Figure 2.

The Structure of Lipid-Linked Oligosaccharide Unit Glc3Man9GlcNAc2.

Two N-acetylglucosamine (N1 and N2, gray squares) and five mannose residues (M3 to M7, gray, pink, and blue circles) are added subsequently to the polyisoprenoid lipid dolichol (Dol) on the cytoplasmic side of the ER. The above lipid-linked oligosaccharide (LLO) unit Man5GlcNAc2 is then flipped over to face the ER lumen, where four additional mannose (M8 to M11, pink circles) and three glucose residues (G12 to G14, blue and gray triangles) are added to form the LLO unit Glc3Man9GlcNAc2. The moieties containing 14 monosaccharide units are then transferred from the lipid Dol to proteins and then subjected to further glucose and/or mannose trimming at bond sites indicated by arrows. The enzymes that catalyze the respective hydrolysis reactions are indicated. The folding status of the protein is recognized either by the CNX/CRT through residues indicated in blue or EDEM through residues indicated in pink. The properly folded proteins are secreted out or the unfolded/misfolded proteins are retrieved for degradation through ERAD.