Beta-1,3-glucuronyltransferase 1 (Glucuronosyltransferase P); beta-1,3- glucuronyltransferase 2 (B3GAT1,2)

The human natural killer-1 (HNK-1) carbohydrate epitope, composed of a unique acidic trisaccharide HSO₃-3GlcAβ1-3Galβ1-4GlcNAc-, is highly expressed in the nervous system. The HNK-1 epitope is found in N-linked and O-mannose-linked glycans of glycoproteins and also in some glycolipids (Chou et al. 1986; Kouno et al. 2011; Yuen et al. 1997). Intriguingly, limited glycoproteins are modified with the HNK-1 epitope such as the immunoglobulin-superfamily cell adhesion molecules (NCAM, L1, MAG, P0, etc.), extracellular matrix proteins (tenascin-R, phosphacan, etc.), and a glutamate receptor subunit (GluA2) (Kizuka and Oka 2012), suggesting that the HNK-1 glycan is expressed in a tightly regulated manner in the neural cells. As for physiological function, HNK-1 glycan is required for memory and learning, revealed by studies using enzyme-deficient mice (Senn et al. 2002; Yamamoto et al. 2002; Yoshihara et al. 2009). The unique structure of the HNK-1 glycan is attributed to a sulfated GlcA residue that is rarely seen in other N-linked and O-mannose-linked glycans, and key enzymes for the biosynthesis are two glucuronyltransferases, GlcAT-P and GlcAT-S (B3GAT1 and 2, respectively), and a sulfotransferase, HNK-1ST (see Chap. 92, “Carbohydrate Sulfotransferase 10 (CHST10)"). In this chapter, enzymatic features of the two glucuronyltransferases are further described, which should help us to understand the functions and expression mechanisms of the HNK-1 glycan.