Golgi glycosylation

Abstract

Glycosylation is a very common modification of protein and lipid, and most glycosylation reactions occur in the Golgi. Although the transfer of initial sugar(s) to glycoproteins or glycolipids occurs in the ER or on the ER membrane, the subsequent addition of the many different sugars that make up a mature glycan is accomplished in the Golgi. Golgi membranes are studded with glycosyltransferases, glycosidases, and nucleotide sugar transporters arrayed in a generally ordered manner from the cis-Golgi to the trans-Golgi network (TGN), such that each activity is able to act on specific substrate(s) generated earlier in the pathway. The spectrum of glycosyltransferases and other activities that effect glycosylation may vary with cell type, and thus the final complement of glycans on glycoconjugates is variable. In addition, glycan synthesis is affected by Golgi pH, the integrity of Golgi peripheral membrane proteins, growth factor signaling, Golgi membrane dynamics, and cellular stress. Knowledge of Golgi glycosylation has fostered the development of assays to identify mechanisms of intracellular vesicular trafficking and facilitated glycosylation engineering of recombinant glycoproteins.

Figure 1.

Glycans that mature in the Golgi. The diagram depicts simple N- and O-glycans attached to glycoproteins, proteoglycans, glycosphingolipids, and a GPI anchor in the plasma membrane. Rather rare O-glycans are found attached to EGF-like repeats (EGF; pink) or thrombospondin repeats (TSR; gray) with a particular consensus sequence. The WxxW motif in a TSR is C-mannosylated. Core regions boxed in teal are sugars added in the ER. The remaining sugars in each class of glycan are added during passage through the cis-, medial-, and trans-Golgi network (TGN) compartments of the Golgi. Abbreviations are: Man, mannose; Gal, galactose; Glc, glucose; GlcNAc, N-acetylglucosamine; GlcNH₂, Glucosamine; GlcA, glucuronic acid; IdoA, iduronic acid; GalNAc, N-acetylgalactosamine; Xyl, xylose; Fuc, Fucose; Sia, sialic acid; 3S, 3-O-sulfated; 6S, 6-O-sulfated, PO₄⁻, phosphate. (Modified from Figure 1.6 in Essentials of glycobiology, with permission from Varki and Sharon 2009.)

Figure 2.

Glycosyltransferases and nucleotide sugar transporters of the Golgi. The diagram depicts a variety of nucleotide sugar transporters that transfer a nucleotide sugar from the cytoplasm into the Golgi lumen in exchange for a nucleotide monophosphate generated by hydrolysis of the nucleotide diphosphate released after transfer of the sugar to an acceptor. A typical type II transmembrane glycosyltransferase (a sialyltransferase; gray) with its short cytoplasmic tail, transmembrane domain, extended stem region, and globular catalytic domain is shown binding CMP-Sia from which it transfers Sia to Gal on a complex N-glycan on a glycoprotein (green). Abbreviations are defined in the legend to Figure 1. (Modified from Figure 4.4 in Essentials of glycobiology, with permission from Freeze and Elbein 2009.)

Figure 3.

Compartmentalization of sugar addition and removal in the Golgi. Although Golgi membranes are dynamically recycling, compartments corresponding to cis-, medial-, and trans-Golgi and the TGN are identified by resident proteins that mostly localize to a particular compartment (Nilsson et al. 1993a; Rabouille et al. 1995). The diagram shows in which Golgi compartment particular sugars are added or generated (green) or may be removed (red). Compartments are shown overlapping to signify that distinctions are not precise. Sugars added in the ER or ERGIC compartments before the cis-Golgi are boxed in teal in Figure 1. Abbreviations are defined in the legend to Figure 1.

Figure 4.

Golgi glycans as tags. Glycoproteins in the ER, ERGIC, and cis-Golgi have high mannose N-glycans that are all susceptible to release by Endo H. Processing α-mannosidases generate the Man₅GlcNAc₂Asn N-glycan shown above in the medial-Golgi to which GlcNAcT-I transfers a GlcNAc on the left terminal Man residue. Hybrid N-glycans result from the extension of this GlcNAc but the Man₅GlcNAc₂Asn core is not further processed and remains sensitive to Endo H. Complex N-glycans are formed by the removal of two Man residues following the action of GlcNAcT-I and the addition of a second GlcNAc by GlcNAcT-II. Further extension gives rise to the biantennary complex N-glycan shown in the diagram. Complex N-glycans are resistant to Endo H. Both high mannose and complex N-glycans are sensitive to removal by N-glycanase, whose action generates an Asp in place of Asn, a change that may be used to identify N-glycosylation sites by mass spectrometry.