Molecular cloning and expression of glucuronyltransferase I involved in the biosynthesis of the glycosaminoglycan-protein linkage region of proteoglycans

We isolated a cDNA encoding a novel glucuronyltransferase from human placenta cDNA with the use of the degenerate reverse transcriptase-polymerase chain reaction method. Degenerate primers ware designed based upon the amino acid sequence alignment of rat glucuronyltransferase (GlcAT-P) involved in the biosynthesis of the carbohydrate epitope HNK-1 with putative proteins in Caenorhabditis elegans and Schistosoma mansoni. The new cDNA sequence revealed an open reading frame coding for a protein of 335 amino acids with a type II transmembrane protein topology. The amino acid sequence displayed 43% identity to the rat GlcAT-P, and the highest sequence identity was found in the COOH-terminal catalytic domain. The expression of a soluble recombinant form of the protein in COS-1 cells produced an active glucuronyltransferase with marked specificity for a glycoserine Galβ1-3Galβ1-4Xylβ1-O-Ser. In contrast, asialoorosomucoid, which contains the Galβ1-4GlcNAc sequence and is a good acceptor substrate for the GlcAT-P, did not serve as an acceptor. The reaction product was sensitive to β-glucuronidase digestion and co- chromatographed with authentic GlcAβ1-3Galβ1-3Galβ1-4Xylβ1-O-Ser in high- performance liquid chromatography, suggesting that the enzyme is a β1,3- glucuronyltransferase. These results indicate that this new member of the glucuronyltransferase gene family is the enzyme previously described as glucuronyltransferase I that forms the glycosaminoglycan-protein linkage region, Glcβ1-3Galβ1-3Galβ1-4Xylβ1-O-Ser, of proteoglycans.