Identification of a Drosophila gene encoding xylosylprotein β4-galactosyltransferase that is essential for the synthesis of glycosaminoglycans and for morphogenesis

In mammals, the xylosylprotein β4-galactosyltransferase termed β4GalT7 (XgalT-1, EC 2.4.1.133) participates in proteoglycan biosynthesis through the transfer of galactose to the xylose that initiates each glycosaminoglycan chain. A Drosophila cDNA homologous to mammalian β4-galactosyltransferases was identified using a human β4GalT7 cDNA as a probe in a BLAST analysis of expressed sequence tags. The Drosophila cDNA encodes a type II membrane protein with 322 amino acids and shows 49% identity to human β4GalT7. Extracts from L cells transfected with the cDNA exhibited marked galactosyltransferase activity specific for a xylopyranoside acceptor. Moreover, transfection with the cloned cDNA restored glycosaminoglycan synthesis in β4GalT7-deficient Chinese hamster ovary cells. In transfectant lysates the properties of Drosophila and human β4GalT7 resembled each other, except that Drosophila β4GalT7 showed a less restricted specificity and was active at a wider range of temperatures. Drosophila β4GalT7 is expressed throughout development, with higher expression levels in adults. Reduction of Drosophila β4GalT7 levels using expressed RNA interference (RNAi) in imaginal discs resulted in an abnormal wing and leg morphology similar to that of flies with defective Hedgehog and Decapentaplegic signaling, which are known to depend on intact proteoglycan biosynthesis. Immunohistochemical analysis of tissues confirmed that both heparan sulfate and chondroitin sulfate biosynthesis were impaired. Our results demonstrate that Drosophila β4GalT7 has the in vitro and in vivo properties predicted for an ortholog of human β4GalT7 and is essential for normal animal development through its role in proteoglycan biosynthesis.