Fig. 1

The process of N-linked glycosylation and O-GalNAc glycosylation. The processing of N-glycans begins with an LLO synthesized by linking the cytoplasmic face oligosaccharide intermediate precursor of the ER with dolichol, and the approximate steps are shown in this figure: under the replacement of multienzyme complexes and three mannosyltransferases, one GlcNAc and five Man residues are sequentially transferred from UDP-GlcNAc and GDP-Man to the substrate to generate Man5GlcNAc2-P-Dol.Next, under the action of invertase, LLO undergoes transbilayer translocation across the ER membrane and successively completes four mannosylations in the ER lumen to produce Man9GlcNAc2-P-Dol, and under OST catalysis, the oligosaccharide fraction of LLO is transferred to newly synthesized proteins containing the Asn-X-Ser/Thr (X is any amino acid except Pro) sequence. N-glycosidic bonds are formed between the terminal carbon of GlcNAc and the nitrogen atom of the asparagine side chain of the protein to synthesize N-glycoproteins. Further, N-glycoproteins controlled by ER quality leave the ER, enter the Golgi complex, and are additionally modified to form complex N-glycoproteins that are ultimately transported to the cell membrane. O-GalNAc glycosylation is initiated by GalNAc-Ts catalyzing the addition of GalNAc to peptides, roughly as follows: GalNAc residues are transferred from UDP-GalNAc to the Ser/Thr side chain catalyzed by GalNAc-Ts, and individual GalNAc residues are linked to Ser/Thr α-linkages via αO-glycosidic bonds to form Tn antigens, and subsequently, other glycosyltransferases rapidly extend Tn to other more complex O-glycans. Further, Tn can also be extended to generate four major O-GalNAc cores and four rare cores, core 1, core2, core3, and core 4, as shown respectively, and this type of glycosylation modifies most secreted and cell surface proteins