Like ST6GalNAc-III, ST6GalNAc-IV is a relatively small sialyltransferase (302 amino acids in length) compared with other sialyltransferases characterized to date. ST6GalNAc-IV has a type II membrane protein topology. The transmembrane domain of mouse ST6GalNAc-IV is 23 amino acids long, from positions 14 to 36. The stem region of mouse ST6GalNAc-IV is very short, and there are only 38 amino acid residues between the transmembrane domain and sialylmotif L, while mouse ST6GalNAc-I, -II, and -III have 261, 123, and 53 amino acid residues, respectively. ST6GalNAc-III-VI are members of one ST6GalNAc subfamily that can synthesize the ganglioside GD1316 from GM1b (Sjoberg et al. 1996; Lee et al. 1999; Okajima et al. 1999, 2000; Ikehara et al. 1999). ST6GalNAc-IV prefers O-glycans to glycolipids as substrates, while ST6GalNAc-III, -V, and -VI prefer glycolipids to O-glycans. 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-linked glycan chains of glycoproteins. 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). The overall amino acid sequence identity of mouse ST6GalNAc-IV is 43.0% to mouse ST6GalNAc-III, 44.8% to mouse ST6GalNAc-V, and 41.2% to mouse ST6GalNAc-VI, but ST6GalNAc-IV shows no significant similarity to other sialyltransferases except in sialylmotifs.
KeywordsSialic Acid Acceptor Substrate mRNA Differential Display GalNAc Residue Precise Biological Function
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