T6SS nanoweapon. The spike, which punctures the bacterial cell envelope allowing effector transport, consists of a torch-like VgrG trimer on which sits a PAAR sharpening the VgrG tip. We show how various effectors can be fused directly to either of the two spike components and reach the supernatant or are delivered into target prey cells. Further, we reveal that effectors specifically bind to different parts of VgrG proteins and that these interactions not only lead to effector stabilisation, but also to their secretion. In summary, this work will give sound evidence that an effector delivered by the T6SS requires a chaperone module for stabilisation and an adaptor module for its connection to the T6SS spike.
Results from this thesis highlight the various modes that are at P. aeruginosa’ s disposal to deliver its many T6SS effectors, which confer a significant advantage during bacterial competition. Overall, this work advances our understanding on the delivery mechanisms of T6SS effectors and thus, in general, of this fantastic nanomachine.