The complex-type of oligosaccharides are synthesised through elongation by glycosyltransferases after trimming of the precursor oligosaccharides transferred to proteins in the endoplasmic reticulum. N-Acetylglucosaminyltransferases (GnTs) take part in the formation of branches in the biosynthesis of complex-type sugar chains. In vertebrates, six GnTs, designated as GnT-I to -VI, which catalyse the transfer of GlcNAc to the core mannose residues of Asn-linked sugar chains, have been identified. GnT-IV () catalyzes the transfer of GlcNAc from UDP-GlcNAc to the GlcNAc1-2Man1-3 arm of core oligosaccharide [Gn2(22)core oligosaccharide]and forms a GlcNAc1-4(GlcNAc1-2)Man1-3 structure on the core oligosaccharide (Gn3(2,4,2)core oligosaccharide) [].GnT-IV (also known as alpha-1,3-mannosyl-glycoprotein 4-beta-N-acetylglucosaminyltransferase or MGAT4) and related proteins, such MGAT4D and MGAT4E, constitute the glycosyl transferase family 54. MGAT4D does not show catalytic glucosyltransferase activity and is thought to be an inhibitor of MGAT1 (GnT-I) [].
