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The Drosophila Gene brainiac Encodes a Glycosyltransferase Putatively Involved in Glycosphingolipid Synthesis*

The Drosophila genesfringe and brainiac exhibit sequence similarities to glycosyltransferases. Drosophila and mammalian fringe homologs encode UDP-N-acetylglucosamine:fucose-O-Ser β1,3-N-acetylglucosaminyltransferases that modulate the function of Notch family receptors. The biological function of brainiac is less well understood. brainiac is a member of a large homologous mammalian β3-glycosyltransferase family with diverse functions. Eleven distinct mammalian homologs have been demonstrated to encode functional enzymes forming β1–3 glycosidic linkages with different UDP donor sugars and acceptor sugars. The putative mammalian homologs with highest sequence similarity tobrainiac encode UDP-N-acetylglucosamine:β1,3-N-acetylglucosaminyltransferases (β3GlcNAc-transferases), and in the present study we show thatbrainiac also encodes a β3GlcNAc-transferase that uses β-linked mannose as well as β-linked galactose as acceptor sugars. The inner disaccharide core structures of glycosphingolipids in mammals (Galβ1–4Glcβ1-Cer) and insects (Manβ1–4Glcβ1-Cer) are different. Both disaccharide glycolipids served as substrates for brainiac, but glycolipids of insect cells have so far only been found to be based on the GlcNAcβ1–3Manβ1–4Glcβ1-Cer core structure. Infection of High FiveTM cells with baculovirus containing full coding brainiac cDNA markedly increased the ratio of GlcNAcβ1–3Manβ1–4Glcβ1-Cer glycolipids compared with Galβ1–4Manβ1–4Glcβ1-Cer found in wild type cells. We suggest that brainiac exerts its biological functions by regulating biosynthesis of glycosphingolipids.