EXTL2, a Member of the EXT Family of Tumor Suppressors,Controls Glycosaminoglycan Biosynthesis in a Xylose Kinase-dependent Manner |
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Authors: | Satomi Nadanaka Shaobo Zhou Shoji Kagiyama Naoko Shoji Kazuyuki Sugahara Kazushi Sugihara Masahide Asano Hiroshi Kitagawa |
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Affiliation: | From the ‡Department of Biochemistry, Kobe Pharmaceutical University, 4-19-1 Motoyamakita-machi, Higashinada-ku, Kobe 658-8558, Japan and ;the §Division of Transgenic Animal Science, Kanazawa University Advanced Science Research Center, Takara-machi, Kanazawa, 920-8640, Japan |
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Abstract: | Mutant alleles of EXT1 or EXT2, two members of the EXT gene family, are causative agents in hereditary multiple exostoses, and their gene products function together as a polymerase in the biosynthesis of heparan sulfate. EXTL2, one of three EXT-like genes in the human genome that are homologous to EXT1 and EXT2, encodes a transferase that adds not only GlcNAc but also N-acetylgalactosamine to the glycosaminoglycan (GAG)-protein linkage region via an α1,4-linkage. However, both the role of EXTL2 in the biosynthesis of GAGs and the biological significance of EXTL2 remain unclear. Here we show that EXTL2 transfers a GlcNAc residue to the tetrasaccharide linkage region that is phosphorylated by a xylose kinase 1 (FAM20B) and thereby terminates chain elongation. We isolated an oligosaccharide from the mouse liver, which was not detected in EXTL2 knock-out mice. Based on structural analysis by a combination of glycosidase digestion and 500-MHz 1H NMR spectroscopy, the oligosaccharide was found to be GlcNAcα1-4GlcUAβ1–3Galβ1–3Galβ1–4Xyl(2-O-phosphate), which was considered to be a biosynthetic intermediate of an immature GAG chain. Indeed, EXTL2 specifically transferred a GlcNAc residue to a phosphorylated linkage tetrasaccharide, GlcUAβ1–3Galβ1–3Galβ1–4Xyl(2-O-phosphate). Remarkably, the phosphorylated linkage pentasaccharide generated by EXTL2 was not used as an acceptor for heparan sulfate or chondroitin sulfate polymerases. Moreover, production of GAGs was significantly higher in EXTL2 knock-out mice than in wild-type mice. These results indicate that EXTL2 functions to suppress GAG biosynthesis that is enhanced by a xylose kinase and that the EXTL2-dependent mechanism that regulates GAG biosynthesis might be a “quality control system” for proteoglycans. |
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Keywords: | Glycobiology Glycosaminoglycan Glycosyltransferases Proteoglycan Proteoglycan Synthesis |
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