| Abstract |
Breastfeeding is one of the main factors guiding the composition of the infant gut microbiota in the first months of life. This process is shaped in part by the high amounts of human milk oligosaccharides that serve as a carbon source for saccharolytic bacteria such as Bifidobacterium species. Infant-borne bifidobacteria have developed various molecular strategies for utilizing these oligosaccharides as a carbon source. We hypothesized that these species also interact with N-glycans found in host glycoproteins that are structurally similar to free oligosaccharides in human milk. Endo-β-N-acetylglucosaminidases were identified in certain isolates of Bifidobacterium longum subsp. longum, B. longum subsp. infantis, and Bifidobacterium breve, and their presence correlated with the ability of these strains to deglycosylate glycoproteins. An endoglycosidase from B. infantis ATCC 15697, EndoBI-1, was active toward all major types of N-linked glycans found in glycosylated proteins. Its activity was not affected by core fucosylation or extensive fucosylation, antenna number, or sialylation, releasing several N-glycans from human lactoferrin and immunoglobulins A and G. Extensive N-deglycosylation of whole breast milk was also observed after coincubation with this enzyme. Mutation of the active site of EndoBI-1 did not abolish binding to N-glycosylated proteins, and this mutant specifically recognized Man(3)GlcNAc(2)(α1-6Fuc), the core structure of human N-glycans. EndoBI-1 is constitutively expressed in B. infantis, and incubation of the bacterium with human or bovine lactoferrin led to the induction of genes associated to import and consumption of human milk oligosaccharides, suggesting linked regulatory mechanisms among these glycans. This work reveals an unprecedented interaction of bifidobacteria with host N-glycans and describes a novel endoglycosidase with broad specificity on diverse N-glycan types, potentially a useful tool for glycoproteomics studies.
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