O-linked glycosylation is the attachment of a sugar molecule to the oxygen atom of serine (Ser) or threonine (Thr) residues in a protein. O-glycosylation is a post-translational modification that occurs after the protein has been synthesised. In eukaryotes, it occurs in the endoplasmic reticulum, Golgi apparatus and occasionally in the cytoplasm; in prokaryotes, it occurs in the cytoplasm. Several different sugars can be added to the serine or threonine, and they affect the protein in different ways by changing protein stability and regulating protein activity. O-glycans, which are the sugars added to the serine or threonine, have numerous functions throughout the body, including trafficking of cells in the immune system, allowing recognition of foreign material, controlling cell metabolism and providing cartilage and tendon flexibility. Because of the many functions they have, changes in O-glycosylation are important in many diseases including cancer, diabetes and Alzheimer's. O-glycosylation occurs in all domains of life, including eukaryotes, archaea and a number of pathogenic bacteria including Burkholderia cenocepacia, Neisseria gonorrhoeae and Acinetobacter baumannii.
Common types of O-glycosylation
O-N-acetylgalactosamine (O-GalNAc)
Addition of N-acetylgalactosamine (GalNAc) to a serine or threonine occurs in the Golgi apparatus, after the protein has been folded. The process is performed by enzymes known as GalNAc transferases (GALNTs), of which there are 20 different types. The initial O-GalNAc structure can be modified by the addition of other sugars, or other compounds such as methyl and acetyl groups. These modifications produce 8 core structures known to date. Different cells have different enzymes that can add further sugars, known as glycosyltransferases, and structures therefore change from cell to cell. Common sugars added include galactose, N-acetylglucosamine, fucose and sialic acid. These sugars can also be modified by the addition of sulfates or acetyl groups.
Biosynthesis GalNAc is added onto a serine or threonine residue from a precursor molecule, through the activity of a GalNAc transferase enzyme. This precursor is necessary so that the sugar can be transported to where it will be added to the protein. The specific residue onto which GalNAc will be attached is not defined, because there are numerous enzymes that can add the sugar and each one will favour different residues. However, there are often proline (Pro) residues near the threonine or serine. Once this initial sugar has been added, other glycosyltransferases can catalyse the addition of additional sugars. Two of the most common structures formed are Core 1 and Core 2. Core 1 is formed by the addition of a galactose sugar onto the initial GalNAc. Core 2 consists of a Core 1 structure with an additional N-acetylglucosamine (GlcNAc) sugar. A poly-N-acetyllactosamine structure can be formed by the alternating addition of GlcNAc and galactose sugars onto the GalNAc sugar. Terminal sugars on O-glycans are important in recognition by lectins and play a key role in the immune system. Addition of fucose sugars by fucosyltransferases forms Lewis epitopes and the scaffold for blood group determinants. Addition of a fucose alone creates the H-antigen, present in people with blood type O. By adding a galactose onto this structure, the B-antigen of blood group B is created. Alternatively, adding a GalNAc sugar will create the A-antigen for blood group A.
Functions O-GalNAc sugars are important in a variety of processes, including leukocyte circulation during an immune response, fertilisation, and protection against invading microbes. O-GalNAc sugars are common on membrane glycoproteins, where they help increase rigidity of the region close to the membrane so that the protein extends away from the surface. For example, the low-density lipoprotein receptor (LDL) is projected from the cell surface by a region rigidified by O-glycans. In order for leukocytes of the immune system to move into infected cells, they have to interact with these cells through receptors. Leukocytes express ligands on their cell surface to allow this interaction to occur. P-selectin glycoprotein ligand-1 (PSGL-1) is such a ligand, and contains a lot of O-glycans that are necessary for its function. O-glycans near the membrane maintain the elongated structure and a terminal sLex epitope is necessary for interactions with the receptor. Mucins are a group of heavily O-glycosylated proteins that line the gastrointestinal and respiratory tracts to protect these regions from infection. Mucins are negatively charged, which allows them to interact with water and prevent it from evaporating. This is important in their protective function as it lubricates the tracts so bacteria cannot bind and infect the body. Changes in mucins are important in numerous diseases, including cancer and inflammatory bowel disease. Absence of O-glycans on mucin proteins changes their 3D shape dramatically and often prevents correct function.
O-N-acetylglucosamine (O-GlcNAc)
Addition of N-acetylglucosamine (O-GlcNAc) to serine and threonine residues usually occurs on cytoplasmic and nuclear proteins that remain in the cell, compared to O-GalNAc modifications which usually occur on proteins that will be secreted. O-GlcNAc modifications were only recently discovered, but the number of proteins with known O-GlcNAc modifications is increasing rapidly. It is the first example of glycosylation that does not occur on secretory proteins.
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