O-linked glycosylation

O-linked glycosylation is a form of glycosylation occurring in the Golgi apparatus.

O-N-acetylgalactosamine (O-GalNAc)
O-linked glycosylation occurs at a later stage during protein processing, probably in the Golgi apparatus. This is the addition of N-acetyl-galactosamine to serine or threonine residues by the enzyme UDP-N-acetyl-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase, followed by other carbohydrates (such as galactose and sialic acid). This process is important for certain types of proteins such as proteoglycans, which involves the addition of glycosaminoglycan chains to an initially unglycosylated "proteoglycan core protein." These additions are usually serine O-linked glycoproteins, which seem to have one of two main functions. One function involves secretion to form components of the extracellular matrix, adhering one cell to another by interactions between the large sugar complexes of proteoglycans. The other main function is to act as a component of mucosal secretions, and it is the high concentration of carbohydrates that tends to give mucus its "slimy" feel. Proteins that circulate in the blood are not normally O-glycosylated, with the exception of IgA1 and IgD (two types of antibody) and C1-inhibitor.

O-fucose
O-fucose is added between the second and third conserved cysteines of EGF-like repeats in the Notch protein, and other substrates by GDP-fucose protein O-fucosyltransferase 1, and to Thrombospondin repeats by GDP-fucose protein O-fucosyltransferase 2. In the case of EGF-like repeats, the O-fucose may be further elongated to a tetrasaccharide by sequential addition of N-acetylglucosamine (GlcNAc), galactose, and sialic acid, and for Thrombospondin repeats, may be elongated to a disaccharide by the addition of glucose. Both of these fucosyltransferases have been localized to the endoplasmic reticulum, which is unusual for glycosyltransferases, most of which function in the Golgi apparatus.

O-glucose
O-glucose is added between the first and second conserved cysteines of EGF-like repeats in the Notch protein, and possibly other substrates by protein:O-glucosyltransferase (Poglut). This enzyme is known as Rumi in Drosophila, and is also localized to the ER like the O-fucosyltransferases. The O-glucose modification appears to be necessary for proper folding of the EGF-like repeats of the Notch protein, and increases secretion of this receptor.

O-N-acetylglucosamine (O-GlcNAc)
O-GlcNAc is added to serines or threonines by O-GlcNAc transferase. O-GlcNAc appears to occur on most serines and threonines that would otherwise be phosphorylated by serine/threonine kinases. Thus, if phosphorylation occurs, O-GlcNAc does not, and vice versa. This is an incredibly important finding because phosphorylation/dephosphorylation has become a scientific paradigm for the regulation of signaling within cells. A massive amount of cancer research is focused on phosphorylation. Ignoring the involvement of this form of glycosylation, which clearly appears to act in concert with phosphorylation, means that a lot of current research is missing at least half of the picture. O-GlcNAc addition and removal also appears to be a key regulator of the pathways that are disrupted in diabetes mellitus. The gene encoding the O-GlcNAcase enzyme has been linked to non-insulin dependent diabetes mellitus. It is the terminal step in a nutrient-sensing hexosamine signaling pathway.

O-N-acetylglucosamine In Other Contexts
Recently, O-GlcNAc was reported to occur between the fifth and sixth conserved cysteines in some EGF-like repeats from the Notch protein. It would seem unlikely that this modification would be due to the same enzyme involved with addition of O-GlcNAc to cytoplasmic and nuclear localized proteins. Considering that O-fucose and O-glucose addition to EGF-like repeats is due to ER localized enzymes, presumably an ER localized protein O-GlcNAc transferase exists.

O-mannose
During O-mannosylation, a mannose residue is transferred from mannose-p-dolichol to a serine/threonine residue in secretory pathway proteins. O-mannosylation is common to both prokaryotes and eukaryotes.

Collagen Glycosylation
Many lysines in collagen are hydroxylated to form hydroxylysine, and many of these hydroxylysines are then glycosylated by the addition of galactose. This galactose monosaccharide can then be further elongated by the addition of a glucose. This glycosylation is required for the proper functioning of collagen. Glycosylation of hydroxlysine occurs in the ER.

Hydroxyproline Glycosylation
Proline is also hydroxylated in collagen, however, no glycosylation occurs here as the hydroxyprolines are necessary for hydrogen bonding in the collagen triple helix. There is one protein named Skp1 in Dictyostelium discoideum that carries a GlcNAc on hydroxyproline, but this would appear to be an extremely rare form of glycosylation. Otherwise, only plants appear to carry glycans on hydroxyproline, with both galactose and arabinose glycans being reported in the literature.

Glycosylation of Glycogenin
Liver and muscle glycogenin carries a glucose on a tyrosine side chain. This is the only known example of glycosylated tyrosine in nature.

Glycosylation of Ceramide
Either a galactose or a glucose can be added to a hydroxyl on the lipid ceramide. The glucose can be further elongated to a disaccharide by the addition of a galactose.

Proteoglycans
The large and complex glycans that modify proteoglycans are initiated by addition of xylose to serine. This is the only form of glycan so far reported to begin with xylose addition directly to protein apart from the xylose seen on phospho-serine in Dictyostelium discoideum described below.