Free oligosaccharide regulation during mammalian protein N-glycosylation

During protein N-glycosylation in mammalian cells, free oligosaccharides(fOS) are generated from lipid-linked oligosaccharides by apyrophosphatase activity and oligosaccharyltransferase and frommisfolded glycoproteins by peptide:N-glycanase in both the ERand cytoplasm. Trafficking machinery comprising oligosaccharide-specificER and lysosomal transporters, an endo-β-N-acetyl-glucosaminidase,and the cytosolic M2C1 mannosidase drives a flux of fOS fromthe ER to cytoplasm and from the cytoplasm into lysosomes wherefOS are degraded. Transport of fOS out of the ER is normallyefficient and if inhibited causes fOS to be secreted via theGolgi apparatus. By contrast, fOS clearance from the cytosolinto lysosomes is less efficient resulting in low micromolarconcentrations of fOS in the cytoplasm. Structural analysisof cytosolic fOS reveals oligosaccharide families whose relativeabundance highlights the importance of different ER-associateddegradation (ERAD) pathways for misfolded glycoproteins andsuggests that in liver cells substantial amounts of glycoproteinsdestined for ERAD may transit early compartments of the Golgiapparatus. Glycoprotein quality control and ERAD are controlledby N-glycan/lectin interactions and the fOS trafficking pathwaywould seem to ensure that fOS do not interfere with these processeswhich occur in both the ER and cytoplasm. Although Saccharomycescerevisiae strains harbouring mutations in genes of the yeastfOS metabolic pathway do not display obvious phenotypes, mammalianfOS are quantitatively more important and the processes leadingto their regulation are more complex, raising the possibilitythat distinct phenotypes will be seen in mammalian cells oranimals in which fOS metabolism is modified.