The Drosophila melanogaster brainiac protein is a glycolipid-specific β1,3N-acetylglucosaminyltransferase

Mutations at the Drosophila melanogaster brainiac locus lead to defective formation of the follicular epithelium during oogenesis and to neural hyperplasia. The brainiac gene encodes a type H transmembrane protein structurally similar to mammalian beta1,3-glycosyltransferases. We have cloned the brainiac gene from D. melanogaster genomic DNA and expressed it as a FLAG-tagged recombinant protein in Sf9 insect cells. Glycosyltransferase assays showed that brainiac is capable of transferring N-acetylglucosamine (GlcNAc) to beta-linked mannose (Man), with a marked preference for the disaccharide Man(beta1,4)Glc, the core of arthro-series glycolipids. The activity of brainiac toward arthro-series glycolipids was confirmed by showing that the enzyme efficiently utilized glycolipids from insects as acceptors whereas it did not with glycolipids from mammalian cells. Methylation analysis of the brainiac reaction product revealed a beta1,3 linkage between GlcNAc and Man, proving that brainiac is a beta1,3GlcNAc-transferase. Human beta1,3GlcNAc-transferases structurally related to brainiac were unable to transfer GlcNAc to Man(beta1,4)Glc-based acceptor substrates and failed to rescue a homozygous lethal brainiac allele, indicating that these proteins are paralogous and not orthologous to brainiac.