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. Author manuscript; available in PMC: 2017 Oct 1.
Published in final edited form as: Prog Lipid Res. 2016 Aug 12;64:69–84. doi: 10.1016/j.plipres.2016.08.003

Figure 6. Glycolipid scrambling is necessary for protein N-glycosylation in the ER.

Figure 6

G3M9-DLO, the oligosaccharide donor for protein N-glycosylation, is synthesized in the ER in a multi-step, topologically split pathway. The first 7 steps convert dolichyl-P (dol-P) to M5-DLO on the cytoplasmic face of the ER. Then, M5-DLO is flipped into the ER lumen and extended in 7 further steps to G3M9-DLO. The sugar donors for these luminal reactions are MPD and GPD that are synthesized on the cytoplasmic face of the ER and must be flipped to the luminal side. In addition to its role in N-glycan biosynthesis, MPD is required in the ER lumen for GPI anchor biosynthesis, O-mannosylation, and C-mannosylation. Oligosaccharyltransferase (OST) transfers the oligosaccharide from G3M9-DLO to Asn residues within glycosylation sequences in translocating proteins as they emerge from the translocon into the ER lumen. The dolichyl-PP product of the OST reaction is recycled. The multistep synthesis and transfer of the oligosaccharide require at least 40 gene products.