Figure 2.

Elaboration of N-linked Glycans in the Golgi cisternae. N-glycan processing in the Golgi gives rise to three main classes of glycans: high-mannose, hybrid and complex glycans. Glycoproteins exit the ERGIC and reach the Golgi apparatus with eight or nine mannose residues. The Golgi apparatus is organized into discrete cisternae where the enzymes needed for glycan elaboration are specifically compartmentalized. Mannose trimming enzymes are located in the cis and medial-Golgi. The addition of GlcNAc is an essential step to generate hybrid and complex N-linked glycans in the trans-Golgi. Hybrid N-glycans are generated by the extension of the α3-arm that received GlcNAc with the addition of Gal, Fuc and SA. Removal of two terminal of the five Man residues, allows branching with an additional GlcNAc residue and synthesis of biantennary complex N-glycans with Gal, Fuc and SA. The polarized distribution of glycosylation enzymes is maintained through retrograde transport of COPI coated vesicles. A proper cisternal pH (different shades of orange) is needed to control retrograde sorting of enzymes and the association of glycoenzymes with the trafficking cargo. GlcNAc, N-acetylglucosamine; Man, mannose; Gal, galactose; NeuAc, N-Acetylneuraminic acid; Fuc, Fucose.