Biosynthesis of the linkage region of glycosaminoglycans -: Cloning and activity of galactosyltransferase II, the sixth member of the β1,3-galactosyltransferase family (β3GalT6)

A family of five beta1,3-galactosyltransferases has been characterized that catalyze the formation of Gal beta1, 3GlcNAc beta and Gal beta1,3GalNAc beta linkages present in glycoproteins and glycolipids (beta 3Ga1T1, -2, -3, -4, and -5). We now report a new member of the family (beta 3GalT6), involved in glycosaminoglycan biosynthesis. The human and mouse genes were located on chromosomes 1p36.3 and 4E2, respectively, and homologs are found in Drosophila melanogaster and Caenorhabditis elegans. Unlike other members of the family, beta 3GalT6 showed a broad mRNA expression pattern by Northern blot analysis. Although a high degree of homology across several subdomains exists among other members of the beta3-galactosyltransferase family, recombinant enzyme did not utilize glucosamine- or galactosamine-containing acceptors. Instead, the enzyme transferred galactose from UDP-galactose to acceptors containing a terminal beta -linked galactose residue. This product, Gal beta1,3Gal beta is found in the linkage region of heparan sulfate and chondroitin sulfate (GlcA beta1, 3Gal beta1, 3Gal beta1, 4Xyl beta -O-Ser), indicating that beta 3GalT6 is the so-called galactosyltransferase II involved in glycosaminoglycan biosynthesis. Its identity was confirmed in vivo by siRNA-mediated inhibition of glycosaminoglycan synthesis in HeLa S3 cells. Its localization in the medial Golgi indicates that this is the major site for assembly of the linkage region.