ST6 N-Acetylgalactosaminide Alpha-2,6-Sialyltransferase 4 (ST6GALNAC4)

Abstract

ST6GalNAc-III, ST6GalNAc-IV, ST6GalNAc-V, and ST6GalNAc-VI are members of NeuAcα2-3Galβ1-3GalNAc α2,6-sialyltransferases. These members except ST6GalNAc-IV can efficiently synthesize the ganglioside GD1α from GM1b (Sjoberg et al. 1996; Lee et al. 1999; Okajima et al. 1999, 2000; Ikehara et al. 1999; Harduin-Lepers et al. 2000; Tsuchida et al. 2005; Ko et al. 2010). Compared with other members, ST6GalNAc-IV prefers O-linked glycans of glycoproteins to glycolipids as substrates. Therefore, ST6GalNAc-IV may be the main candidate for synthesizing the NeuAcα2-3Galβ1-3(NeuAcα2-6)GalNAc residue, which is usually found in the O-glycans. ST6GalNAc-IV is also considered to be involved in the early alteration of the sialylation pattern of cell surface molecules in activated lymphocytes (Kaufmann et al. 1999). Like ST6GalNAc-III, ST6GalNAc-IV is a relatively small sialyltransferase (302 amino acids in length for human, mouse, and pig enzymes) compared with other animal sialyltransferases because of the short stem region. However, isoforms of ST6GalNAc-IV with different amino acid length are also reported (350 and 360 amino acids in length for mouse isoforms). The overall amino acid sequence identity of mouse ST6GalNAc-IV is 88.4 % to human ST6GalNAc-IV, 86.1 % to pig ST6GalNAc-IV, 43.0 % to mouse ST6GalNAc-III, 44.8 % to mouse ST6GalNAc-V, and 41.2 % to mouse ST6GalNAc-VI, respectively, but ST6GalNAc-IV shows no significant similarity to other sialyltransferases except in sialylmotifs.