Abstract
The transport of sugar residues into the endoplasmic reticulum (ER) during glycoprotein synthesis was studied by examining the transmembrane orientations of the oligosaccharide-lipid precursors of asparagine-linked oligosaccharides. Using the lectin concanavalin A, the lipid-linked oligosaccharides Man 3–5GIcNAc 2 were found on the cytoplasmic side of ER-derived vesicles in vitro while lipid-linked Man 6–9GIcNAc 2 and GIc 1–3Man 9GIcNAc 2 were found facing the lumen. These results suggest that Man 5GlcNAc 2-lipid is synthesized on the cytoplasmic side of the ER membrane and then translocated to the luminal side. Glc 3Man 9GIcNAc 2-lipid is then completed on the luminal side where it serves as the donor in peptide glycosylation. Translocation of Man 5GlcNAc 2-lipid offers a mechanism for the export of sugar residues from the cytoplasm during glycoprotein synthesis. This translocation may be the reason for the participation of lipid-linked mono- and oligosaccharides in glycoprotein synthesis.
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