Glycome Atlas
process
Also known as O-glycosylation, mucin-type O-glycosylation, O-GalNAc glycosylation
GalNAc (α1-O) → Ser/Thr; Gal (β1-3) → GalNAc
How to read these diagrams (SNFG)
Each shape is a class of sugar and each colour a specific one. Structures read right to left, with the reducing end (the point of attachment) on the right.
Plain-language answer
O-linked glycosylation is a way cells attach sugar chains to proteins at the amino acids serine and threonine. Unlike the N-linked pathway, there is no fixed recognition pattern, and the chain is built up one sugar at a time in the Golgi starting with a sugar called GalNAc. This is the type of glycosylation that gives mucus proteins their dense sugar coating.1
Dense O-glycans are what make mucus slippery and protective, lining the gut, airways, and other surfaces. The exact O-glycan patterns also change in cancer and inflammation, which is why they are studied as markers and targets.1
Technical detail
O-linked glycosylation, in its predominant mucin-type form, is the enzyme-directed, Golgi-localized addition of N-acetylgalactosamine to serine and threonine hydroxyls by polypeptide GalNAc-transferases, followed by stepwise elaboration into core 1 through core 4 structures, without any consensus acceptor sequon and without a lipid-linked precursor.1
Mucin-type O-glycosylation is initiated by a large family of polypeptide GalNAc-transferases that transfer GalNAc from UDP-GalNAc to serine or threonine. There is no defined consensus sequence, so acceptor site selection is governed by the overlapping and distinct specificities of the individual transferases and by local peptide context. This contrasts with N-linked glycosylation, which requires the N-X-S/T sequon and a preassembled dolichol precursor.1
The initiating GalNAc, the Tn antigen, is extended by specific glycosyltransferases into a set of common core structures designated cores 1 through 4. These cores are further elongated and terminated with residues such as galactose, fucose, and sialic acid, generating the diverse O-glycan repertoire that densely decorates mucins and many cell surface glycoproteins. Because assembly is residue by residue and enzyme-driven, the output reflects the glycosyltransferase complement expressed by the cell.1
Human relevance
Truncated mucin-type O-glycans such as the Tn and sialyl-Tn antigens accumulate in many carcinomas, reflecting disrupted O-glycan processing and serving as recognized tumor-associated markers.1
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References