Scattered Golgi Elements during Microtubule Disruption Are Initially Enriched in Trans-Golgi Proteins

We have addressed the question of whether or not Golgi fragmentation, as exemplified by that occurring during drug-induced microtubule depolymerization, is accompanied by the separation of Golgi subcompartments one from another. Scattering kinetics of Golgi subcompartments during microtubule disassembly and reassembly following reversible nocodazole exposure was inferred from multimarker analysis of protein distribution. Stably expressed α-2,6-sialyltransferase and N-acetylglucosaminyltransferase-I (NAGT-I), both C-terminally tagged with the myc epitope, provided markers for the trans-Golgi/trans-Golgi network (TGN) and medial-Golgi, respectively, in Vero cells. Using immunogold labeling, the chimeric proteins were polarized within the Golgi stack. Total cellular distributions of recombinant proteins were assessed by immunofluorescence (anti-myc monoclonal antibody) with respect to the endogenous protein, β-1,4-galactosyltransferase (GalT, trans-Golgi/TGN, polyclonal antibody). ERGIC-53 served as a marker for the intermediate compartment). In HeLa cells, distribution of endogenous GalT was compared with transfected rat α-mannosidase II (medial-Golgi, polyclonal antibody). After a 1-h nocodazole treatment, Vero α-2,6-sialyltransferase and GalT were found in scattered cytoplasmic patches that increased in number over time. Initially these structures were often negative for NAGT-I, but over a two- to threefold slower time course, NAGT-I colocalized with α-2,6-sialyltransferase and GalT. Scattered Golgi elements were located in proximity to ERGIC-53-positive structures. Similar trans-first scattering kinetics was seen with the HeLa GalT/α-mannosidase II pairing. Following nocodazole removal, all cisternal markers accumulated at the same rate in a juxtanuclear Golgi. Accumulation of cisternal proteins in scattered Golgi elements was not blocked by microinjected GTPγS at a concentration sufficient to inhibit secretory processes. Redistribution of Golgi proteins from endoplasmic reticulum to scattered structures following brefeldin A removal in the presence of nocodazole was not blocked by GTPγS. We conclude that Golgi subcompartments can separate one from the other. We discuss how direct trafficking of Golgi proteins from the TGN/trans-Golgi to endoplasmic reticulum may explain the observed trans-first scattering of Golgi transferases in response to microtubule depolymerization.     

In Vivo Synthesis of Mammalian-Like, Hybrid-Type N-Glycans in Pichia pastoris

The Pichia pastoris N-glycosylation pathway is only partially homologous to the pathway in human cells. In the Golgi apparatus, human cells synthesize complex oligosaccharides, whereas Pichia cells form mannose structures that can contain up to 40 mannose residues. This hypermannosylation of secreted glycoproteins hampers the downstream processing of heterologously expressed glycoproteins and leads to the production of protein-based therapeutic agents that are rapidly cleared from the blood because of the presence of terminal mannose residues. Here, we describe engineering of the P. pastoris N-glycosylation pathway to produce nonhyperglycosylated hybrid glycans. This was accomplished by inactivation of OCH1 and overexpression of an α-1,2-mannosidase retained in the endoplasmic reticulum and N-acetylglucosaminyltransferase I and β-1,4-galactosyltransferase retained in the Golgi apparatus. The engineered strain synthesized a nonsialylated hybrid-type N-linked oligosaccharide structure on its glycoproteins. The procedures which we developed allow glycan engineering of any P. pastoris expression strain and can yield up to 90% homogeneous protein-linked oligosaccharides.     

UDP-N-acetylglucosamine Transporter (SLC35A3) Regulates Biosynthesis of Highly Branched N-Glycans and Keratan Sulfate

SLC35A3 is considered the main UDP-N-acetylglucosamine transporter (NGT) in mammals. Detailed analysis of NGT is restricted because mammalian mutant cells defective in this activity have not been isolated. Therefore, using the siRNA approach, we developed and characterized several NGT-deficient mammalian cell lines. CHO, CHO-Lec8, and HeLa cells deficient in NGT activity displayed a decrease in the amount of highly branched tri- and tetraantennary N-glycans, whereas monoantennary and diantennary ones remained unchanged or even were accumulated. Silencing the expression of NGT in Madin-Darby canine kidney II cells resulted in a dramatic decrease in the keratan sulfate content, whereas no changes in biosynthesis of heparan sulfate were observed. We also demonstrated for the first time close proximity between NGT and mannosyl (α-1,6-)-glycoprotein β-1,6-N-acetylglucosaminyltransferase (Mgat5) in the Golgi membrane. We conclude that NGT may be important for the biosynthesis of highly branched, multiantennary complex N-glycans and keratan sulfate. We hypothesize that NGT may specifically supply β-1,3-N-acetylglucosaminyl-transferase 7 (β3GnT7), Mgat5, and possibly mannosyl (α-1,3-)-glycoprotein β-1,4-N-acetylglucosaminyltransferase (Mgat4) with UDP-GlcNAc.     

Galatosylation and sialylation of mammalian glycoproteins produced by baculovirus-madiated gene expression in insect cells

The baculovirus expression vector system (BEVS) is used extensively for the production of proteins from exogenous cDNAs. However, BEVS is not ideal for pharmaceutical production of glycoproteins owing to the properties of the N-glycans in the expressed products and that insect cells lack several of the enzymes required for mammalian-type N-glycan synthesis. This study describes the effective mammalian-like production of glycoproteins, such as β-1,4-galactosyltransferase and α-2,6-sialyltransferase, in the insect cell line Sf9.Revisions requested 13 April 2005; Revisions received 17 May 2005     

The Novel Membrane Protein TMEM59 Modulates Complex Glycosylation,Cell Surface Expression,and Secretion of the Amyloid Precursor Protein

Ectodomain shedding of the amyloid precursor protein (APP) by the two proteases α- and β-secretase is a key regulatory event in the generation of the Alzheimer disease amyloid β peptide (Aβ). At present, little is known about the cellular mechanisms that control APP shedding and Aβ generation. Here, we identified a novel protein, transmembrane protein 59 (TMEM59), as a new modulator of APP shedding. TMEM59 was found to be a ubiquitously expressed, Golgi-localized protein. TMEM59 transfection inhibited complex N- and O-glycosylation of APP in cultured cells. Additionally, TMEM59 induced APP retention in the Golgi and inhibited Aβ generation as well as APP cleavage by α- and β-secretase cleavage, which occur at the plasma membrane and in the endosomes, respectively. Moreover, TMEM59 inhibited the complex N-glycosylation of the prion protein, suggesting a more general modulation of Golgi glycosylation reactions. Importantly, TMEM59 did not affect the secretion of soluble proteins or the α-secretase like shedding of tumor necrosis factor α, demonstrating that TMEM59 did not disturb the general Golgi function. The phenotype of TMEM59 transfection on APP glycosylation and shedding was similar to the one observed in cells lacking conserved oligomeric Golgi (COG) proteins COG1 and COG2. Both proteins are required for normal localization and activity of Golgi glycosylation enzymes. In summary, this study shows that TMEM59 expression modulates complex N- and O-glycosylation and suggests that TMEM59 affects APP shedding by reducing access of APP to the cellular compartments, where it is normally cleaved by α- and β-secretase.     

Glycosyltransferase activities of Ehrlich ascites tumor cells: Detection,isolation, and characterization using oligosaccharide-Synsorb beads

Detergent extracts of Ehrlich tumor cell membranes exhibit a host of glycosyltransferase activities which have been investigated using oligosaccharides immobilized to Synsorb beads as acceptors. Glycosidase digestions in combination with methylation analysis of the insoluble products have demonstrated the presence of an α(1,3)-galactosyltransferase and a β(1,3)-N-acetylglucosaminyltransferase, enzymes that utilize N-acetyllactosamine as their acceptor substrate. The two enzymes are presumably involved in the biosynthesis of α-d-galactosyl-terminated poly-N-acetyllactosamine glycans that occur on the surface of Ehrlich cells. In addition, a β-galactosyltransferase acting on N-acetylglucosamine and a separate β-N-acetylglucosaminyltransferase that is capable of incorporating GlcNAc into the trisaccharide β-d-GlcNAc(1,3)-β-d-Gal(1,4)-β-d-Glc-Synsorb have been identified. The Ehrlich cell α- and β-galactosyltransferases have been separated by chromatography on β-GlcNAc-Synsorb beads. In the presence of MnCl₂ and UDP the β-galactosyltransferase is specifically adsorbed to the monosaccharide column whereas the α-galactosyltransferase passes through unretarded.     

Golgi Phosphoprotein 3 Triggers Signal-mediated Incorporation of Glycosyltransferases into Coatomer-coated (COPI) Vesicles

Newly synthesized membrane and secreted proteins undergo a series of posttranslational modifications in the Golgi apparatus, including attachment of carbohydrate moieties. The final structure of so-formed glycans is determined by the order of execution of the different glycosylation steps, which seems intimately related to the spatial distribution of glycosyltransferases and glycosyl hydrolases within the Golgi apparatus. How cells achieve an accurate localization of these enzymes is not completely understood but might involve dynamic processes such as coatomer-coated (COPI) vesicle-mediated trafficking. In yeast, this transport is likely to be regulated by vacuolar protein sorting 74 (Vps74p), a peripheral Golgi protein able to interact with COPI coat as well as with a binding motif present in the cytosolic tails of some mannosyltransferases. Recently, Golgi phosphoprotein 3 (GOLPH3), the mammalian homolog of Vps74, has been shown to control the Golgi localization of core 2 N-acetylglucosamine-transferase 1. Here, we highlight a role of GOLPH3 in the spatial localization of α-2,6-sialyltransferase 1. We show, for the first time, that GOLPH3 supports incorporation of both core 2 N-acetylglucosamine-transferase 1 and α-2,6-sialyltransferase 1 into COPI vesicles. Depletion of GOLPH3 altered the subcellular localization of these enzymes. In contrast, galactosyltransferase, an enzyme that does not interact with GOLPH3, was neither incorporated into COPI vesicles nor was dependent on GOLPH3 for proper localization.     

Down-regulated expression of plant-specific glycoepitopes in alfalfa

Compared with other plant expression systems used for pharmaceutical protein production, alfalfa offers the advantage of very homogeneous N -glycosylation. Therefore, this plant was selected for further attempts at glycoengineering. Two main approaches were developed in order to humanize N -glycosylation in alfalfa. The first was a knock-down of two plant-specific N -glycan maturation enzymes, β1,2-xylosyltransferase and α1,3-fucosyltransferases, using sense, antisense and RNA interference strategies. In a second approach, with the ultimate goal of rebuilding the whole human sialylation pathway, human β1,4-galactosyltransferase was expressed in alfalfa in a native form or in fusion with a targeting domain from N -acetylglucosaminyltransferase I, a glycosyltransferase located in the early Golgi apparatus in Nicotiana tabacum . Both knock-down and knock-in strategies strongly, but not completely, inhibited the biosynthesis of α1,3-fucose- and β1,2-xylose-containing glycoepitopes in transgenic alfalfa. However, recombinant human β1,4-galactosyltransferase activity in transgenic alfalfa completely prevented the accumulation of the Lewis a glycoepitope on complex N -glycans.     

Engineering of Sialylated Mucin-type O-Glycosylation in Plants

Proper N- and O-glycosylation of recombinant proteins is important for their biological function. Although the N-glycan processing pathway of different expression hosts has been successfully modified in the past, comparatively little attention has been paid to the generation of customized O-linked glycans. Plants are attractive hosts for engineering of O-glycosylation steps, as they contain no endogenous glycosyltransferases that perform mammalian-type Ser/Thr glycosylation and could interfere with the production of defined O-glycans. Here, we produced mucin-type O-GalNAc and core 1 O-linked glycan structures on recombinant human erythropoietin fused to an IgG heavy chain fragment (EPO-Fc) by transient expression in Nicotiana benthamiana plants. Furthermore, for the generation of sialylated core 1 structures constructs encoding human polypeptide:N-acetylgalactosaminyltransferase 2, Drosophila melanogaster core 1 β1,3-galactosyltransferase, human α2,3-sialyltransferase, and Mus musculus α2,6-sialyltransferase were transiently co-expressed in N. benthamiana together with EPO-Fc and the machinery for sialylation of N-glycans. The formation of significant amounts of mono- and disialylated O-linked glycans was confirmed by liquid chromatography-electrospray ionization-mass spectrometry. Analysis of the three EPO glycopeptides carrying N-glycans revealed the presence of biantennary structures with terminal sialic acid residues. Our data demonstrate that N. benthamiana plants are amenable to engineering of the O-glycosylation pathway and can produce well defined human-type O- and N-linked glycans on recombinant therapeutics.     

Expression of glycoproteins bearing complex human-like glycans with galactose terminal in Hansenula polymorpha

Glycoproteins derived from Hansenula polymorpha can not be used for therapeutic purposes due to their high-mannose type asparagine-linked (N-linked) glycans, which result in immune reactions and poor pharmacokinetic behaviors in human body. Previously, we reported that the trimannosyl core N-linked glycans (Man₃GlcNAc₂) intermediate can be generated in endoplasmic reticulum in HpALG3 and HpALG11 double-mutant H. polymorpha. Here, we describe the further modification of the glycosylation pathway in this double-defect strain to express glycoproteins with complex human-like glycans. After eliminating the impact of HpOCH1, three glycosyltransferases were introduced into this triple-mutant strain. When human β-1,2-N-acetylglucosaminyltransferase I (hGnTI) was efficiently targeted in early Golgi, more than 95 % glycans attached to the glycoproteins were added one N-acetylglucosamine (GlcNAc). With subsequently introduction of rat β-1,2-N-acetylglucosaminyltransferase II (rGnTII) and human β-1,4-galactosyltransferase I (hGalTI), several glycoengineered strains can produce glycoproteins bearing glycans with terminal N-acetylglucosamine or galactose. The expression of glycoproteins with glycan Gal₂GlcNAc₂Man₃GlcNAc₂ represents a significant step toward the ability to express fully humanized glycoproteins in H. polymorpha. Furthermore, several shake-flask and bioreactor fermentation experiments indicated that, although the cells do display a reduction in growth rate, the glycoengineered strains are still suitable for high-density fermentation.     

P1 Trisaccharide (Galα1,4Galβ1,4GlcNAc) Synthesis by Enzyme Glycosylation Reactions Using Recombinant Escherichia coli

The frequency of Escherichia coli infection has lead to concerns over pathogenic bacteria in our food supply and a demand for therapeutics. Glycolipids on gut cells serve as receptors for the Shiga-like toxin produced by E. coli. Oligosaccharide moiety analogues of these glycolipids can compete with receptors for the toxin, thus acting as antibacterials. An enzymatic synthesis of the P1 trisaccharide (Galα1,4Galβ1,4GlcNAc), one of the oligosaccharide analogues, was assessed in this study. In the proposed synthetic pathway, UDP-glucose was generated from sucrose with an Anabaena sp. sucrose synthase and then converted with an E. coli UDP-glucose 4-epimerase to UDP-galactose. Two molecules of galactose were linked to N-acetylglucosamine subsequently with a Helicobacter pylori β-l,4-galactosyltransferase and a Neisseria meningitidis α-1,4-galactosyltransferase to produce one molecule of P1 trisaccharide. The four enzymes were coexpressed in a single genetically engineered E. coli strain that was then permeabilized and used to catalyze the enzymatic reaction. P1 trisaccharide was accumulated up to 50 mM (5.4 g in a 200-ml reaction volume), with a 67% yield based on the consumption of N-acetylglucosamine. This study provides an efficient approach for the preparative-scale synthesis of P1 trisaccharide with recombinant bacteria.     

Metabolic Mechanism of Mannan in a Ruminal Bacterium,Ruminococcus albus,Involving Two Mannoside Phosphorylases and Cellobiose 2-Epimerase: DISCOVERY OF A NEW CARBOHYDRATE PHOSPHORYLASE, β-1,4-MANNOOLIGOSACCHARIDE PHOSPHORYLASE*

Ruminococcus albus is a typical ruminal bacterium digesting cellulose and hemicellulose. Cellobiose 2-epimerase (CE; EC 5.1.3.11), which converts cellobiose to 4-O-β-d-glucosyl-d-mannose, is a particularly unique enzyme in R. albus, but its physiological function is unclear. Recently, a new metabolic pathway of mannan involving CE was postulated for another CE-producing bacterium, Bacteroides fragilis. In this pathway, β-1,4-mannobiose is epimerized to 4-O-β-d-mannosyl-d-glucose (Man-Glc) by CE, and Man-Glc is phosphorolyzed to α-d-mannosyl 1-phosphate (Man1P) and d-glucose by Man-Glc phosphorylase (MP; EC 2.4.1.281). Ruminococcus albus NE1 showed intracellular MP activity, and two MP isozymes, RaMP1 and RaMP2, were obtained from the cell-free extract. These enzymes were highly specific for the mannosyl residue at the non-reducing end of the substrate and catalyzed the phosphorolysis and synthesis of Man-Glc through a sequential Bi Bi mechanism. In a synthetic reaction, RaMP1 showed high activity only toward d-glucose and 6-deoxy-d-glucose in the presence of Man1P, whereas RaMP2 showed acceptor specificity significantly different from RaMP1. RaMP2 acted on d-glucose derivatives at the C2- and C3-positions, including deoxy- and deoxyfluoro-analogues and epimers, but not on those substituted at the C6-position. Furthermore, RaMP2 had high synthetic activity toward the following oligosaccharides: β-linked glucobioses, maltose, N,N′-diacetylchitobiose, and β-1,4-mannooligosaccharides. Particularly, β-1,4-mannooligosaccharides served as significantly better acceptor substrates for RaMP2 than d-glucose. In the phosphorolytic reactions, RaMP2 had weak activity toward β-1,4-mannobiose but efficiently degraded β-1,4-mannooligosaccharides longer than β-1,4-mannobiose. Consequently, RaMP2 is thought to catalyze the phosphorolysis of β-1,4-mannooligosaccharides longer than β-1,4-mannobiose to produce Man1P and β-1,4-mannobiose.     

Characterization of Five β-Glycoside Hydrolases from Cellulomonas fimi ATCC 484

The Gram-positive bacterium Cellulomonas fimi produces a large array of carbohydrate-active enzymes. Analysis of the collection of carbohydrate-active enzymes from the recent genome sequence of C. fimi ATCC 484 shows a large number of uncharacterized genes for glycoside hydrolase (GH) enzymes potentially involved in biomass utilization. To investigate the enzymatic activity of potential β-glucosidases in C. fimi, genes encoding several GH3 enzymes and one GH1 enzyme were cloned and recombinant proteins were expressed in Escherichia coli. Biochemical analysis of these proteins revealed that the enzymes exhibited different substrate specificities for para-nitrophenol-linked substrates (pNP), disaccharides, and oligosaccharides. Celf_2726 encoded a bifunctional enzyme with β-d-xylopyranosidase and α-l-arabinofuranosidase activities, based on pNP-linked substrates (CfXyl3A). Celf_0140 encoded a β-d-glucosidase with activity on β-1,3- and β-1,6-linked glucosyl disaccharides as well as pNP-β-Glc (CfBgl3A). Celf_0468 encoded a β-d-glucosidase with hydrolysis of pNP-β-Glc and hydrolysis/transglycosylation activities only on β-1,6-linked glucosyl disaccharide (CfBgl3B). Celf_3372 encoded a GH3 family member with broad aryl-β-d-glycosidase substrate specificity. Celf_2783 encoded the GH1 family member (CfBgl1), which was found to hydrolyze pNP-β-Glc/Fuc/Gal, as well as cellotetraose and cellopentaose. CfBgl1 also had good activity on β-1,2- and β-1,3-linked disaccharides but had only very weak activity on β-1,4/6-linked glucose.     

The GH130 Family of Mannoside Phosphorylases Contains Glycoside Hydrolases That Target β-1,2-Mannosidic Linkages in Candida Mannan

The depolymerization of complex glycans is an important biological process that is of considerable interest to environmentally relevant industries. β-Mannose is a major component of plant structural polysaccharides and eukaryotic N-glycans. These linkages are primarily cleaved by glycoside hydrolases, although recently, a family of glycoside phosphorylases, GH130, have also been shown to target β-1,2- and β-1,4-mannosidic linkages. In these phosphorylases, bond cleavage was mediated by a single displacement reaction in which phosphate functions as the catalytic nucleophile. A cohort of GH130 enzymes, however, lack the conserved basic residues that bind the phosphate nucleophile, and it was proposed that these enzymes function as glycoside hydrolases. Here we show that two Bacteroides enzymes, BT3780 and BACOVA_03624, which lack the phosphate binding residues, are indeed β-mannosidases that hydrolyze β-1,2-mannosidic linkages through an inverting mechanism. Because the genes encoding these enzymes are located in genetic loci that orchestrate the depolymerization of yeast α-mannans, it is likely that the two enzymes target the β-1,2-mannose residues that cap the glycan produced by Candida albicans. The crystal structure of BT3780 in complex with mannose bound in the −1 and +1 subsites showed that a pair of glutamates, Glu²²⁷ and Glu²⁶⁸, hydrogen bond to O₁ of α-mannose, and either of these residues may function as the catalytic base. The candidate catalytic acid and the other residues that interact with the active site mannose are conserved in both GH130 mannoside phosphorylases and β-1,2-mannosidases. Functional phylogeny identified a conserved lysine, Lys¹⁹⁹ in BT3780, as a key specificity determinant for β-1,2-mannosidic linkages.     

Discovery of Two β-1,2-Mannoside Phosphorylases Showing Different Chain-Length Specificities from Thermoanaerobacter sp. X-514

We characterized Teth514_1788 and Teth514_1789, belonging to glycoside hydrolase family 130, from Thermoanaerobacter sp. X-514. These two enzymes catalyzed the synthesis of 1,2-β-oligomannan using β-1,2-mannobiose and d-mannose as the optimal acceptors, respectively, in the presence of the donor α-d-mannose 1-phosphate. Kinetic analysis of the phosphorolytic reaction toward 1,2-β-oligomannan revealed that these enzymes followed a typical sequential Bi Bi mechanism. The kinetic parameters of the phosphorolysis of 1,2-β-oligomannan indicate that Teth514_1788 and Teth514_1789 prefer 1,2-β-oligomannans containing a DP ≥3 and β-1,2-Man₂, respectively. These results indicate that the two enzymes are novel inverting phosphorylases that exhibit distinct chain-length specificities toward 1,2-β-oligomannan. Here, we propose 1,2-β-oligomannan:phosphate α-d-mannosyltransferase as the systematic name and 1,2-β-oligomannan phosphorylase as the short name for Teth514_1788 and β-1,2-mannobiose:phosphate α-d-mannosyltransferase as the systematic name and β-1,2-mannobiose phosphorylase as the short name for Teth514_1789.     

Identification of difructose dianhydride I synthase/hydrolase from an oral bacterium establishes a novel glycoside hydrolase family

Fructooligosaccharides and their anhydrides are widely used as health-promoting foods and prebiotics. Various enzymes acting on β-D-fructofuranosyl linkages of natural fructan polymers have been used to produce functional compounds. However, enzymes that hydrolyze and form α-D-fructofuranosyl linkages have been less studied. Here, we identified the BBDE_2040 gene product from Bifidobacterium dentium (α-D-fructofuranosidase and difructose dianhydride I synthase/hydrolase from Bifidobacterium dentium [αFFase1]) as an enzyme with α-D-fructofuranosidase and α-D-arabinofuranosidase activities and an anomer-retaining manner. αFFase1 is not homologous with any known enzymes, suggesting that it is a member of a novel glycoside hydrolase family. When caramelized fructose sugar was incubated with αFFase1, conversions of β-D-Frup-(2→1)-α-D-Fruf to α-D-Fruf-1,2′:2,1′-β-D-Frup (diheterolevulosan II) and β-D-Fruf-(2→1)-α-D-Fruf (inulobiose) to α-D-Fruf-1,2′:2,1′-β-D-Fruf (difructose dianhydride I [DFA I]) were observed. The reaction equilibrium between inulobiose and DFA I was biased toward the latter (1:9) to promote the intramolecular dehydrating condensation reaction. Thus, we named this enzyme DFA I synthase/hydrolase. The crystal structures of αFFase1 in complex with β-D-Fruf and β-D-Araf were determined at the resolutions of up to 1.76 Å. Modeling of a DFA I molecule in the active site and mutational analysis also identified critical residues for catalysis and substrate binding. The hexameric structure of αFFase1 revealed the connection of the catalytic pocket to a large internal cavity via a channel. Molecular dynamics analysis implied stable binding of DFA I and inulobiose to the active site with surrounding water molecules. Taken together, these results establish DFA I synthase/hydrolase as a member of a new glycoside hydrolase family (GH172).     

Discovery of β-1,4-d-Mannosyl-N-acetyl-d-glucosamine Phosphorylase Involved in the Metabolism of N-Glycans

A gene cluster involved in N-glycan metabolism was identified in the genome of Bacteroides thetaiotaomicron VPI-5482. This gene cluster encodes a major facilitator superfamily transporter, a starch utilization system-like transporter consisting of a TonB-dependent oligosaccharide transporter and an outer membrane lipoprotein, four glycoside hydrolases (α-mannosidase, β-N-acetylhexosaminidase, exo-α-sialidase, and endo-β-N-acetylglucosaminidase), and a phosphorylase (BT1033) with unknown function. It was demonstrated that BT1033 catalyzed the reversible phosphorolysis of β-1,4-d-mannosyl-N-acetyl-d-glucosamine in a typical sequential Bi Bi mechanism. These results indicate that BT1033 plays a crucial role as a key enzyme in the N-glycan catabolism where β-1,4-d-mannosyl-N-acetyl-d-glucosamine is liberated from N-glycans by sequential glycoside hydrolase-catalyzed reactions, transported into the cell, and intracellularly converted into α-d-mannose 1-phosphate and N-acetyl-d-glucosamine. In addition, intestinal anaerobic bacteria such as Bacteroides fragilis, Bacteroides helcogenes, Bacteroides salanitronis, Bacteroides vulgatus, Prevotella denticola, Prevotella dentalis, Prevotella melaninogenica, Parabacteroides distasonis, and Alistipes finegoldii were also suggested to possess the similar metabolic pathway for N-glycans. A notable feature of the new metabolic pathway for N-glycans is the more efficient use of ATP-stored energy, in comparison with the conventional pathway where β-mannosidase and ATP-dependent hexokinase participate, because it is possible to directly phosphorylate the d-mannose residue of β-1,4-d-mannosyl-N-acetyl-d-glucosamine to enter glycolysis. This is the first report of a metabolic pathway for N-glycans that includes a phosphorylase. We propose 4-O-β-d-mannopyranosyl-N-acetyl-d-glucosamine:phosphate α-d-mannosyltransferase as the systematic name and β-1,4-d-mannosyl-N-acetyl-d-glucosamine phosphorylase as the short name for BT1033.     

Limited Addition of the 6-Arm β1,2-linked N-Acetylglucosamine (GlcNAc) Residue Facilitates the Formation of the Largest N-Glycan in Plants

The most abundant N-glycan in plants is the paucimannosidic N-glycan with core β1,2-xylose and α1,3-fucose residues (Man₃XylFuc(GlcNAc)₂). Here, we report a mechanism in Arabidopsis thaliana that efficiently produces the largest N-glycan in plants. Genetic and biochemical evidence indicates that the addition of the 6-arm β1,2-GlcNAc residue by N-acetylglucosaminyltransferase II (GnTII) is less effective than additions of the core β1,2-xylose and α1,3-fucose residues by XylT, FucTA, and FucTB in Arabidopsis. Furthermore, analysis of gnt2 mutant and 35S:GnTII transgenic plants shows that the addition of the 6-arm non-reducing GlcNAc residue to the common N-glycan acceptor GlcNAcMan₃(GlcNAc)₂ inhibits additions of the core β1,2-xylose and α1,3-fucose residues. Our findings indicate that plants limit the rate of the addition of the 6-arm GlcNAc residue to the common N-glycan acceptor as a mechanism to facilitate formation of the prevalent N-glycans with Man₃XylFuc(GlcNAc)₂ and (GlcNAc)₂Man₃XylFuc(GlcNAc)₂ structures.     

A Gene of the β3-Glycosyltransferase Family Encodes N-Acetylglucosaminyltransferase II Function in Trypanosoma brucei

The bloodstream form of the human pathogen Trypanosoma brucei expresses oligomannose, paucimannose, and complex N-linked glycans, including some exceptionally large poly-N-acetyllactosamine-containing structures. Despite the presence of complex N-glycans in this organism, no homologues of the canonical N-acetylglucosaminyltransferase I or II genes can be found in the T. brucei genome. These genes encode the activities that initiate the elaboration of the Manα1–3 and Manα1–6 arms, respectively, of the conserved trimannosyl-N-acetylchitobiosyl core of N-linked glycans. Previously, we identified a highly divergent T. brucei N-acetylglucosaminyltransferase I (TbGnTI) among a set of putative T. brucei glycosyltransferase genes belonging to the β3-glycosyltransferase superfamily (Damerow, M., Rodrigues, J. A., Wu, D., Güther, M. L., Mehlert, A., and Ferguson, M. A. (2014) J. Biol. Chem. 289, 9328–9339). Here, we demonstrate that TbGT15, another member of the same β3-glycosyltransferase family, encodes an equally divergent N-acetylglucosaminyltransferase II (TbGnTII) activity. In contrast to multicellular organisms, where GnTII activity is essential, TbGnTII null mutants of T. brucei grow in culture and are still infectious to animals. Characterization of the large poly-N-acetyllactosamine containing N-glycans of the TbGnTII null mutants by methylation linkage analysis suggests that, in wild-type parasites, the Manα1–6 arm of the conserved trimannosyl core may carry predominantly linear poly-N-acetyllactosamine chains, whereas the Manα1–3 arm may carry predominantly branched poly-N-acetyllactosamine chains. These results provide further detail on the structure and biosynthesis of complex N-glycans in an important human pathogen and provide a second example of the adaptation by trypanosomes of β3-glycosyltransferase family members to catalyze β1–2 glycosidic linkages.     

Enantioselective Dehydrochlorination of δ-Hexachlorocyclohexane and δ-Pentachlorocyclohexene by LinA1 and LinA2 from Sphingobium indicum B90A

δ-Hexachlorocyclohexane (δ-HCH), one of the prevalent isomers of technical HCH, was enantioselectively dehydrochlorinated by the dehydrochlorinases LinA1 and LinA2 from Sphingobium indicum B90A to the very same δ-pentachlorocyclohexene enantiomer. Racemic δ-pentachlorocyclohexene, however, was transformed with opposite enantioselectivities by the two enzymes. A transformation pathway based on an anti-1,2-elimination, followed by a syn-1,4-elimination and a subsequent syn-1,2-elimination is postulated.