Cloning of a novel member of the UDP-galactose:β-N-acetylglucosamine β1,4-galactosyltransferase family, β4Gal-T4, involved in glycosphingolipid biosynthesis

A novel putative member of the human UDP-galactose:beta-N-acetylglucosamine beta 1,4-galactosyltransferase family, designated beta 4Gal-T4, was identified by BLAST analysis of expressed sequence tags. The sequence of beta 4Gal-T4 encoded a type II membrane protein with significant sequence similarity to other beta 1,4-galactosyltransferases. Expression of the full coding sequence and a secreted form of beta 4Gal-T4 in insect cells showed that the gene product had beta 1,4 galactosyltransferase activity. Analysis of the substrate specificity of the secreted form revealed that the enzyme catalyzed glycosylation of glycolipids with terminal beta-GlcNAc; however, in contrast to beta 4Gal-T1, -T2, and -T3, this enzyme did not transfer galactose to asialo-agalacto-fetuin, asialo-agalactotransferrin, or ovalbumin. The catalytic activity of beta 4Gal-T4 with monosaccharide acceptor substrates, N-acetylglucosamine as well as glucose, was markedly activated in the presence of alpha-lactalbumin. The genomic organization of the coding region of beta 4Gal-T4 was contained in six exons. All intron/exon boundaries were similarly positioned in beta 4Gal-T1, -T2, and -T3. beta 4Gal-T4 represents a new member of the beta 4-galactosyltransferase family. Its kinetic parameters suggest unique functions in the synthesis of neolactoseries glycosphingolipids.