N-linked glycosylation and GE.

<p>A dolichol phosphate-linked precursor consisting of 2 membrane-linked N-acteylglucosamine (GlcNac) moieties and 5 mannose (Man) forms on the cytoplasmic surface of the endoplasmic reticulum (ER), then flips to the lumen. Further mannose moieties are added to create 3 termini (D1-D3), to which glucose moieties are added by glucosyltransferase (GlcT). This is transferred to Asn-X-Ser/Thr sequons of a nascent protein by oligosaccharyltransferase (OST). The 3 terminal glucose residues are then removed by α-glucosidase to form Man₉GlcNAc₂ (Man9)—a step that is inhibited by <a target="_blank">N-Butyldeoxynojirimycin</a> (NB-DNJ). The terminal D2 mannose is then cleaved by α-mannosidase 1 (ERMAN1)—a step that can be inhibited by kifunensine. In the cis-Golgi, mannose is trimmed by α-mannosidases 1A, 1B and 1C (MAN1A-C) to form Man₅GlcNAc₂. GlcNAc is then transferred to the α-1,3 D1 arm by N-acetylglucosaminyltransferase 1 (GNT1; inactive in GNT1- cells). In the medial Golgi, there is a bifurcation in the pathway. In one fork, D2 and D3 mannose subunits are removed by α-mannosidase II (MAN2)—a step that is blocked by swainsonine (swain). GlcNAc moieties may then be added to the trimmed α-1,6 arm by GNT2 to initiate a biantennary glycan, followed by the addition of a core fucose moiety by fucosyltransferase (FUCT8), a step that is blocked by 2-deoxy-2-fluoro-1-fucose (2FF). Further GlcNAc termini may then be added by GNT4 and GNT5 to form tri- and tetra-antennary glycans. These may be galactosylated by β-1,4 galactosyltransferases (B4GALT1), a step that is blocked by 2-deoxy-2-fluoro-d-galactose (2FG). Terminal SA may then be added by β-galactoside α-2,3-sialyltransferase (ST3GAL4) or β-galactoside α-2,6-sialyltransferase (ST6GAL1). Polysialic chains may form by the addition of α-2,8-linked SA by α-2,8-sialyltransferase (ST8SIA4). SAs can be cleaved by NA. In the alternative fork, α-1,6 arm mannose is not removed, but the α-1,3 arm may be modified with galactose and SA, forming hybrid glycans, sometimes with the addition of a bisecting GlcNAc subunit by GNT3 and no fucosylation. Non-fucosylated high mannose and hybrid glycans can be cleaved by endoglycosidase H (endo H).