Control of glycoprotein synthesis. Bovine colostrum UDP-N-acetylglucosamine:alpha-D-mannoside beta 2-N-acetylglucosaminyltransferase I. Separation from UDP-N-acetylglucosamine:alpha-D-mannoside beta 2-N-acetylglucosaminyltransferase II, partial purification, and substrate specificity

Determination, expression and characterization of an UDP-N-acetylglucosamine:α-1,3-D-mannoside β-1,2-N-acetylglucosaminyltransferase I (GnT-I) from the Pacific oyster, Crassostrea gigas

Molluscs are intermediate hosts for several parasites. The recognition processes, required to evade the host's immune response, depend on carbohydrates. Therefore, the investigation of mollusc glycosylation capacities is of high relevance to understand the interaction of parasites with their host. UDP-N-acetylglucosamine:α-1,3-D-mannoside β-1,2-N-acetylglucosaminyltransferase I (GnT-I) is the key enzyme for the biosynthesis of hybrid and complex type N-glycans catalysing the transfer of N-acetylglucosamine from UDP-N-acetylglucosamine to the α-1,3 Man antenna of Man5GlcNAc2. Thereby, the enzyme produces a suitable substrate for further enzymes, such as α-mannosidase II, GlcNAc-transferase II, galactosyltransferases or fucosyltransferases. The sequence of GnT- I from the Pacific oyster, Crassostrea gigas, was obtained by homology search using the corresponding human enzyme as the template. The obtained gene codes for a 445 amino acids long type II transmembrane glycoprotein and shared typical structural elements with enzymes from other species. The enzyme was expressed in insect cells and purified by immunoprecipitation using protein A/G-plus agarose beads linked to monoclonal His-tag antibodies. GnT-I activity was determined towards the substrates Man5-PA, MM-PA and GnM-PA. The enzyme displayed highest activity at pH 7.0 and 30 °C, using Man5-PA as the substrate. Divalent cations were indispensable for the enzyme, with highest activity at 40 mM Mn2+, while the addition of EDTA or Cu2+ abolished the activity completely. The activity was also reduced by the addition of UDP, UTP or galactose. In this study we present the identification, expression and biochemical characterization of the first molluscan UDP-N-acetylglucosamine:α-1,3-D-mannoside β-1,2-N-acetylglucosaminyltransferase I, GnT-I, from the Pacific oyster Crassostrea gigas.

N-linked glycans: an underappreciated key determinant of T cell development, activation, and function

N-linked glycosylation is a post-translational modification that results in the decoration of newly synthesized proteins with diverse types of oligosaccharides that originate from the amide group of the amino acid asparagine. The sequential and collective action of multiple glycosidases and glycosyltransferases are responsible for determining the overall size, composition, and location of N-linked glycans that become covalently linked to an asparagine during and after protein translation. A growing body of evidence supports the critical role of N-linked glycan synthesis in regulating many features of T cell biology, including thymocyte development and tolerance, as well as T cell activation and differentiation. Here, we provide an overview of how specific glycosidases and glycosyltransferases contribute to the generation of different types of N-linked glycans and how these post-translational modifications ultimately regulate multiple facets of T cell biology.

Sequential in vitro enzymatic N-glycoprotein modification reveals site-specific rates of glycoenzyme processing

N-glycosylation is an essential eukaryotic posttranslational modification that affects various glycoprotein properties, including folding, solubility, protein–protein interactions, and half-life. N-glycans are processed in the secretory pathway to form varied ensembles of structures, and diversity at a single site on a glycoprotein is termed ‘microheterogeneity’. To understand the factors that influence glycan microheterogeneity, we hypothesized that local steric and electrostatic factors surrounding each site influence glycan availability for enzymatic modification. We tested this hypothesis via expression of reporter N-linked glycoproteins in N-acetylglucosaminyltransferase MGAT1-null HEK293 cells to produce immature Man₅GlcNAc₂ glycoforms (38 glycan sites total). These glycoproteins were then sequentially modified in vitro from high mannose to hybrid and on to biantennary, core-fucosylated, complex structures by a panel of N-glycosylation enzymes, and each reaction time course was quantified by LC-MS/MS. Substantial differences in rates of in vitro enzymatic modification were observed between glycan sites on the same protein, and differences in modification rates varied depending on the glycoenzyme being evaluated. In comparison, proteolytic digestion of the reporters prior to N-glycan processing eliminated differences in in vitro enzymatic modification. Furthermore, comparison of in vitro rates of enzymatic modification with the glycan structures found on the mature reporters expressed in WT cells correlated well with the enzymatic bottlenecks observed in vivo. These data suggest higher order local structures surrounding each glycosylation site contribute to the efficiency of modification both in vitro and in vivo to establish the spectrum of microheterogeneity in N-linked glycoproteins.

Glycan-protein interactions determine kinetics of N-glycan remodeling

A hallmark of N-linked glycosylation in the secretory compartments of eukaryotic cells is the sequential remodeling of an initially uniform oligosaccharide to a site-specific, heterogeneous ensemble of glycostructures on mature proteins. To understand site-specific processing, we used protein disulfide isomerase (PDI), a model protein with five glycosylation sites, for molecular dynamics (MD) simulations and compared the result to a biochemical in vitro analysis with four different glycan processing enzymes. As predicted by an analysis of the accessibility of the N-glycans for their processing enzymes derived from the MD simulations, N-glycans at different glycosylation sites showed different kinetic properties for the processing enzymes. In addition, altering the tertiary structure of the glycoprotein PDI affected its N-glycan remodeling in a site-specific way. We propose that the observed differential N-glycan reactivities depend on the surrounding protein tertiary structure and lead to different glycan structures in the same protein through kinetically controlled processing pathways.

Atomistic glycoprotein simulations reveal a site-specific availability of glycan substrates in time-resolved mass spectrometry of maturating enzyme kinetics.

Complex N-Glycans Are Important for Normal Fruit Ripening and Seed Development in Tomato

Complex N-glycan modification of secretory glycoproteins in plants is still not well understood. Essential in animals, where a lack of complex N-glycans is embryo-lethal, their presence in plants seemed less relevant for a long time mostly because Arabidopsis thaliana cgl1 mutants lacking N-acetyl-glucosaminyltransferase I (GNTI, the enzyme initiating complex N-glycan maturation in the Golgi apparatus) are viable and showed only minor impairments regarding stress tolerance or development. A different picture emerged when a rice (Oryza sativa) gntI T-DNA mutant was found to be unable to reach the reproductive stage. Here, we report on tomato (Solanum lycopersicum) lines that showed severe impairments upon two RNA interference (RNAi) approaches. Originally created to shed light on the role of core α1,3-fucose and β1,2-xylose residues in food allergy, plants with strongly reduced GNTI activity developed necrotic fruit-attached stalks and early fruit drop combined with patchy incomplete ripening. Correspondingly, semiquantitative RT-PCR of the abscission zone (az) revealed an increase of abscission markers. Also, GNTI-RNA interference (RNAi) plants were more susceptible to sporadic infection. To obtain vital tomatoes with comparable low allergenic potential, Golgi α-mannosidase II (MANII) was chosen as the second target. The resulting phenotypes were oppositional: MANII-reduced plants carried normal-looking fruits that remained attached for extended time without signs of necrosis. Fruits contained no or only few, but enlarged, seeds. Furthermore, leaves developed rolled-up rims simultaneously during the reproductive stage. Trials to cross MANII-reduced plants failed, while GNTI-reduced plants could be (back-)crossed, retaining their characteristic phenotype. This phenotype could not be overcome by ethephon or indole-3-acetic acid (IAA) application, but the latter was able to mimic patchy fruit ripening in wild-type. Phytohormones measured in leaves and 1-aminocyclopropane-1-carboxylic acid (ACC) contents in fruits showed no significant differences. Together, the findings hint at altered liberation/perception of protein-bound N-glycans, known to trigger auxin-like effects. Concomitantly, semiquantitative RT-PCR analysis revealed differences in auxin-responsive genes, indicating the importance of complex N-glycan modification for hormone signaling/crosstalk. Another possible role of altered glycoprotein life span seems subordinate, as concluded from transient expression of Arabidopsis KORRIGAN KOR1-GFP fusion proteins in RNAi plants of Nicotiana benthamiana. In summary, our analyses stress the importance of complex N-glycan maturation for normal plant responses, especially in fruit-bearing crops like tomato.

Complex N-glycans: the story of the "yellow brick road"

The synthesis of complex asparagine-linked glycans (N-glycans) involves a multi-step process that starts with a five mannose N-glycan structure: [Manα1-6(Manα1-3)Manα1-6][Manα1-3]-R where R = Manβ1-4GlcNAcβ1-4GlcNAcβ1-Asn-protein. N-acetylglucosaminyltransferase I (GlcNAc-TI) first catalyzes addition of GlcNAc in β1-2 linkage to the Manα1-3-R terminus of the five-mannose structure. Mannosidase II then removes two Man residues exposing the Manα1-6 terminus that serves as a substrate for GlcNAc-T II and addition of a second GlcNAcβ1-2 residue. The resulting structure is the complex N-glycan: GlcNAcβ1-2Manα1-6(GlcNAcβ1-2Manα1-3)-R. This structure is the precursor to a large assortment of branched complex N-glycans involving four more N-acetylglucosaminyltransferases. This short review describes the experiments (done in the early 1970s) that led to the discovery of GlcNAc-TI and II.

Reduction of N-linked xylose and fucose by expression of rat beta1,4-N-acetylglucosaminyltransferase III in tobacco BY-2 cells depends on Golgi enzyme localization domain and genetic elements used for expression

Plant-specific N-glycosylation, such as the introduction of core alpha1,3-fucose and beta1,2-xylose residues, is a major obstacle to the utilization of plant cell- or plant-derived recombinant therapeutic proteins. The beta1,4-N-acetylglucosaminyltransferase III (GnTIII) introduces a bisecting GlcNAc residue into N-glycans, which exerts a high level of substrate mediated control over subsequent modifications, for example inhibiting mammalian core fucosylation. Based on similar findings in plants, we used Nicotianatabacum BY-2 cells to study the effects of localization and expression levels of GnTIII in the remodeling of the plant N-glycosylation pathway. The N-glycans produced by the cells expressing GnTIII were partially bisected and practically devoid of the paucimannosidic type which is typical for N-glycans produced by wildtype BY-2 suspension cultured cells. The proportion of human-compatible N-glycans devoid of fucose and xylose could be increased from an average of 4% on secreted protein from wildtype cells to as high as 59% in cells expressing chimeric GnTIII, named GnTIII(A.th.) replacing its native localization domain with the cytoplasmic tail, transmembrane, and stem region of Arabidopsis thaliana mannosidase II. The changes in N-glycosylation observed were dependent on the catalytic activity of GnTIII, as the expression of catalytically inactive GnTIII mutants did not show a significant effect on N-glycosylation.

Efficient introduction of a bisecting GlcNAc residue in tobacco N-glycans by expression of the gene encoding human N-acetylglucosaminyltransferase III

In this study, we show that introduction of human N-acetylglucosaminyltransferase (GnT)-III gene into tobacco plants leads to highly efficient synthesis of bisected N-glycans. Enzymatically released N-glycans from leaf glycoproteins of wild-type and transgenic GnT-III plants were profiled by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) in native form. After labeling with 2-aminobenzamide, profiling was performed using normal-phase high-performance liquid chromatography with fluorescence detection, and glycans were structurally characterized by MALDI-TOF/TOF-MS and reverse-phase nano-liquid chromatography-MS/MS. These analyses revealed that most of the complex-type N-glycans in the plants expressing GnT-III were bisected and carried at least two terminal N-acetylglucosamine (GlcNAc) residues in contrast to wild-type plants, where a considerable proportion of N-glycans did not contain GlcNAc residues at the nonreducing end. Moreover, we have shown that the majority of N-glycans of an antibody produced in a plant expressing GnT-III is also bisected. This might improve the efficacy of therapeutic antibodies produced in this type of transgenic plant.

Essential and mutually compensatory roles of {alpha}-mannosidase II and {alpha}-mannosidase IIx in N-glycan processing in vivo in mice

Many proteins synthesized through the secretory pathway receive posttranslational modifications, including N-glycosylation. alpha-Mannosidase II (MII) is a key enzyme converting precursor high-mannose-type N-glycans to matured complex-type structures. Previous studies showed that MII-null mice synthesize complex-type N-glycans, indicating the presence of an alternative pathway. Because alpha-mannosidase IIx (MX) is a candidate enzyme for this pathway, we asked whether MX functions in N-glycan processing by generating MII/MX double-null mice. Some double-nulls died between embryonic days 15.5 and 18.5, but most survived until shortly after birth and died of respiratory failure, which represents a more severe phenotype than that seen in single-nulls for either gene. Structural analysis of N-glycans revealed that double-nulls completely lack complex-type N-glycans, demonstrating a critical role for at least one of these enzymes for effective N-glycan processing. Recombinant mouse MX and MII showed identical substrate specificities toward N-glycan substrates, suggesting that MX is an isozyme of MII. Thus, either MII or MX can biochemically compensate for the deficiency of the other in vivo, and either of two is required for late embryonic and early postnatal development.

The synthesis of a series of modified mannotrisaccharides as probes of the enzymes involved in the early stages of mammalian complex N-glycan formation

A series of mannotrisaccharides were synthesized by two distinct chemical pathways as probes of the enzymes involved in the early stages of mammalian complex N-glycan formation. Methyl (alpha-D-mannopyranosyl)-(1-->3)-[(alpha-D-mannopyranosyl)-(1-->6)]-beta-D-mannopyranoside (6) and methyl (2-deoxy-2-fluoro-alpha-D-mannopyranosyl)-(1-->3)-[(2-deoxy-2-fluoro-alpha-D-mannopyranosyl)-(1-->6)]-beta-D-mannopyranoside (8) were rapidly synthesized from unprotected methyl beta-D-mannopyranoside (12). Methyl (2-deoxy-2-fluoro-alpha-D-mannopyranosyl)-(1-->3)-[(alpha-D-mannopyranosyl)-(1-->6)]-beta-D-mannopyranoside (7) and methyl (alpha-D-mannopyranosyl)-(1-->3)-[(2-deoxy-2-fluoro-alpha-D-mannopyranosyl)-(1-->6)]-beta-D-mannopyranoside (9) were synthesized from the common orthogonally protected precursor methyl 2-O-acetyl-4,6-O-benzylidene-beta-D-mannopyranoside (15). The 2-deoxy-2-fluoro substitution common to trisaccharides 7-9 renders these analogues resistant to enzyme action in two distinct ways. Firstly the fluorine serves as a non-nucleophilic isostere for the acceptor hydroxyl in studies with glycosyl transferases GnT-I and GnT-II (7 and 9, respectively). Secondly it should render trisaccharide 8 stable to hydrolysis by the mannosidases Man-II and Man-III by inductive destabilization of their oxocarbenium ion-like transition states. These analogues should be useful for structural studies on these enzymes.

Use of synthetic oligosaccharide substrate analogs to map the active sites of N-acetylglucosaminyltransferases I and II

Tables III and IV summarize substrate analog data presented in Tables I and II, respectively. Data for GlcNAc-T I shown in Tables I and III correlate very well with the crystal structure for GlcNAc-T I. This indicates that substitution of the various hydroxyl groups by hydrogen, [table: see text] O-methyl, and maybe even larger O-alkyl groups does not cause appreciable changes to either the overall conformation of the oligosaccharide or the binding mode, thus supporting this approach of chemical modification of oligosaccharide substrates for mapping of the binding site. There is as yet no crystal structure for GlcNAc-T II. These studies indicate both advantages and disadvantages of this approach for elucidating the catalytic and binding sites of an enzyme. Substrate analog data indicate which chemical groups in the substrate are essential for catalysis and binding and suggest the type of linkage involved (hydrogen bond donor or acceptor). However, no information has been obtained on the protein groups involved in these interactions. If a crystal structure is available, the substrate analog conclusions are primarily confirmatory. However, whether or not a crystal structure is available, this approach can be very helpful in the design of specific inhibitors.

Down-regulation of the alpha-Gal epitope expression in N-glycans of swine endothelial cells by transfection with the N-acetylglucosaminyltransferase III gene. Modulation of the biosynthesis of terminal structures by a bisecting GlcNAc

The down-regulation of the alpha-Gal epitope (Galalpha1,3Galbeta-R) in swine tissues would be highly desirable, in terms of preventing hyperacute rejection in pig-to-human xenotransplantation. In an earlier study, we reported that the introduction of the beta1,4-N-acetylglucosaminyltransferase (GnT) III gene into swine endothelial cells resulted in a substantial reduction in the expression of the alpha-Gal epitope. In this study, we report on the mechanism for this down-regulation of the alpha-Gal epitope by means of structural and kinetic analyses. The structural analyses revealed that the amount of N-linked oligosaccharides bearing the alpha-Gal epitopes in the GnT-III-transfected cells was less than 10% that in parental cells, due to the alteration of the terminal structures as well as a decrease in branch formation. In addition, it appeared that the addition of a bisecting GlcNAc, which is catalyzed by GnT-III, leads to a more efficient sialylation rather than alpha-galactosylation. In vitro kinetic analyses showed that the bisecting GlcNAc has an inhibitory effect on alpha-galactosylation, but does not significantly affect the sialylation. These results suggest that the bisecting GlcNAc in the core is capable of modifying the biosynthesis of the terminal structures via its differential effects on the capping glycosyltransferase reactions. The findings may contribute to the development of a novel strategy to eliminate carbohydrate xenoantigens.

Recycling cell surface glycoproteins undergo limited oligosaccharide reprocessing in LEC1 mutant Chinese hamster ovary cells

The ability of particular cell surface glycoproteins to recycle and become exposed to individual Golgi enzymes has been demonstrated. This study was designed to determine whether endocytic trafficking includes significant reentry into the overall oligosaccharide processing pathway. The Lec1 mutant of Chinese hamster ovary (CHO) cells lack N -acetylglucosaminyltransferase I (GlcNAc-TI) activity resulting in surface expression of incompletely processed Man5GlcNAc2 N -linked oligosaccharides. An oligosaccharide tracer was created by exoglycosylation of cell surface glycoproteins with purified porcine GlcNAc-TI and UDP-[3H]GlcNAc. Upon reculturing, all cell surface glycoproteins that acquired [3H]GlcNAc were acted upon by intracellular mannosidase II, the next enzyme in the Golgi processing pathway of complex N -linked oligosaccharides (t1/2= 3-4 h). That all radiolabeled cell surface glycoproteins were included in this endocytic pathway indicates a common intracellular compartment into which endocytosed cell surface glycoproteins return. Significantly, no evidence was found for continued oligosaccharide processing consistent with transit through the latter cisternae of the Golgi apparatus. These data indicate that, although recycling plasma membrane glycoproteins can be reexposed to individual Golgi-derived enzymes, significant reentry into the overall contiguous processing pathway is not evident.

Synthetic substrate analogues for UDP-GlcNAc: Man alpha 1-3R beta 1-2-N-acetylglucosaminyltransferase I. Substrate specificity and inhibitors for the enzyme

UDP-GlcNAc:Man alpha 1-3R beta 1-2-N-acetylglucosaminyltransferase I (GlcNAc-T I; EC 2.4.1.101) catalyses the conversion of [Man alpha 1-6(Man alpha 1-3)Man alpha 1-6][Man alpha 1-3]Man beta-O-R to [Man alpha 1-6(Man alpha 1-3)Man alpha 1-6] [GlcNAc beta 1-2Man alpha 1-3]Man beta-O-R (R = 1-4GlcNAc beta 1-4GlcNAc- Asn-X) and thereby controls the conversion of oligomannose to complex and hybrid asparagine-linked glycans (N-glycans). GlcNAc-T I also catalyses the conversion of Man alpha 1-6(Man alpha 1-3)Man beta-O-octyl to Man alpha 1-6(GlcNAc beta 1-2Man alpha 1-3)Man beta-O-octyl. We have therefore tested a series of synthetic analogues of Man"alpha 1-6(Man'alpha 1-3)Man beta-O-octyl as substrates and inhibitors for rat liver GlcNAc-T I. The 2"-deoxy and the 3"-, 4"- and 6"-O-methyl derivatives are all good substrates confirming previous observations that the hydroxyl groups of the Man"alpha 1-6 residue do not play major roles in the binding of substrate to enzyme. In contrasts, all four hydroxyl groups on the Man'alpha 1-3 residue are essential since the corresponding deoxy derivatives either do not bind (2'- and 3'-deoxy) or bind very poorly (4'- and 6'-deoxy) to the enzyme. The 2'- and 3'-O-methyl derivatives also do not bind to the enzyme. However, the 4'-O-methyl derivative is a substrate (KM = 2.6 mM) and the 6'-O-methyl compound is a competitive inhibitor (Ki = 0.76 mM). We have therefore synthesized various 4'- and 6'-O-alkyl derivatives, some with reactive groups attached to an O-pentyl spacer, and tested these compounds as reversible and irreversible inhibitors of GlcNAc-T I. The 6'-O-(5-iodoacetamido-pentyl) compound is a specific time dependent inhibitor of the enzyme. Four other 6'-O-alkyl compounds showed competitive inhibition while the remaining compounds showed little or no binding indicating that the electronic properties of the attached O-pentyl groups influence binding.

Synthesis of uridine-5-propylamine derivatives and their use in affinity chromatography of N-acetylglucosaminyltransferases I and II

The C-5 substituted uridine derivatives UDP-5-propylamine (7) and UDP-GlcNAc-5-propylamine (8) were synthesized in good yields by Heck alkylation of the 5-mercuriuridines, followed by hydrogenation. The products were characterized by 1H and 13C NMR spectroscopy, electrospray mass spectrometry and UV spectrophotometry. The amines are of interest for the preparation of affinity probes for glycosyltransferases. The benzoylbenzamides of 7 and 8 show strong competitive inhibition of N-acetylglucosaminyltransferase I and II with Ki values ranging from 30 to 100 microM (without irradiation) and may be useful as active site-directed photoaffinity labels. A conjugate of 8 and Sepharose was used for affinity chromatographic purification of N-acetylglucosaminyltransferases I and II. The results indicate that this affinity gel is a stable alternative to the commonly used but unstable UDP-GlcNAc-5-Hg-thiopropyl conjugate.

Synthetic substrate analogues for UDP-GlcNAc: Man alpha 1-6R beta(1-2)-N-acetylglucosaminyltransferase II. Substrate specificity and inhibitors for the enzyme

UDP-GlcNAc:Man alpha 1-6R beta(1-2)-N-acetylglucosaminyltransferase II (GlcNAc-T II; EC 2.4.1.143) is a key enzyme in the synthesis of complex N-glycans. We have tested a series of synthetic analogues of the substrate Man"'alpha 1-6(GlcNAc"beta 1-2Man'alpha 1-3)Man beta-O-octyl as substrates and inhibitors for rat liver GlcNAc-T II. The enzyme attaches N-acetylglucosamine in beta 1-2 linkage to the 2"'-OH of the Man"'alpha 1-6 residue. The 2"'-deoxy analogue is a competitive inhibitor (Ki = 0.13 mM). The 2"'-O-methyl compound does not bind to the enzyme presumably due to steric hindrance. The 3"'-, 4"'- and 6"'-OH groups are not essential for binding or catalysis since the 3"'-, 4"'- and 6"'-deoxy and -O-methyl derivatives are all good substrates. Increasing the size of the substituent at the 3"'-position to pentyl and substituted pentyl groups causes competitive inhibition (Ki = 1.0-2.5 mM). We have taken advantage of this effect to synthesize two potentially irreversible GlcNAc-T II inhibitors containing a photolabile 3"'-O-(4,4-azo)pentyl group and a 3"'-O-(5-iodoacetamido)pentyl group respectively. The data indicate that none of the hydroxyls of the Man"'alpha 1-6 residue are essential for binding although the 2"'- and 3"'-OH face the catalytic site of the enzyme. The 4-OH group of the Man beta-O-octyl residue is not essential for binding or catalysis since the 4-deoxy derivative is a good substrate; the 4-O-methyl derivative does not bind.(ABSTRACT TRUNCATED AT 250 WORDS)

Structures and functions of the sugar chains of glycoproteins

Most proteins within living organisms contain sugar chains. Recent advancements in cell biology have revealed that many of these sugar chains play important roles as signals for cell-surface recognition phenomena in multi-cellular organisms. In order to elucidate the biological information included in the sugar chains and link them with biology, a novel scientific field called 'glycobiology' has been established. This review will give an outline of the analytical techniques for the structural study of the sugar chains of glycoproteins, the structural characteristics of the sugar chains and the biosynthetic mechanism to produce such characteristics. Based on this knowledge, functional aspects of the sugar chains of glycohormones and of those in the immune system will be described to help others understand this new scientific field.

The metastatic potential of rat prostate tumor variant R3327-MatLyLu is correlated with an increased activity of N-acetylglucosaminyl transferase III and V

Enzyme activities of N-acetylglucosaminyltransferase (GlcNAc-Tase) I-V involved in N-linked complex-type carbohydrate synthesis were determined in a non-metastatic hormone-dependent rat prostate tumor (R3327-H) and a related, hormone-independent variant metastasizing to lymph nodes and lungs (R3327-MatLyLu). In the metastasizing variant a significantly increased activity of both GlcNAc-Tase III and GlcNAc-Tase V was observed, whereas the activities of GlcNAc-Tase I and II were essentially unchanged. The increase in activity of GlcNAc-Tase III is particularly noteworthy since it indicates that elevated expression of this enzyme cannot be considered as an exclusive marker of hepatic malignancy.

Control of glycoprotein synthesis: substrate specificity of rat liver UDP-GlcNAc:Man alpha 3R beta 2-N-acetylglucosaminyltransferase I using synthetic substrate analogues

UDP-GlcNAc: Man alpha 3R beta 2-N-acetylglucosaminyltransferase I (GlcNAc-T I; EC 2.4.1.101) is the key enzyme in the synthesis of complex and hybrid N-glycans. Rat liver GlcNAc-T I has been purified more than 25,000-fold (M(r) 42,000). The Vmax for the pure enzyme with [Man alpha 6(Man alpha 3)Man alpha 6](Man alpha 3)Man beta 4GlcNAc beta 4GlcNAc beta-Asn as substrate was 4.6 mumol min-1 mg-1. Structural analysis of the enzyme product by proton nuclear magnetic resonance spectroscopy proved that the enzyme adds an N-acetylglucosamine (GlcNAc) residue in beta 1-2 linkage to the Man alpha 3Man beta-terminus of the substrate. Several derivatives of Man alpha 6(Man alpha 3)Man beta-R, a substrate for the enzyme, were synthesized and tested as substrates and inhibitors. An unsubstituted equatorial 4-hydroxyl and an axial 2-hydroxyl on the beta-linked mannose of Man alpha 6(Man alpha 3)Man beta-R are essential for GlcNAc-T I activity. Elimination of the 4-hydroxyl of the alpha 3-linked mannose (Man) of the substrate increases the KM 20-fold. Modifications on the alpha 6-linked mannose or on the core structure affect mainly the KM and to a lesser degree the Vmax, e.g., substitutions of the Man alpha 6 residue at the 2-position by GlcNAc or at the 3- and 6-positions by mannose lower the KM, whereas various other substitutions at the 3-position increase the KM slightly. Man alpha 6(Man alpha 3)4-O-methyl-Man beta 4GlcNAc was found to be a weak inhibitor of GlcNAc-T I.

N-acetylglucosaminyltransferase III in human serum, and liver and hepatoma tissues: increased activity in liver cirrhosis and hepatoma patients

An N-acetylglucosaminyltransferase III which catalyzes the addition of N-acetylglucosamine through a beta 1-4 linkage (bisecting N-acetylglucosamine) to the beta-linked mannose of the trimannosyl core structure of N-linked oligosaccharides of glycoproteins was measured in human serum, and liver and hepatoma tissues. The enzyme activity in serum was significantly elevated in patients with hepatomas and liver cirrhosis, and the activity markedly decreased on the transcatheter arterial embolization treatment. High activities were also found in the hepatoma and cirrhotic liver tissues, indicating that the serum activity reflected the activity in the tissues. The assaying of the enzyme activity in serum appears to be useful for the detection and monitoring of primary hepatomas.

Biosynthesis of galactogen: identification of a beta-(1----6)-D-galactosyltransferase in Helix pomatia albumen glands

A beta-(1----6)-D-galactosyltransferase has been purified over 2000-fold by affinity chromatography on UDP-p-aminophenyl-Sepharose. The enzyme, from a pellet fraction (8000 x g) of Helix pomatia albumen gland, catalyzes transfer of D-galactose from UDP-galactose to a (1----6) linkage on acceptor H. pomatia galactogen. Three other polymers served as acceptors: beef lung galactan, Lymnaea stagnalis galactogen and arabinogalactan from larch wood. To determine the linkage specificity of the enzyme, it was incubated with UDP-D-galactose and acceptor galactogen that had been tritiated previously by treatment with galactose oxidase and [3H]KBH4. The [3H]galactogen reaction product was recovered, methylated, hydrolyzed and acetylated; tritiated derivatives were identified by mass spectroscopy of effluent fractions separated by gas chromatography. This analysis revealed that (1----6)-linked galactosyl groups had been added to the enzyme-treated acceptor galactogen. Also identified was a hydrolytic enzyme that removed terminal alpha 1,2-linked L-galactosyl residues from H. pomatia galactogen.

The biosynthesis of highly branched N-glycans: studies on the sequential pathway and functional role of N-acetylglucosaminyltransferases I, II, III, IV, V and VI

At least 6 N-acetylglucosaminyltransferases (GlcNAc-T I, II, III, IV, V and VI) are involved in initiating the synthesis of the various branches found in complex asparagine-linked oligosaccharides (N-glycans), as indicated below: GlcNAc beta 1-6 GlcNAc-T V GlcNAc beta 1-4 GlcNAc-T VI GlcNAc beta 1-2Man alpha 1-6 GlcNAc-T II GlcNAc beta 1-4Man beta 1-4-R GlcNAc T III GlcNAc beta 1-4Man alpha 1-3 GlcNAc-T IV GlcNAc beta 1-2 GlcNAc-T I where R is GlcNAc beta 1-4(+/- Fuc alpha 1-6)GlcNAcAsn-X. HPLC was used to study the substrate specificities of these GlcNAc-T and the sequential pathways involved in the biosynthesis of highly branched N-glycans in hen oviduct (I. Brockhausen, J.P. Carver and H. Schachter (1988) Biochem. Cell Biol. 66, 1134-1151). The following sequential rules have been established: GlcNAc-T I must act before GlcNAc-T II, III and IV; GlcNAc-T II, IV and V cannot act after GlcNAc-T III, i.e., on bisected substrates; GlcNAc-T VI can act on both bisected and non-bisected substrates; both Glc-NAc-T I and II must act before GlcNAc-T V and VI; GlcNAc-T V cannot act after GlcNAc-T VI. GlcNAc-T V is the only enzyme among the 6 transferases cited above which can be essayed in the absence of Mn2+. In studies on the possible functional role of N-glycan branching, we have measured GlcNAc-T III in pre-neoplastic rat liver nodules (S. Narasimhan, H. Schachter and S. Rajalakshmi (1988) J. Biol. Chem. 263, 1273-1281). The nodules were initiated by administration of a single dose of carcinogen 1,2-dimethyl-hydrazine.2 HCl 18 h after partial hepatectomy and promoted by feeding a diet supplemented with 1% orotic acid for 32-40 weeks. The nodules had significant GlcNAc-T III activity (1.2-2.2 nmol/h/mg), whereas the surrounding liver, regenerating liver 24 h after partial hepatectomy and control liver from normal rats had negligible activity (0.02-0.03 nmol/h/mg). These results suggest that GlcNAc-T III is induced at the pre-neoplastic stage in liver carcinogenesis and are consistent with the reported presence of bisecting GlcNAc residues in N-glycans from rat and human hepatoma gamma-glutamyl transpeptidase and their absence in enzyme from normal liver of rats and humans (A. Kobata and K. Yamashita (1984) Pure Appl. Chem. 56, 821-832).

[Conformation analysis of modified tetrasaccharide sequences of the N-glycoprotein type--problem of the alpha-(1 to 6)-glycosidic bond]

The conformational analysis of the recently synthesized tetrasaccharides alpha-D-Manp (1----3)-[alpha-D-Manp-(1----6)]-4-deoxy-beta-D-lyx-hexp+ ++-(1----4)-D-GlcNAc (2) and alpha-D-Manp-(1----3)-[alpha-D-Manp-(1----6)]-beta-D-Talp -(1----4)-D-GlcNAc (3) will be described. The preferred solution conformation of 2 and 3 is a gt-conformation, which is nearly identical with the preferred conformation of the naturally occurring tetrasaccharide alpha-D-Manp-(1----3)-[alpha-D-Manp-(1----6)]-beta-D-Manp -(1----4)-D-GlcNAc (1). The main structural feature is the backfolding of the alpha-(1----6)-linked D-Man to the reducing D-GlcNAc unit. Conformational analysis of the tetrasaccharides alpha-D-Manp-(1----3)-[alpha-D-Manp-(1----6)]-beta-D-Manp -(1----4)-1,6- anhydro-beta-D-GlcNAc (4), alpha-D-Manp-(1----3)-alpha-D-Manp-(1----6)]-4-deoxy-beta-D- lyx-hexp-(1----4)- 1,6-anhydro-beta-D-GlcNAc (5), and alpha-D-Manp-(1----3)-[alpha-D-Manp-(1----6)]-beta-D-Talp -(1----4)- 1,6-anhydro-beta-D-GlcNAc (6) gave additional proof for this backfolding. The substitution of the reducing unit leads to a smaller amount of gt- and a greater amount of gg-conformers. The method used for conformational analysis of 2-6 is a combination of n.m.r.-experiments and HSEA-calculations with the program GESA. Concerning the application of new 2D-techniques, the COLOC-experiment turned out to be extremely useful in sequencing oligosaccharides.

The beta 1----2-D-xylose and alpha 1----3-L-fucose substituted N-linked oligosaccharides from Erythrina cristagalli lectin. Isolation, characterisation and comparison with other legume lectins

The carbohydrate moieties of Erythrina cristagalli lectin were released as oligosaccharides by hydrazinolysis, followed by N-acetylation and reduction with NaB3H4. Fractionation of the tritium-labelled oligosaccharide mixture by Bio-Gel P-4 column chromatography and high-voltage borate electrophoresis revealed that it is composed of five neutral oligosaccharides. Structural studies by sequential exoglycosidase digestion in combination with methylation analysis and two-dimensional 1H-NMR showed that the major component was the fucose-containing heptasaccharide Man alpha 3(Man alpha 6)(Xyl beta 2)Man beta 4GlcNAc beta 4(Fuc alpha 3)GlcNAcol. This is the first report of such a structure in plant lectins. Small amounts of the corresponding afucosyl hexasaccharide were also identified, as well as three other minor components. The structure of the heptasaccharide shows the twin characteristics of a newly established family of N-linked glycans, found to date only in plants. The characteristics are substitution of the common pentasaccharide core [Man alpha 3(Man alpha 6)Man beta 4GlcNAc beta 4GlcNAc] by a D-xylose residue linked beta 1----2 to the beta-mannosyl residue and an L-fucose residue linked alpha 1----3 to the reducing terminal N-acetylglucosamine residue. The oligosaccharide heterogeneity pattern for Erythrina cristagalli lectin was also found for the lectins from four other Erythrina species and the lectins of two other legumes, Sophora japonica and Lonchocarpus capassa.

The effect of a "bisecting" N-acetylglucosaminyl group on the binding of biantennary, complex oligosaccharides to concanavalin A, Phaseolus vulgaris erythroagglutinin (E-PHA), and Ricinus communis agglutinin (RCA-120) immobilized on agarose

The effect of a "bisecting" 2-acetamido-2-deoxy-beta-D-glucopyranosyl group, linked (1----4) to the beta-D-mannopyranosyl group of asparagine-linked complex and hybrid oligosaccharides, on the binding of [14C]acetylated glycopeptides to columns of immobilized concanavalin A (Con A), Phaseolus vulgaris erythroagglutinin (E-PHA), and Ricinus communis agglutinin-120 (RCA-120) was investigated. The presence of this "bisecting" GlcNAc group caused significant inhibition of the binding to ConA-agarose of biantennary complex glycopeptides in which the two branches are terminated at their nonreducing ends by two GlcNAc groups, or by a Gal and a GlcNAc group, or by two Gal groups, or by a Man and a GlcNAc group. Binding of biantennary, complex glycopeptides to E-PHA-agarose required a "bisecting" GlcNAc group, a Gal group at the nonreducing terminus of the alpha-D-Man-p-(1----6) branch, and a terminal or internal GlcNAc residue linked beta-(1----2) to the alpha-D-Manp-(1----3) branch. Binding to RCA-120-agarose occurred only when at least one nonreducing terminal Gal group was present, and increased as the proportion of terminal Gal groups increased; the presence of a "bisecting" GlcNAc group caused either enhancement or inhibition of these binding patterns. It is concluded that a "bisecting" GlcNAc group affects the binding of glycopeptides to all three lectin columns.

Accumulation of UDP-GlcNAc into intracellular vesicles and occurrence of a carrier-mediated transport. Study with plasma-membrane-permeabilized mouse thymocytes

Metabolic labelling of mouse thymocytes with radioactive mannose or glucosamine leads to the formation of labelled GDP-Man and UDP-GlcNAc. Using isotonic ammonium chloride treatment which renders the plasma membrane of thymocytes permeable to sugar nucleotides, we demonstrate that, in contrast to GDP-Man, a pool of UDP-GlcNAc remains associated with the cells after plasma membrane permeabilization. These observations are confirmed in experiments in which permeabilized thymocytes are incubated with exogenous labelled GDP-Man and UDP-GlcNAc, and we show that only UDP-GlcNAc is accumulated into sealed intracellular vesicles. This accumulation is a saturable process which can be inhibited by UDP, demonstrating the occurrence of a specific carrier. This transport mechanism can be blocked by covalent attachment of a non-permeant inhibitor UDP-dialdehyde without affecting the N-acetylglucosaminyltransferase itself. The fact that this carrier-mediated transport is not inhibited by tunicamycin indicates that this translocation process of UDP-GlcNAc does not involve lipid intermediates.

Control of glycoprotein synthesis. Bovine milk UDPgalactose:N-acetylglucosamine beta-4-galactosyltransferase catalyzes the preferential transfer of galactose to the GlcNAc beta 1,2Man alpha 1,3- branch of both bisected and nonbisected complex biantennary asparagine-linked oligosaccharides

Bovine milk UDPgalactose:N-acetylglucosamine beta-4-galactosyltransferase has been used to investigate the effect of a bisecting GlcNAc residue (linked beta 1,4 to the beta-linked mannose of the trimannosyl core of asparagine-linked complex oligosaccharides) on galactosylation of biantennary complex oligosaccharides. Columns of immobilized lectins (concanavalin A, erythroagglutinating phytohemagglutinin, and Ricinus communis agglutinin 120) were used to separate the various products of the reactions. Preferential galactosylation of the GlcNAc beta 1,2Man alpha 1,3 arm occurred both in the absence and in the presence of a bisecting GlcNAc residue; the ratio of the rates of galactosylation of the Man alpha 1,3 arm to the Man alpha 1,6 arm was 6.5 in the absence of a bisecting GlcNAc and 2.8 in its presence. The bisecting GlcNAc residue reduced galactosylation of the Man alpha 1,3 arm by about 78% probably due to steric hindrance of the GlcNAc beta 1,2Man alpha 1,3 beta 1,4 region of the substrate by the bisecting GlcNAc. This steric hindrance prevents the action of four other enzymes involved in assembly of complex asparagine-linked oligosaccharides and indicates the importance of the bisecting GlcNAc residue in the control of glycoprotein biosynthesis. The Man alpha 1,3 arm of biantennary oligosaccharides is believed to be freely accessible to enzyme action whereas the Man alpha 1,6 arm is believed to be folded back toward the core. This may explain the preferential action of Gal-transferase on the Man alpha 1,3 arm of both bisected and nonbisected oligosaccharides.

Structures of N-glycans of a ricin-resistant mutant of baby hamster kidney cells. Synthesis of high-mannose and hybrid N-glycans

The asparagine-linked glycopeptides (N-glycans) of a ricin-resistant mutant of baby hamster kidney (BHK) cells, RicR21, have been isolated and fractionated from a Pronase digest of disrupted cells by concanavalin A (Con A)-Sepharose chromatography, ion-exchange chromatography, and lentil lectin chromatography. The structures of all the major N-glycans have been determined by 500-MHz H NMR spectroscopy. RicR21 synthesizes only hybrid and high-mannose N-glycans. All the hybrid structures contain only three mannose residues. The major hybrid glycopeptide has the following structure: (Formula: see text). There is also about 15% of the nonfucosylated species present. Only a small amount (less than or equal to 5%) of the asialo hybrid is produced. Branched hybrid N-glycans are also present in RicR21 cells, containing two complex antenna linked beta 1----2 and beta 1----4 to the Man alpha 1----3 arm; about 70% of this species is core fucosylated. Man6GlcNAc2 glycopeptide is the most abundant (about 70%) of the high-mannose N-glycans. These studies account for the very poor ricin binding property of this mutant, as the sialic acid residues of the major hybrid N-glycan are exclusively linked alpha 2----3 to galactose and ricin is unable to bind to alpha 2----3-substituted galactosyl residues [Baenziger, J. U., & Fiete, D. (1979) J. Biol. Chem. 254, 9795-9799].

Galactosyltransferases: physical, chemical, and biological aspects

Galactosyltransferases (GTs) are one of the members of a family of enzymes called glycosyltransferases involved in the biosynthesis of complex carbohydrates. These enzymes catalyze the transfer of galactose from UDP-galactose to an acceptor (glycoprotein, glycolipid) containing terminal N-acetylglucosamine or N-acetylgalactosamine residue. GTs occur in soluble (milk, serum, effusions, etc.) and insoluble (membrane) forms. The GT activities on the outer surface of the cells have been correlated with a host of cellular interactions, including fertilization, cell migration, embryonic induction, chondrogenesis, contact inhibition of growth, cell adhesion, hemostasis, intestinal cell differentiation, and immune recognition. GTs have been purified to homogeneity using affinity chromatography. Most GTs are found active in the pH range 6 to 8 and at temperatures between 35 to 40 degrees C. Manganese is an essential co-factor for GT activity. Isoenzymes of GT have been recognized, especially in tumor tissues, malignant effusions, and sera of cancer patients using polyacrylamide gel electrophoresis in the presence and absence of SDS. Depending on the source of the enzyme, the molecular weights of GTs range between 40,000 to 80,000 daltons. Carcinoma-associated GT isoenzyme has been reported to have a higher molecular weight than the normal GT isoenzyme. Development of monoclonal antibody against the cancer-specific GT isoenzyme will provide help in the development of an immunoassay for the measurement of this isoenzyme in the sera and an aid in the radioimmunolocalization of the tumors in cancer patients.