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(C4H4O3)(Pent)-Hex. In addition, samples from a wzzP/porT double mutant that produced free truncated O-PS were specifically hydrolyzed to cleave lipid A prior to MS analysis. In these samples A-LS was found attached to a limited number of O-PS repeating units that in turn were associated with a putative core oligosaccharide that included the LPS-specific sugar, 3-deoxy-d-manno-octulosonic acid (Kdo). The proposed structure of A-LPS explains all 11 genes specific to A-LPS biosynthesis, and provides the first structural evidence that cargo proteins such as the gingipains are anchored to the cell surface via a complete LPS molecule.
Veith et al. (Wed,) studied this question.