Glycosylation

1. N-Linked Glycosylation

Attachment begins in the ER at the moment of translation. The oligosaccharyltransferase (OST)complex transfers a preformed 14-sugar oligosaccharide Glc₃Man₉GlcNAc₂ from a dolichol-phosphate lipid anchor to Asn residues in Asn-X-Ser/Thrmotifs (X ≠ Pro). The common precursor is then trimmed:

1. ER: glucosidases I, II remove all three Glc; mannosidases trim one Man. Output: Man₈GlcNAc₂.
2. cis-Golgi: mannosidase I trims further (Man₅).
3. medial-Golgi: GnT-I adds GlcNAc; mannosidase II trims the final mannoses; additional GlcNAcs added by GnT-II.
4. trans-Golgi: galactose added by β4GalT; sialic acid by ST3/6Gal sialyltransferases; fucose by FucTs; sulphation by sulphotransferases.

Three major N-glycan classes result: high-mannose (retained-ER forms, lysosomal enzymes with mannose-6-phosphate signal), complex (branched, sialylated), hybrid (mixture). Branching (bi-, tri-, tetraantennary) is controlled by the GnT-I/II/IV/V series and influences lectin binding and protein function.

2. O-Linked Glycosylation

O-glycans attach to serine or threonine hydroxyls. Multiple types:

• Mucin-type (O-GalNAc): initiated in the Golgi by GalNAc-T enzymes. Extended by sialic acid, galactose, fucose. Produces the mucin-dense O-glycans characteristic of secretory-cell membranes.
• O-Fucose: on EGF-repeat domains of Notch, other signalling receptors; modified by Fringe to regulate Notch-ligand binding.
• O-GlcNAc: nuclear-cytoplasmic (not Golgi), reversible, regulates transcription factors and cytoskeleton.
• O-Mannose: on α-dystroglycan of muscle; mutations in the O-mannose pathway cause muscular dystrophy (Walker-Warburg syndrome, dystroglycanopathies).

3. Blood-Group Antigens

The ABO blood groups are trans-Golgi glycan modifications. A common precursor (H antigen = Fuc-Gal-GlcNAc) is either unmodified (O blood group), GalNAc-modified (A blood group, by A-transferase), or Gal-modified (B blood group, by B-transferase). A and B transferases differ by only four amino acids. The corresponding ABO gene alleles are a textbook example of how single Golgi-enzyme polymorphism generates antigenic diversity. Landsteiner’s 1901 blood-group discovery (Nobel 1930) was — though he did not know it — a discovery of Golgi glycosylation variation.

4. Sialyl-Lewis X and Selectin Ligands

Leukocyte rolling on inflamed endothelium is driven by selectins (E, L, P) binding sialyl-Lewis X (sLe^x)tetrasaccharides on leukocyte surfaces. sLe^x synthesis requires α1-3-fucosyltransferases (FUT4, FUT7) in the Golgi; its presentation on appropriate protein scaffolds (PSGL-1, ESL-1) is developmentally and inflammation-regulated. This single Golgi modification underlies vertebrate immune-cell trafficking. Genetic absence (LAD-II, leukocyte-adhesion deficiency-II) causes severe recurrent infection.

5. Mannose-6-Phosphate: The Lysosomal Zip Code

Lysosomal hydrolases carry a unique glycan signal — mannose-6-phosphate (M6P) — that directs them from the TGN to endosomes and thence to lysosomes. Two-step biosynthesis in the cis-Golgi:

1. GNPTAB recognises a folding signal on soluble lysosomal hydrolases and adds GlcNAc-1-P to mannose residues.
2. NAGPA (uncovering enzyme) removes GlcNAc, exposing the M6P signal.

M6P receptors (CI-MPR and CD-MPR) in the TGN bind M6P-bearing cargo and package it into clathrin-coated vesicles for delivery to endosomes. Loss-of-function of GNPTAB causes I-cell disease(mucolipidosis II): lysosomal hydrolases are secreted instead of targeted to the lysosome, producing a severe multisystem storage disorder. This is one of the clearest examples of a Golgi-specific disease.

6. Proprotein Convertases and Sulphation

In the trans-Golgi and TGN, proprotein convertases (furin, PC1/PC2, PCSK9) cleave immature precursors into active hormones and receptors. Furin cleaves the R-X-K/R-R motif; PCSK9 regulates LDL-receptor turnover (therapeutic target: alirocumab, evolocumab). Tyrosine sulphation by TPST1/2 in the trans-Golgi produces high-affinity binding sites for chemokines and other receptors.