Initiation and maturation of glycoproteins occur in the ER-Golgi complex. Nucleotide sugar donors transport to the ER, and the folding of glycoproteins takes place in the ER. Trimming, extension, and terminal glycosylation occur in the Golgi apparatus. Glycans participate in the nascent protein folding and intracellular trafficking. The secretory vesicles transport the glycoproteins to the cell surface. Glycoproteins possess specific functions on cell surfaces such as cell-cell adhesion, ligand-receptor interaction, protecting from pathogens, providing receptors for toxins, etc.