ALPHA-LACTALBUMIN AFFECTS THE ACCEPTOR SPECIFICITY OF LYMNAEA-STAGNALIS ALBUMIN GLAND UDP-GALNAC-GLCNAC-BETA-R BETA-1-]4-N-ACETYLGALACTOSAMINYLTRANSFERASE - SYNTHESIS OF GALNAC-BETA-1-]4GLC

The N,N'-diacetyllactosediamine (lacdiNAc) pathway of complex-type oli gosaccharide synthesis is controlled by a UDP-GalNAc:GlcNAc beta-R bet a 1-->4-N-acetylgalactosaminyltransferase (beta 4-GalNAcT) that acts a nalogously to the common UDP-Gal:GlcNAc beta-R beta 1-->4 galactosyltr ansferase (beta 4-GalT). LacdiNAc-based chains particularly occur in i nvertebrates and cognate beta 4-GalNAcTs have been identified in the s nail Lymnaea stagnalis, in two schistosomal species, and in several le pidopteran insect cell lines, Because of the similarity in reactions c atalyzed by both enzymes, we investigated whether L. stagnalis albumen gland beta 4-GalNAcT would share with mammalian beta 4-GalT the prope rty of interacting with alpha-lactalbumin (alpha-LA), a protein that o nly occurs in the lactating mammary gland, to form a complex in which the specificity of the enzyme is changed, It was found that, under con ditions where beta 4-GalT forms the lactose synthase complex with alph a-LA, the snail beta 4-GalNAcT was induced by this protein to act on G lc with a > 100-fold increased efficiency, resulting in the formation of the lactose analog GalNAc beta 1-->4Glc. This forms the second exam ple of a glycosyltransferase, the specificity of which can be altered by a modifier protein. So far, however, no protein fraction could he i solated from L. stagnalis that could likewise interact with the beta 4 -GalNAcT, Neither had lysozyme c, a protein that is homologous to alph a-LA, an effect on the specificity of the enzyme, These results raise the question of how the capability to interact with alpha-LA has been conserved in the snail enzyme during evolution without any apparent se lective pressure. They also suggest that snail beta 4-GalNAcT and mamm alian beta 4-GalT show similarity at a molecular level and allows the identification of the beta 4-GalNAcT as a candidate member of the beta 4-GalT family.