IDENTIFICATION AND CHARACTERIZATION OF A UDP-GALNAC-GLCNAC-BETA-R BETA-1-]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-acetylgalactosaminyltransferase 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, glycopeptides and glycoproteins carrying a terminally beta-linked N-acetylglucosamine (GlcNAc) residue, regardless of the underlying structure. Analysis of the products obtained with GlcNAc and a desialylated and degalactosylated diantennary glycopeptide by 400 MHz H-1-NMR spectroscopy revealed that a GalNAc beta 1-->4GlcNAc (N,N'-diacetyllactosediamine, lacdiNAc) unit was formed. The enzyme can therefore be described as a UDP-GalNAc:GlcNAc beta-R beta 1-->4-N-acetylgalactosaminytransferase (beta 4-GalNAcT). Using specific accepters, the enzyme could be distinguished from all other beta 4-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 beta 4-GalNAcT (except for the sugar-donor specificity) strongly resemble those of the beta 4-galactosyltransferase of higher animals, an enzyme which is known to control the synthesis of Gal1-->4GlcNAc (lacNAc)-type oligosaccharide chains. By analogy, the beta 4-GalNAcT is concluded to control the key step in the synthesis of lacdiNAc-type chains. LacdiNAc-type glycans are also common to the mollusc Lymnaea stagnalis, which is the intermediate host of T.ocellata. It is proposed that the schistosome beta 4-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.