Protein O-glycosylation in the Bacteroidota phylum

Glycans play crucial roles in bacteria, such as providing structural integrity or enabling interactions with the ecosystem. They can be linked to lipids, peptides, or proteins. In proteins, they modify either asparagine (N-glycosylation) or serine or threonine (O-glycosylation). Species of the Bacteroidota phylum, a major component of the human microbiome and marine and soil ecosystems, have a unique type of O-glycosylation that modifies multiple noncytoplasmic proteins containing a specific amino acid sequence. Only a small number of species have currently been characterized, but within one species, generally all proteins are modified with the same glycan structure. Most species share a common inner part but differ in the sugar composition and branching of the outer part of their glycan. This suggests that the biosynthesis of the glycan occurs in two separate steps. Both the inner core and the outer glycan are likely assembled from nucleotide-activated monosaccharides on undecaprenyl phosphate on the cytoplasmic side of the inner membrane, prior to being flipped to the periplasm and transferred to the protein. A genomic locus responsible for the biosynthesis of the outer glycan has been identified, containing some conserved genes across species. Despite substantial progress in the characterization of this O-glycosylation system, its function, the overall diversity of glycan structures across the phylum, and the complete biosynthetic pathway remain mostly unknown. Due to the importance of this group of species for the human gut microbiome, elucidating these aspects can open up strategies to modulate the composition of the microbiome community toward a healthy state.