S2.7 Molecular cloning and expression of cDNA encoding rat UDP-N-acetylglucosamine:?-6-d-mannoside?-1,2-N-acetylglucosaminyltransferase II

Synthesis of pentasaccharide analogues of the N-glycan substrates of N-acetylglucosaminyltransferases III, IV and V using tetrasaccharide precursors and recombinant beta-(1-->2)-N-acetylglucosaminyltransferase II

Recombinant human UDP-GlcNAc: alpha-Man-(1-->6)R beta-(1-->2)-N-acetylglucosaminyltransferase II (EC 2.4.1.143, GlcNAc-T II) was produced in the Sf9 insect cell/baculovirus expression system as a fusion protein with a (His)6 tag and partially purified by affinity chromatography on a metal chelating column. The partially purified enzyme was used to catalyze the transfer of GlcNAc from UDP-GlcNAc to R-alpha-Man(1-->6)(beta-GlcNAc(1-->2)alpha-Man(1-->3))beta-Man-O-octyl to form beta-GlcNAc(1-->2)R-alpha-Man(1-->6)(beta-GlcNAc(1-->2)alpha- Man(1-->3))beta-Man-O-octyl where there is either no modification of the alpha-Man(1-->6) residue (7), or where R is 3-deoxy (8), 4-deoxy (9) or 6-deoxy (10). The yields ranged from 64-80%. Products were characterized by 1H and 13C nuclear magnetic resonance spectroscopy and fast atom bombardment mass spectrometry. Compounds 7-10 are pentasaccharide analogues of the biantennary N-glycan substrates of N-acetylglucosaminyltransferases III, IV and V.

In the biosynthesis of N-glycans in connective tissue of the snail Lymnaea stagnalis of incorporation GlcNAc by beta 2GlcNAc-transferase I is an essential prerequisite for the action of beta 2GlcNAc-transferase II and beta 2Xyl-transferase

Using a series of relevant substrates, connective tissue of the snail Lymnaea stagnalis was shown to contain beta 1-2 xylosyltransferase (beta 2Xyl-T), beta 1-2 N-acetylglucosaminyltransferase I (beta 2GlcNAc-T I), and beta 1-2 N-acetylglucosaminyltransferase II (beta 2GlcNAc-T II) activities. These enzymes are probably involved in the biosynthesis of the N-linked carbohydrate chains, like those present in hemocyanin. The products formed by incubation of GlcNAc beta 1-2Man alpha 1-6(GlcNAc beta 1-2Man alpha 1-3)Man beta 1-R [where R = -4GlcNAc beta 1-4GlcNAc or O-(CH2)7CH3] with UDP-Xyl and connective tissue microsomes have been purified and characterized by 1H-NMR spectroscopy in conjunction with methylation analysis to be GlcNAc beta 1-2Man alpha 1-6(GlcNAc beta 1-2Man alpha 1-3)(Xyl beta 1-2)Man beta 1-R. Substrate specificity studies focused on connective tissue beta 2Xyl-T show that the minimal structure requirements are fulfilled in GlcNAc beta 1-2Man alpha 1-3Man beta 1-O-(CH2)7CH3. The enzyme activity can therefore be characterized as UDP-Xyl:Glc-NAc beta 1-2Man alpha 1-3Man beta-R (Xyl to Man beta) beta 1-2 xylosyltransferase. In substrate-specificity studies directed to connective tissue beta 2GlcNAc-T I, it could be demonstrated that the enzyme is active towards acceptors having at the minimum a Man alpha 1-3Man beta-R sequence, and that introduction of a beta Xyl residue at C2 of beta Man totally abolishes the enzyme activity. Xylose-containing oligosaccharides are not acceptors for beta 2GlcNAc-T I. In combination with the substrate specificity of beta Xyl-T, this shows that in snail connective tissue beta 2GlcNAc-T I must act before beta 2Xyl-T. The connective tissue beta 2GlcNAc-T II activity follows the earlier established biosynthetic routes. Based on the substrate specificities of the various connective tissue glycosyltransferases known so far, and the structures isolated from L. stagnalis hemocyanin, a partial biosynthetic scheme for N-glycosylation in snail connective tissue is proposed.

The human UDP-N-acetylglucosamine: alpha-6-D-mannoside-beta-1,2- N-acetylglucosaminyltransferase II gene (MGAT2). Cloning of genomic DNA, localization to chromosome 14q21, expression in insect cells and purification of the recombinant protein

UDP-GlcNAc:alpha-6-D-mannoside [GlcNAc to Man alpha 1-6] beta-1,2-N-acetylglucosaminyltransferase II (GlcNAc-T II, EC 2.4.1.143) is a Golgi enzyme catalyzing an essential step in the conversion of oligomannose to complex N-glycans. A 1.2-kb probe from a rat liver cDNA encoding GlcNAc-T II was used to screen a human genomic DNA library in lambda EMBL3. Southern analysis of restriction endonuclease digests of positive phage clones identified two hybridizing fragments (3.0 and 3.5 kb) which were subcloned into pBlueScript. The inserts of the resulting plasmids (pHG30 and pHG36) are over-lapping clones containing 5.5 kb of genomic DNA. The pHG30 insert (3.0 kb) contains a 1341-bp open reading frame encoding a 447-amino-acid protein, 250 bp of G + C-rich 5'-upstream sequence and 1.4 kb of 3'-downstream sequence. The pHG36 insert (3.5 kb) contains 2.75 kb of 5'-upstream sequence and 750 bp of the 5'-end of the open reading frame. The protein sequence showed the domain structure typical of all previously cloned glycosyltransferases, i.e. a short 9-residue putative cytoplasmic N-terminal domain, a 20-residue hydrophobic non-cleavable putative signal-anchor domain and a 418-residue C-terminal catalytic domain. Northern analysis of human tissues showed a major message at 3 kb and minor signals at 2 and 4.5 kb. There is no sequence similarity to any previously cloned glycosyltransferases including human UDP-GlcNAc:alpha-3-D-mannoside [GlcNAc to Man alpha 1-3] beta-1,2-N-acetylglucosaminyltransferase I (GlcNAc-T I) which has 445 amino acids with a 418-residue C-terminal catalytic domain. The human GlcNAc-T I and II genes (MGAT1 and MGAT2) map to chromosome bands 5q35 and 14q21, respectively, by fluorescence in situ hybridization. The entire coding regions of human GlcNAc-T I and II are each on a single exon. There is 92% identity between the amino acid sequences of the catalytic domains of human and rat GlcNAc-T II. Southern analysis of restriction enzyme digests of human genomic DNA indicates that there is only a single copy of the MGAT2 gene. The full-length coding region of GlcNAc-T II has been expressed in the baculovirus/Sf9 insect cell system, the recombinant enzyme has been purified to near homogeneity with a specific activity of about 20 mumol.min-1.mg-1 and the product synthesized by the recombinant enzyme has been identified by high-resolution 1H-NMR spectroscopy and mass spectrometry.