Mannosyl (alpha-1,3-)-glycoprotein beta-1,2-NAcetylglucosaminyltransferase (MGAT1)

Early structural studies on glycoproteins revealed bi-, tri-, and tetra-antennary N-glycans in which GlcNAc residues were linked to a conserved trimannosyl core, prompting the search for the GlcNAc-transferases that catalyzed the addition of each GlcNAc residue. Mannosyl (alpha-1,3-)-glycoprotein beta-1,2-N-acetylglucosaminyltransferase I (MGAT1), originally termed N-acetylglucosaminyltransferase I, abbreviated GlcNAc-TI, was the first N-glycan branching GlcNAc-transferase for which an assay was developed (Gottlieb et al. 1975; Stanley et al. 1975). MGAT1 catalyzes the transfer of GlcNAc from UDP-GlcNAc to the terminal α1,3-linked Man in Man₅GlcNAc₂Asn to initiate the synthesis of hybrid and complex N-linked glycans in multicellular organisms (reviewed in Kornfeld and Kornfeld 1985). It is not found in yeast or bacteria. The human gene MGAT1 resides on chromosome 5q35 (Kumar et al. 1992), covering 25.12kb, from 180,242,651 to 180,217,536 (NCBI 37, August 2010) on the reverse strand (Thierry-Mieg and Thierry-Mieg 2006). The mouse gene, Mgat1, is on chromosome 11 (Pownall et al. 1992). Northern blot analyses revealed two transcripts of ~2.9 and ~3.3kb present in most mammalian tissues, with the shorter transcript predominating in liver, and the longer transcript in brain (Yang et al. 1994; Yip et al. 1997). However, the human MGAT1 locus is complex with 30 introns, seven predicted alternative promoters, ten validated poly[A] addition sites >30 transcripts that encode 11 protein isoforms, with three containing the complete coding sequence (Thierry-Mieg and Thierry-Mieg 2006). The coding region is in a single exon and the Mgat1 gene is ubiquitously expressed.