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O-Glycan Synthesis and Modification

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In mammals, there are a several different types of O-glycosylation.  These include mucin and nonmucin O-glycans.

Mucin glycoproteins:

  • Tend to be heavily O-glycosylated.
  • Are found in mucous secretions and as transmembrane glycoproteins on the cell surface.
  • Addition of O-glycans on the glycoprotein is initiated in the Golgi apparatus.
  • Synthesis starts with the addition of an N-acetylgalactosamine (GalNAc) residue to the OH of either a serine or threonine on the glycoprotein.
  • There are 8 O-glycan core structures found in mucins (Table 1).
  • The growing glycan chain is extended with the addition of other monosaccharides (1, Fig. 1)

 

Figure 1: A representation of the synthesis of O-glycans in the Golgi. Glycosyltransferases use activated sugars (nucleotide-sugars) as donors for the extension reactions.

O-glycan Structure
Core 1 or T antigen
Core 2
Core 3
Core 4
Core 5
Core 6
Core 7
Core 8
Table 1: Structures of O-glycan cores found on mucin glycopeptides.     

 


Non-mucin glycans are more varied:

  • O-fucose and O-glucose residues are:
    • transferred to consensus cysteine in certain proteins in the ER
    • essential for protein interaction and signal transduction (2)
  • O-GlcNAc can modify nuclear and cytosolic proteins. This:
    • occurs at serine or threonine residues
    • is a highly dynamic modification
    • plays an important role in cell signaling
    • modulates protein function much like phosphorylation (3, Fig. 2)
    • can compete directly with phosphate residues for occupancy of serine or threonine residues on the protein

Figure 2: Demonstrates the transient O-GlcNAc modification of some proteins. O-GlcNAc transferase (OGT) transfers a GlcNAc residue from UDP-GlcNAc to a serine or threonine in the target protein.

References

  1. Gill DJ, et al. (2011) Trends Cell Biol. 21(3):149-58. PMID: 21145746
  2. Luther KB, Haltiwanger RS. (2009) Int J Biochem Cell Biol. 41(5):1011-24. PMID: 18952191
  3. Zeidan Q, Hart GW. (2010) J Cell Sci. 123(Pt 1):13-22. PMID: 20016062