Fig. 2
Protein N- and O-glycosylation in quality control of protein folding and degradation. Glycosylation of protein mainly accomplished through the synergistic action of ER and Golgi apparatus. N-glycosylation involves adding glycans to amide group of Asn(N) residues within ER and Golgi apparatus, whereas O-glycosylation refers to attaching a single glycan to hydroxyl oxygens of Ser (S) or Thr(T) residues in Golgi apparatus. Based on their side chain branches, N-glycans can be categorized into three main types like high mannose, complex and hybrid N-glycans, whereas O-glycans are synthesized by first attaching GalNAc to the Ser/Thr side chain, and then extending them with additional sugars such as Gal, GlcNAc, sialic acid, and Fuc resulting in either linear or branched glycan. However, due to variability in O-glycosylation and an absence of consensus sequences, only four core types are shown in the figure. During glycoprotein synthesis, nascent polypeptides are translated and subsequently translocated through Sect. 61 pore while OST transfers Glc3Man9GlcNAc2 from dolichol phosphate onto unfold proteins. Then, native folded glycoproteins are then generated with the help of various glycosyltransferases, glycosidases, molecular chaperones and lectins. At this point, properly folded glycoproteins proceed to Golgi apparatus for further modifications via COPII, meanwhile, un/misfolded proteins will be degraded through ERAD pathway. ER, endoplasmic reticulum; OST, oligosaccharyltransferase; ERMan I, ER mannosidase I; Man I/II, mannosidase I/II; Glc I, glucosidases I; Glc II, glucosidases II; Gal-T, galactosyltransferases; GalNAc-T, GalNAc-transferase; Sialyl-T, sialyltransferase; GlcNAc-T, GlcNAc-transferase; ERp57, Endoplasmic Reticulum Protein57