Oligosaccharide-assemblage

Glycoproteins with complex oligosaccharide side chains are synthesized in the ER and modified in the Golgi.

Typically, the entire  N-linked oligosaccharide precursor is added to an ER protein. Then, glucosidase I and glucosidase II remove one and two units of glucose, respectively.

After an ER mannosidase trims a specific mannose, the protein is transferred to the Golgi.

Within the Golgi lumen, mannosidase I removes three mannoses from different parts of the precursor. Here, additional sugar units such as N-acetylglucosamine are added.

Next, mannosidase II removes two mannoses from the precursor yielding a final core comprising three mannoses.

A nucleotide sugar transporter adds incoming sugars bound to nucleotide phosphates such as UDP-N-acetylglucosamine. The nucleotide phosphates released during the sugar addition are recycled to the cytosol to bring in additional sugars through the nucleotide sugar transporter, such as galactose and sialic acid.

This forms a complex oligosaccharide side chain on the protein.