SecNine · Mechanism of Type 9 Secretion: an unusual protein transport system involved in pathogenesis and motility

The recently-discovered Type IX Secretion System (T9SS) is a protein transport pathway that exports proteins across the outer membrane of Gram-negative of the phylum Bacteroidetes. It is an essential pathogenicity determinant in severe periodontal disease and in the major bacterial diseases of farmed fish. It is also required for the unique Bacteroidete gliding motility. The components required to catalyse T9SS export are still not fully defined, but include at least 18 authenticated proteins. Possible functions can currently only be assigned to some of these T9SS components. However, it is clear that the pathway involves a number of interacting protein complexes. Our current understanding of the T9SS is superficial and lacking in molecular level structural and mechanistic detail. In a landmark recent study we have identified and structurally characterized the multi-protein outer membrane protein conducting channel of the T9SS. Our ambitious goal is to apply similar state-of-the-art experimental methods to systematically characterise the full T9SS pathway. We will systematically characterise the component protein complexes that make up the T9SS by:- Defining their composition through co-purification from native sources.- Determining their molecular structures by cryo-EM.- Assessing their dynamic interactions through live cell fluorescence imaging, biochemical experiments, and structural characterization. In addition, we will use single molecule tracking methods to follow substrate molecules through the steps of the T9SS transport cycle in real time in live bacterial cells and apply this methodology to elucidate T9SS mechanism.