Project title: Cellular and structural studies of accessory protein function in the Porphyromonas gingivalis type-IX secretion system
Summary: Porphyromonas gingivalis is a Gram-negative pathogenic bacterium that forms biofilms in the oral cavity and is the causative agent of chronic periodontitis. Unlike dominant pathogens that manipulate their environment to outgrow indigenous microbiota, P. gingivalis is a low-abundance microbe that can orchestrate inflammatory disease by remodelling a normally benign microbiota into a dysbiotic one. It uses a type-IX secretion system (T9SS/PorSS) to export virulence factors (e.g. gingipains: Rgp, Kgp etc) across its outer membrane. These are covalently attached to the cell surface liposaccharide but can also be released from the bacteria in the form of outer membrane vesicles.
The T9SS is formed from a periplasmic inner membrane platform (PorL/PorM), a major outer membrane pore (Sov, PorK, PorN) and an outer membrane sortase (PorU). However, the exact architecture and stoichiometry of the system is unknown. The translocation mechanism is also unclear but it is thought that upon entering the periplasm, the C-terminal domain of cargo recognize the PorK/PorL/PorM/PorN/Sov complex, are secreted across the outer membrane and then the C-terminal domain is cleaved by PorU. This is then followed by the covalent attachment of the cargo to the cell surface liposaccharide. Although it is not certain how this is coupled with the sorting of these proteins into OMVs, OMV proteins are anchored to the vesicle surface via a modified liposaccharide and this modification may act as the signal to instigate OMV formation. In addition to PorK, PorL, PorM, PorN and Sov there are several other outer membrane associated factors, namely PorP, PorQ, PorT, PorV and PorW. Many of these are predicted to form small β-barrel porin-like structures but their function is unknown. However, structure predictions of several of these proteins identify homology with fatty acid transporters and therefore they may have roles that include the modification/recycling of liposaccharides during attachment of cargo to the cell surface or formation of OMVs.