Differential roles of two N-acetylgalactosaminyltransferases, CSGalNAcT-1, and a novel enzyme, CSGalNAcT-2 - Initiation and elongation in synthesis of chondroitin sulfate

By a tblastn search with beta1,4-galactosyltransferases as query sequences, we found an expressed sequence tag that showed similarity in beta1,4-glycosyltransferase motifs. The full-length complementary DNA was obtained by a method of 5'-rapid amplification of complementary DNA ends. The predicted open reading frame encodes a typical type H membrane protein comprising 543 amino acids, the sequence of which was highly homologous to chondroitin sulfate N-acetylgalactosaminyltransferase CSGalNAcT-1), and we designated this novel enzyme CSGalNAcT-2. CSGalNAcT-2 showed much stronger N-acetylgalactosaminyltransferase activity toward glucuronic acid of chondroitin poly- and oligosaccharides, and chondroitin sulfate poly- and oligosaccharides with a beta1-4 linkage, i.e. elongation activity for chondroitin and chondroitin sulfate, but showed much weaker activity toward a tetrasaccharide of the glycosaminoglycan linkage structure (GlcA-Gal-Gal-Xyl-0-methoxyphenyl), i.e. initiation activity, than CSGalNAcT-1. Transfection of the CSGalNAcT-1 gene into Chinese hamster ovary cells yielded a change of glycosaminoglycan composition, i.e. the replacement of heparan sulfate on a syndecan-4/fibroblast growth factor-1 chimera protein by chondroitin sulfate, however, transfection of the CSGalNAcT-2 gene did not. The above results indicated that CSGalNAcT-1 is involved in the initiation of chondroitin sulfate synthesis, whereas CSGalNAcT-2 participates mainly in the elongation, not initiation. Quantitative real-time PCR analysis revealed that CSGalNAcT-2 tran-scripts were highly expressed in the small intestine, leukocytes, and spleen, however, both CSGalNAcTs were ubiquitously expressed in various tissues.