Mannosyl (beta-1,4-)-glycoprotein beta-1,4-N-acetylglucosaminyltransferase (MGAT3); β1,4-NAcetylglucosaminyltransferase III (GnT-III, GlcNAcT-III)

β1,4-N-Acetylglucosaminyltransferase III (β-1,4-mannosyl-glycoprotein β1,4-N-acetylglucosaminyltransferase: EC 2.4.1.144) catalyzes the transfer of GlcNAc to the core β-mannose residue of N-glycans with β1,4-linkage (Schachter 1986). The resulting β1,4-linked GlcNAc is referred to as a bisecting GlcNAc and plays a regulatory role(s) in the biosynthesis of complex and hybrid types of the oligosaccharides. The addition of the bisecting GlcNAc residue to the core β-mannose by the enzyme prevents the actions of other GlcNAc-transferases that serve to form multiantennary sugar chains, therefore leading to decrease in branch formation of N-glycans. It has been considered that this glycosyltransferase is one of the key enzymes in the N-glycan biosynthesis, because of such a unique regulatory function of the enzyme product. The relatively high levels of the activity were found in kidney and brain of mammals (Nishikawa et al. 1988b). Consistent with this distribution of the enzyme, various N-glycans carrying the bisecting GlcNAc, bisected sugar chains, were identified in these tissues (Nakakita et al. 1998; Shimizu et al. 1993; Yamashita et al. 1986, 1983). Expression of the enzyme is enhanced during hepatocarcinogenesis, while the activity is nearly undetectable in normal liver (Narasimhan et al. 1988; Nishikawa et al. 1988a; Miyoshi et al. 1993). Because expression of the enzyme appears to lead to remarkable structural alteration of the sugar chains on the cell surface, it seems that the enzyme is associated with various biological events such as differentiation and carcinogenesis via regulating functions of some glycoproteins such as receptors and adhesion molecules.