The formation of N-glycosidic linkages of glycoproteins involves the ordered assembly of the common Glc3Man9GlcNAc2 core-oligosaccharide on the lipid carrier dolichyl diphosphate. Early mannosylation steps occur on the cytoplasmic side of the endoplasmic reticulum with GDP-Man as donor, the final reactions from Man5GlcNAc2-PP-Dol to Man9Glc-NAc2-PP-Dol on the lumenal side use dolichyl ¦Â-D-mannosyl phosphate. ALG9 mannosyltransferase catalyses the addition of two different ¦Á-1,2-mannose residues – the addition of ¦Á-1,2-mannose to Man6GlcNAc2-PP-Dol (EC 2.4.1.259) and the addition of ¦Á-1,2-mannose to Man8GlcNAc2-PP-Dol (EC 2.4.1.261).
