Practice question

Secretory proteins undergo co-translational modification ensuring solubility, folding, and functional diversity needed for extracellular environment. N-glycosylation begins in rough ER lumen where oligosaccharyltransferase complex scans nascent polypeptide emerging from Sec61 translocon, transferring preassembled 14-sugar oligosaccharide Glc3Man9GlcNAc2 from dolichol phosphate lipid anchor to asparagine in consensus sequon Asn-X-Ser/Thr, followed by trimming by glucosidase I and II and binding to lectin chaperones calnexin-calreticulin that monitor folding, with UGGT reglucosylating misfolded species for another folding attempt. Correctly folded glycoproteins packaged into COPII vesicles transport to Golgi apparatus where sequential cisternae house mannosidases and glycosyltransferases mediating O-glycosylation initiation by GalNAc-T family adding N-acetylgalactosamine to serine/threonine, elongation, branching, sulfation, and terminal sialylation by ST6GAL1 generating complex glycans dictating serum half-life and receptor binding. Mitochondria produce ATP via electron transport chain, lysosomes degrade via cathepsins at low pH, peroxisomes handle oxidative reactions, but bulk glycosylation resides in ER-Golgi secretory pathway essential for antibody effector function and Notch signaling.