Characterization of mucin‐type core‐1 β1‐3 galactosyltransferase homologous enzymes in Drosophila melanogaster

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Abstract
Mucin type O‐glycosylation is a widespread modification of eukaryotic proteins. The transfer of N‐acetylgalactosamine to selected serine or threonine residues is catalyzed by a family of polypeptide N‐acetylgalactosaminyltransferases localized in the Golgi apparatus. The most abundant elongation of O‐glycans is the addition of a β1‐3 linked galactose by the core‐1 β1‐3 galactosyltransferase (core‐1 β3GalT), thereby building the T‐antigen or core‐1 structure Gal(β1‐3)GalNAc(α1‐O). We have isolated four Drosophila melanogaster cDNAs encoding proteins structurally similar to the human core‐1 β3GalT enzyme and expressed them as FLAG‐tagged proteins in Sf9 insect cells. The identity of these D. melanogasterβ3GalT enzymes with a core‐1 β3GalT activity was confirmed by utilization of MUC5AC mucin derived O‐glycopeptide acceptors. In addition to the core‐1 β3GalT activity toward O‐glycoprotein substrates, one member of this enzyme family showed a strong activity towards glycolipid acceptors, thereby building the core‐1 terminated Nz6 glycosphingolipid. Transcripts of the embryonically expressed core‐1 β3GalTs were found in the maternally deposited mRNA, in salivary glands and in the amnioserosa. The presence of multiple core‐1 β3GalT genes in D. melanogaster suggests an increased complexity of core‐1 O‐glycan expression, which is possibly related to multiple developmental and physiological functions attributable to this class of glycans.