Oligosaccharide transport: pumping waste from the ER into lysosomes

N-glycans play important roles during the folding and secretion of glycopro teins. Surprisingly, during the N-glycosylation of glycoproteins, considera ble amounts of unconjugated polymannose-type oligosaccharides ('free OS') a re generated. Although free oligosaccharides have no known function in mamm alian cells, a sophisticated cellular machinery enables them to be cleared from the endoplasmic reticulum (ER) into the cytosol and then re-enter the endomembrane system at the level of the lysosome. One possible function of this pathway is to stop free OS from interfering with the carbohydrate-depe ndent aspects of glycoprotein folding and transport along the secretory pat hway.