Transmembrane movement of oligosaccharide-lipids during glycoprotein synthesis.
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 Man3-5GlcNAc2 were found on the cytoplasmic side of ER-derived vesicles in vitro while lipid-linked Man6-9GlcNAc2 and Glc1-3Man9GlcNAc2 were found facing the lumen. These results suggest that Man5GlcNAc2-lipid is synthesized on the cytoplasmic side of the ER membrane and then translocated to the luminal side. Glc3Man9GlcNAc2-lipid is then completed on the luminal side where it serves as the donor in peptide glycosylation. Translocation of Man5GlcNAc2-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.
D000818:Animals; D001692:Biological Transport; D002451:Cell Compartmentation; D002478:Cells, Cultured; D003208:Concanavalin A; D006224:Cricetinae; D003593:Cytoplasm / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006017:Glycolipids / Q000378:metabolism*; D006023:Glycoproteins / Q000096:biosynthesis*; D011499:Protein Processing, Post-Translational
D006017:Glycolipids; D006023:Glycoproteins; D003208:Concanavalin A