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PorZ, an Essential Component of the Type IX Secretion System of Porphyromonas gingivalis, Delivers Anionic Lipopolysaccharide to the PorU Sortase for Transpeptidase Processing of T9SS Cargo Proteins

    Pathogenic bacteria employ a Type-IX-Secretion-System (T9SS) to export virulence factors across the outer membrane into the extracellular space as part of the infective process. Among such bacteria is the periodontopathogen Porphyromonas gingivalis, a keystone pathogen responsible for severe periodontal disease. In this work, we investigated the cross-talk between two T9SS components, PorU and PorZ, which carry essential functions for T9SS transport.

    Cargo proteins of the type IX secretion system (T9SS) in human pathogens from the Bacteroidetes phylum invariably possess a conserved C-terminal domain (CTD) that functions as a signal for outer membrane (OM) translocation. In Porphyromonas gingivalis, the CTD of cargos is cleaved off after translocation, and anionic lipopolysaccharide (A-LPS) is attached. This transpeptidase reaction anchors secreted proteins to the OM. PorZ, a cell surface-associated protein, is an essential component of the T9SS whose function was previously unknown. We recently solved the crystal structure of PorZ and found that it consists of two β-propeller moieties, followed by a CTD. In this study, we performed structure-based modeling, suggesting that PorZ is a carbohydrate-binding protein. Indeed, we found that recombinant PorZ specifically binds A-LPS in vitro. Binding was blocked by monoclonal antibodies that specifically react with a phosphorylated branched mannan in the anionic polysaccharide (A-PS) component of A-LPS, but not with the core oligosaccharide or the lipid A endotoxin. Examination of A-LPS derived from a cohort of mutants producing various truncations of A-PS confirmed that the phosphorylated branched mannan is indeed the PorZ ligand. Moreover, purified recombinant PorZ interacted with the PorU sortase in an A-LPS-dependent manner. This interaction on the cell surface is crucial for the function of the “attachment complex” composed of PorU, PorZ, and the integral OM β-barrel proteins PorV and PorQ, which is involved in posttranslational modification and retention of T9SS cargos on the bacterial surface.

    Reference:

    PorZ, an Essential Component of the Type IX Secretion System of Porphyromonas gingivalis, Delivers Anionic Lipopolysaccharide to the PorU Sortase for Transpeptidase Processing of T9SS Cargo Proteins

    Mariusz MadejZuzanna NowakowskaMiroslaw KsiazekAnna M. LasicaDanuta MizgalskaMagdalena NowakAnna JaculaMonika BzowskaCarsten ScaveniusJan J. EnghildJoseph Aduse-OpokuMichael A. CurtisF. Xavier Gomis-RüthJan Potempa.

    DOI: 10.1128/mBio.02262-20

    Current state-of-the-art of the architecture and function of the P. gingivalis T9SS.

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