Abstract
Hereditary multiple exostoses gene (EXT) family members encode glycosyltransferases required for heparan sulfate (HS) biosynthesis in humans as well as in Drosophila. In the present study, we identified a novel Drosophila EXT protein with a type II transmembrane topology and demonstrated its glycosyltransferase activities. The truncated soluble form of this new homolog designated DEXT3 transferred N-acetylglucosamine (GlcNAc) through an alpha1,4-linkage not only to N-acetylheparosan oligosaccharides that represent growing HS chains (alpha-GlcNAc transferase II activity) but also to GlcUAbeta1-3Galbeta1-O-C(2)H(4)NHCbz, a synthetic substrate for alpha-GlcNAc transferase I that determines and initiates HS biosynthesis. The results suggest that DEXT3 is the ortholog of human EXTL3 and Caenorhabditis elegans rib-2. Semiquantitative reverse transcriptase-PCR analysis revealed ubiquitous expression of the DEXT3 mRNA. Based on the findings of the present study and those of a recent study where a fly mutant, deficient in the botv gene identical to DEXT3, affected HS proteoglycan-mediated developmental signalings, it is suggested that DEXT3 with the revealed glycosyltransferase activities is critically involved in HS formation in Drosophila. These results suggest the essential roles of DEXT3, its human ortholog EXTL3, and the C. elegans ortholog rib-2 in the biosynthesis of heparan sulfate and heparin, if present, in the respective organisms.
