Cargo Secreted by the Type IX Secretion System of Porphyromonas gingivalis Are Tethered to O-Lipopolysaccharides via a Pentasaccharide Linker.

The Gram-negative oral pathogen, Porphyromonas gingivalis, uses the Type IX Secretion System (T9SS) to secrete major virulence factors (cargo proteins) and anchor them to the cell surface via a novel linking sugar, 2-N-seryl, 3-N-acetylglucuronamide (SAGA), which is a component of a specific type of lipopolysaccharide, A-LPS. The reported structure of the polysaccharide component (A-PS) was a repeating phosphorylated mannan whereas the PS of conventional O-LPS (O-PS) is a repeating Gal-Glu-Rha-GalNAc unit. Here, we have performed extensive mass spectrometric analyses of cargo protein-linked LPS with and without proteinase K treatment to determine the structure of A-LPS. Limited acid hydrolysis of the PS backbone with trifluoromethanesulfonic acid enabled long PS fragments linked to cargo-derived peptides to be identified for the first time. Unexpectedly, rather than finding A-PS units, up to eleven O-PS repeating units were found linked to cargo via a novel pentasaccharide linker designated A-LS, composed of SAGA-Hex-dHex(C