Identification and characterization of a UDP-GalNAc:GlcNAcη-R η→4–N-acetylgalactosaminyltransferase from cercariae of the schistosome Trichobilharzia ocellata. Catalysis of a key step in the synthesis of N,N’-diacetyllactosediamino (lacdiNAc)-type glycans
Three different stages of the avian schistosome Trichobilharzia ocellata appeared to contain a novel N-acetylgalac-tosaminyltransferase activity. To investigate its function in the biosynthesis of schistosome glycoconjugates, the enzyme was partially purified from cercariae, a free-living stage of the parasite, by affinity chromatography on UDP-Sepharose. Acceptor specificity studies showed that the enzyme catalyses the transfer of N-acetylgalactosamine (GalNAc) from UDP-GalNAc to oligosaccharides, glyco-peptides and glycoproteins carrying a terminally η-llnked N-acetylglucosamine (GlcNAc) residue, regardless of the underlying structure. Analysis of the products obtained with GlcNAc and a desialylated and degalactosylated di-antennary giycopeptlde by 400 MHz 1H -NMR spectroscopy revealed that a GalNAcη1→4GlcNAc (N,N’-diacetyl-lactosedlamine, lacdiNAc) unit was formed. The enzyme can therefore be described as a UDP-GalNAc:GlcNAcη-R ηl→4-Nacetylgalactosaminytransferase (η4-GalNAcT). Using specific acceptors, the enzyme could be distinguished from all other (η4-GalNAcTs described to date, including the one from pituitary gland that is Involved in the specific glycosylation of pituitary glycohormones. By contrast, the enzymatic properties of the schistosome η4-GalNAcT (except for the sugar-donor specificity) strongly resemble those of the η4-galactosyltransferase of higher animals, an enzyme which is known to control the synthesis of Gall→4GlcNAc QacNAc)-type oligosaccharide chains. By analogy, the η4-GalNAcT is concluded to control the key step hi the synthesis of lacdiNAc-type chains. LacdiNAc-type glycans are also common to the mollusc Lymnaea stagnate, which is the intermediate host of Tocellata. It is proposed that the schistosome fW-GalNAcT functions in the expression of specific carbohydrate structures that contribute to a molecular mimicry, enabling the schistosome to evade the defence system of the snail host.