Null Mutations in Drosophila N-Acetylglucosaminyltransferase I Produce Defects in Locomotion and a Reduced Life Span

UDP-GlcNAc:α3-d-mannoside β1,2-N-acetylglucosaminyltransferase I (encoded by Mgat1) controls the synthesis of hybrid, complex, and paucimannose N-glycans. Mice make hybrid and complex N-glycans but little or no paucimannose N-glycans. In contrast, Drosophila melanogaster and Caenorhabditis elegans make paucimannose N-glycans but little or no hybrid or complex N-glycans. To determine the functional requirement for β1,2-N-acetylglucosaminyltransferase I in Drosophila, we generated null mutations by imprecise excision of a nearby transposable element. Extracts from Mgat11/Mgat11 null mutants showed no β1,2-N-acetylglucosaminyltransferase I enzyme activity. Moreover, mass spectrometric analysis of these extracts showed dramatic changes in N-glycans compatible with lack of β1,2-N-acetylglucosaminyltransferase I activity. Interestingly, Mgat11/Mgat11 null mutants are viable but exhibit pronounced defects in adult locomotory activity when compared with Mgat11/CyO-GFP heterozygotes or wild type flies. In addition, in null mutants males are sterile and have a severely reduced mean and maximum life span. Microscopic examination of mutant adult fly brains showed the presence of fused β lobes. The removal of both maternal and zygotic Mgat1 also gave rise to embryos that no longer express the horseradish peroxidase antigen within the central nervous system. Taken together, the data indicate that β1,2-N-acetylglucosaminyltransferase I-dependent N-glycans are required for locomotory activity, life span, and brain development in Drosophila. UDP-GlcNAc:α3-d-mannoside β1,2-N-acetylglucosaminyltransferase I (encoded by Mgat1) controls the synthesis of hybrid, complex, and paucimannose N-glycans. Mice make hybrid and complex N-glycans but little or no paucimannose N-glycans. In contrast, Drosophila melanogaster and Caenorhabditis elegans make paucimannose N-glycans but little or no hybrid or complex N-glycans. To determine the functional requirement for β1,2-N-acetylglucosaminyltransferase I in Drosophila, we generated null mutations by imprecise excision of a nearby transposable element. Extracts from Mgat11/Mgat11 null mutants showed no β1,2-N-acetylglucosaminyltransferase I enzyme activity. Moreover, mass spectrometric analysis of these extracts showed dramatic changes in N-glycans compatible with lack of β1,2-N-acetylglucosaminyltransferase I activity. Interestingly, Mgat11/Mgat11 null mutants are viable but exhibit pronounced defects in adult locomotory activity when compared with Mgat11/CyO-GFP heterozygotes or wild type flies. In addition, in null mutants males are sterile and have a severely reduced mean and maximum life span. Microscopic examination of mutant adult fly brains showed the presence of fused β lobes. The removal of both maternal and zygotic Mgat1 also gave rise to embryos that no longer express the horseradish peroxidase antigen within the central nervous system. Taken together, the data indicate that β1,2-N-acetylglucosaminyltransferase I-dependent N-glycans are required for locomotory activity, life span, and brain development in Drosophila. According to recent genome project estimates, the human and fruit fly genomes contain about 24,000 and 14,000 genes, respectively. However, the number of functionally discrete proteins encoded by either of these genomes is probably increased by at least 1 order of magnitude because of post-translational modifications (PTM) 3The abbreviations used are: PTM, post-translational modification; CDG, congenital disorder of glycosylation; FucT, fucosyltransferase; GlcNAcT, N-acetylglucosaminyltransferase; HRP, horseradish peroxidase; MALDI-TOF-MS, matrix-assisted laser desorption/ionization-time of flight mass spectrometry; MS, mass spectrometry; PNGase, protein N-glycanase; GFP, green fluorescent protein; CHAPS, 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonic acid; MES, 4-morpholineethanesulfonic acid; PBS, phosphate-buffered saline; Gnase, β-N-acetylglucosaminidase. and processes such as gene splicing. PTM has been implicated in many important processes, e.g. signal transduction cascades, growth, transformation, and memory formation (1Parekh R.B. Rohlff C. Curr. Opin. Biotechnol. 1997; 8: 718-723Crossref PubMed Scopus (53) Google Scholar, 2Routtenberg A. Rekart J.L. Trends Neurosci. 2005; 28: 12-19Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar). Glycosylation is one of the most common PTM. Protein-bound glycans are often branched and are composed of several different monomeric sugar components connected by one of six to eight different linkages. This type of structure confers on glycans the ability to carry a great deal of information in very compact structures and thereby to mediate many different functions (3Laine R.A. Glycobiology. 1994; 4: 759-767Crossref PubMed Scopus (336) Google Scholar). Our laboratory is interested in the synthesis and function of glycans conjugated to proteins by β-linkage of GlcNAc to the amido group of Asn (N-glycans). The first phase of N-glycan synthesis involves the assembly of a lipid-linked precursor Glc3Man9GlcNAc2-pyrophosphate-dolichol and the oligosaccharyltransferase-catalyzed transfer of the Glc3Man9GlcNAc2-moiety to an Asn residue within an Asn-X-(Ser/Thr) sequon (4Burda P. Aebi M. Biochim. Biophys. Acta. 1999; 1426: 239-257Crossref PubMed Scopus (529) Google Scholar). The second phase involves the processing, within the lumen of the endoplasmic reticulum and Golgi apparatus, of Asn-linked Glc3Man9GlcNAc2 to Man5GlcNAc2 (5Herscovics A. Biochim. Biophys. Acta. 1999; 1473: 96-107Crossref PubMed Scopus (240) Google Scholar, 6Moremen K.W. Biochim. Biophys. Acta. 2002; 1573: 225-235Crossref PubMed Scopus (95) Google Scholar) (Figs. 1 and 2A). The final phase of the pathway (7Schachter H. Glycobiology. 1991; 1: 453-461Crossref PubMed Scopus (169) Google Scholar) occurs in the Golgi apparatus and involves the conversion of Man5GlcNAc2-Asn to hybrid, paucimannose, and complex N-glycans (Fig. 2A). UDP-GlcNAc:α3-d-mannoside β1,2-N-acetylglucosaminyltransferase I (GlcNAcTI, encoded by Mgat1) converts Man5GlcNAc2-Asn to the hybrid N-glycan GlcNAcMan5GlcNAc2-Asn. This is followed by the action of α3,6-mannosidase II to form the hybrid N-glycans GlcNAcMan4GlcNAc2-Asn and GlcNAcMan3GlcNAc2-Asn (Fig. 2A). In vertebrates, GlcNAcMan3GlcNAc2-Asn is converted to complex N-glycans by the action of UDP-GlcNAc:α6-d-mannoside β1,2-N-acetylglucosaminyltransferase II (GlcNAcTII) and other branching GlcNAcTs (7Schachter H. Glycobiology. 1991; 1: 453-461Crossref PubMed Scopus (169) Google Scholar). Further action by other glycosyltransferases (galactosyl-, sialyl-, and fucosyltransferases) on the distal nonreducing ends of the glycan creates a large variety of complex N-glycans.FIGURE 2A, N-glycan synthesis in wild type Drosophila. This scheme is based on the data in Table 1 and on Paschinger et al. (44Paschinger Glycobiology. 2005; PubMed Scopus Google Scholar). The of the N-glycans are in are in by have been by in and are based on other (44Paschinger Glycobiology. 2005; PubMed Scopus Google Scholar). with indicate that The the conversion of N-glycans to hybrid paucimannose N-glycans. The structure in wild type is with a A. Full Text Full Text PDF PubMed Scopus Google Scholar, Glycobiology. 2005; PubMed Scopus Google other structures in large of are with The structures are in of structures have been by but are in the on the of other (44Paschinger Glycobiology. 2005; PubMed Scopus Google Scholar). GlcNAc in to the of to form the hybrid N-glycan M. H. PubMed Scopus Google Scholar, H. PubMed Scopus Google Scholar). are from by the action of α3,6-mannosidase II to form the hybrid N-glycans and K.W. Biochim. Biophys. Acta. 2002; 1573: 225-235Crossref PubMed Scopus (95) Google Scholar). in H. Full Text Full Text PDF PubMed Scopus Google Scholar, C. 2005; Full Text Full Text PDF PubMed Scopus Google the GlcNAc by to form and paucimannose N-glycans. Biophys. PubMed Scopus Google Scholar) on to the synthesis of complex is a pathway in and C. elegans because for The and of the and are (44Paschinger Glycobiology. 2005; PubMed Scopus Google Scholar). (44Paschinger Glycobiology. 2005; PubMed Scopus Google Scholar) and A. Paschinger Full Text Full Text PDF PubMed Scopus Google Scholar) in Drosophila are on on structures with a and to make the of in wild type Drosophila A. 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