Analyses of biosynthesis mutants reveal that the fifth and sixth sugars of the Porphyromonas gingivalis O-glycan are L-fucose and N-acetylgalactosamine respectively

The oral pathogen, Porphyromonas gingivalis has a general O-glycosylation system which it utilises to modify hundreds of proteins localised outside of the cytoplasm. The O-glycan is a heptasaccharide that includes a putative L-fucose and N-acetylgalactosamine (GalNAc) as the 5th and 6th sugar residues respectively. The putative L-fucose is expected to be synthesized as GDP-L-fucose involving the enzymes Gmd (PGN_1078) and Fcl (PGN_1079), while GalNAc is putatively epimerised from GlcNAc by GalE (PGN_1614). In this study we created mutants lacking each of these three enzymes and analysed the resultant glycosylation defects. Immunoblot analysis using antibodies against the model glycoproteins Mfa2, PGN_0742 and PGN_1037 detected bands of reduced size, consistent with glycan truncation. Mass spectrometry analysis of tryptic digests of whole cell lysate proteins revealed that O-glycans in the galE mutant were predominantly pentasaccharides consistent with the 6th sugar being GalNAc. For the gmd and fcl mutants, tetrasaccharides were observed confirming the 5th sugar as L-fucose. The confirmation of these sugars also confirmed PGN_1135 as a GalNAc transferase as the previously analysed PGN_1135 mutant produced the same O-glycan truncation as the galE mutant.