Molecular cloning of a novel alpha 2,3-sialyltransferase (ST3Gal VI) that sialylates type II lactosamine structures on glycoproteins and glycolipids

A novel member of the human CMP-NeuAc:beta-galacto side alpha 2,3-sialyltra nsferase (ST) subfamily, designated ST3Gal VI, was identified based on BLAS T analysis of expressed sequence tags, and a cDNA clone was isolated from a human melanoma line library. The sequence of ST3Gal VI encoded a type II m embrane protein with 2 amino acids of cytoplasmic domain, 32 amino acids of transmembrane region, and a large catalytic domain with 297 amino acids; a nd showed homology to previously cloned ST3Gal III, ST3Gal IV, and ST3Gal V at 34, 38, and 33%, respectively. Extracts from L cells transfected with S T3Gal VI cDNA in a expression vector and a fusion protein with protein A sh owed an enzyme activity of alpha 2,3-sialyltransferase toward Gal beta 1,4G lcNAc structure on glycoproteins and glycolipids, In contrast to ST3Gal III and ST3Gal IV, this enzyme exhibited restricted substrate specificity, i.e . it utilized Gal beta 1,4GlcNAc on glycoproteins, and neolactotetraosylcer amide and neolactohexaosylceramide, but not lactotetraosylceramide, lactosy lceramide, or asialo-GM1. Consequently, these data indicated that this enzy me is involved in the synthesis of sialyl-paragloboside, a precursor of sia lyl-Lewis X determinant.