Characterization of a core alpha1-->3-fucosyltransferase from the snail Lymnaea stagnalis that is involved in the synthesis of complex-type N-glycans

Mollusc N-glycosylation: Structures, Functions and Perspectives

Molluscs display a sophisticated N-glycan pattern on their proteins, which is, in terms of involved structural features, even more diverse than that of vertebrates. This review summarises the current knowledge of mollusc N-glycan structures, with a focus on the functional aspects of the corresponding glycoproteins. Furthermore, the potential of mollusc-derived biomolecules for medical applications is addressed, emphasising the importance of mollusc research.

Multistep Fractionation and Mass Spectrometry Reveal Zwitterionic and Anionic Modifications of the N- and O-glycans of a Marine Snail

Various studies in the past have revealed that molluscs can produce a wide range of rather complex N-glycan structures, which vary from those occurring in other invertebrate animals; particularly methylated glycans have been found in gastropods, and there are some reports of anionic glycans in bivalves. Due to the high variability in terms of previously described structures and methodologies, it is a major challenge to establish glycomic workflows that yield the maximum amount of detailed structural information from relatively low quantities of sample. In this study, we apply differential release with peptide:N-glycosidases F and A followed by solid-phase extraction on graphitized carbon and reversed-phase materials to examine the glycome of Volvarina rubella (C. B. Adams, 1845), a margin snail of the clade Neogastropoda. The resulting four pools of N-glycans were fractionated on a fused core RP-HPLC column and subject to MALDI-TOF MS and MS/MS in conjunction with chemical and enzymatic treatments. In addition, selected N-glycan fractions, as well as O-glycans released by β-elimination, were analyzed by porous graphitized carbon-LC-MS and MS(n). This comprehensive approach enabled us to determine a number of novel modifications of protein-linked glycans, including N-methyl-2-aminoethylphosphonate on mannose and N-acetylhexosamine residues, core β1,3-linked mannose, zwitterionic moieties on core Galβ1,4Fuc motifs, additional mannose residues on oligomannosidic glycans, and bisubstituted antennal fucose; furthermore, typical invertebrate N-glycans with sulfate and core fucose residues are present in this gastropod.

Structural studies of Helix aspersa agglutinin complexed with GalNAc: A lectin that serves as a diagnostic tool

Lectins belong to a differentiated group of proteins known to possess sugar-binding properties. Due to this fact, they are interesting research targets in medical diagnostics. Helix aspersa agglutinin (HAA) is a lectin that recognizes the epitopes containing α-d-N-acetylgalactosamine (GalNAc), which is present at the surface of metastatic cancer cells. Although several reports have already described the use of HAA as a diagnostic tool, this protein was not characterized on the molecular level. Here, we present for the first time the structural information about lectin isolated from mucus of Helix aspersa (garden snail). The amino acid sequence of this agglutinin was determined by Edman degradation and tertiary as well as quaternary structure by X-ray crystallography. The high resolution crystal structure (1.38Å) and MALDI-TOF mass spectrometry analysis provide the detailed information about a large part of the HAA natural glycan chain. The topology of the GalNAc binding cleft and interaction with lectin are very well defined in the structure and fully confirmed by STD HSQC NMR spectroscopy. Together, this provides structural clues regarding HAA specificity and opens possibilities to rational modifications of this important diagnostic tool.

Structural analysis of the endogenous glycoallergen Hev b 2 (endo-β-1,3-glucanase) from Hevea brasiliensis and its recognition by human basophils

Endogenous glycosylated Hev b 2 (endo-β-1,3-glucanase) from Hevea brasiliensis is an important latex allergen that is recognized by IgE antibodies from patients who suffer from latex allergy. The carbohydrate moieties of Hev b 2 constitute a potentially important IgE-binding epitope that could be responsible for its cross-reactivity. Here, the structure of the endogenous isoform II of Hev b 2 that exhibits three post-translational modifications, including an N-terminal pyroglutamate and two glycosylation sites at Asn27 and at Asn314, is reported from two crystal polymorphs. These modifications form a patch on the surface of the molecule that is proposed to be one of the binding sites for IgE. A structure is also proposed for the most important N-glycan present in this protein as determined by digestion with specific enzymes. To analyze the role of the carbohydrate moieties in IgE antibody binding and in human basophil activation, the glycoallergen was enzymatically deglycosylated and evaluated. Time-lapse automated video microscopy of basophils stimulated with glycosylated Hev b 2 revealed basophil activation and degranulation. Immunological studies suggested that carbohydrates on Hev b 2 represent an allergenic IgE epitope. In addition, a dimer was found in each asymmetric unit that may reflect a regulatory mechanism of this plant defence protein.

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.

The production of biopharmaceuticals in plant systems

Biopharmaceuticals present the fastest growing segment in the pharmaceutical industry, with an ever widening scope of applications. Whole plants as well as contained plant cell culture systems are being explored for their potential as cheap, safe, and scalable production hosts. The first plant-derived biopharmaceuticals have now reached the clinic. Many biopharmaceuticals are glycoproteins; as the Golgi N-glycosylation machinery of plants differs from the mammalian machinery, the N-glycoforms introduced on plant-produced proteins need to be taken into consideration. Potent systems have been developed to change the plant N-glycoforms to a desired or even superior form compared to the native mammalian N-glycoforms. This review describes the current status of biopharmaceutical production in plants for industrial applications. The recent advances and tools which have been utilized to generate glycoengineered plants are also summarized and compared with the relevant mammalian systems whenever applicable.

Variation of acharan sulfate and monosaccharide composition and analysis of neutral N-glycans in African giant snail (Achatina fulica)

Acharan sulfate content from African giant snail (Achatina fulica) was compared in eggs and snails of different ages. Acharan sulfate was not found in egg. Acharan sulfate disaccharide -->4)-alpha-D-GlcNpAc (1-->4)-alpha-L-IdoAp2S(1-->, analyzed by SAX (strong-anion exchange)-HPLC was observed soon after hatching and increases as the snails grow. Monosaccharide compositional analysis showed that mole % of glucosamine, a major monosaccharide of acharan sulfate, increased with age while mole % of galactose decreased with age. These results suggest that galactans represent a major energy source during development, while acharan sulfate appearing immediately after hatching, is essential for the snail growth. The structures of neutral N-glycans released from eggs by peptide N-glycosidase F (PNGase F), were next elucidated using ESI-MS/MS, MALDI-MS/MS, enzyme digestion, and monosaccharide composition analysis. Three types of neutral N-glycan structures were observed, truncated (Hex(2-4)-HexNAc(2)), high mannose (Hex(5-9)-HexNAc(2)), and complex (Hex(3)-HexNAc(2-10)) types. None showed core fucosylation.

Neutral N-glycan patterns of the gastropods Limax maximus, Cepaea hortensis, Planorbarius corneus, Arianta arbustorum and Achatina fulica

The N-glycosylation potentials of Limax maximus, Cepaea hortensis, Planorbarius corneus, Arianta arbustorum and Achatina fulica were analysed by investigation of the N-glycan structures of the skin and viscera glycoproteins by a combination of HPLC and mass-spectrometry methods. It is one of the first steps to enlarge the knowledge on the glycosylation abilities of gastropods, which may help to establish new cell culture systems, to uncover new means for pest control for some species, and to identify carbohydrate-epitopes which may be relevant for immune response. All snails analysed contained mainly oligomannosidic and small paucimannosidic structures, often terminated with 3-O-methylated mannoses. The truncated structures carried modifications by beta1-2-linked xylose to the beta-mannose residue, and/or an alpha-fucosylation, mainly alpha1,6-linked to the innermost N-acetylglucosaminyl residue of the core. Many of these structures were missing the terminal N-acetylglucosamine, which has been shown to be a prerequisite for processing to complex N-glycans in the Golgi. In some species (Planorbarius corneus and Achatina fulica) traces of large structures, terminated by 3-O-methylated galactoses and carrying xylose and/or fucose residues, were also detected. In Planorbarius viscera low amounts of terminal alpha1-2-fucosylation were determined. Combining these results, gastropods seem to be capable to produce all kinds of structures ranging from those typical in mammals through to structures similar to those found in plants, insects or nematodes. The detailed knowledge of this very complex glycosylation system of the gastropods will be a valuable tool to understand the principle rules of glycosylation in all organisms.

Molecular cloning and characterization of the Caenorhabditis elegans alpha1,3-fucosyltransferase family

The genome of Caenorhabditis elegans encodes five genes with homology to known alpha1,3 fucosyltransferases (alpha1,3FTs), but their expression and functions are poorly understood. Here we report the molecular cloning and characterization of these C. elegans alpha1,3FTs (CEFT-1 through -5). The open-reading frame for each enzyme predicts a type II transmembrane protein and multiple potential N-glycosylation sites. We prepared recombinant epitope-tagged forms of each CEFT and found that they had unusual acceptor specificity, cation requirements, and temperature sensitivity. CEFT-1 acted on the N-glycan pentasaccharide core acceptor to generate Manalpha1-3(Manalpha1-6)Manbeta1-4GlcNAcbeta1-4(Fucalpha1-3)GlcNAcbeta1-Asn. In contrast, CEFT-2 did not act on the pentasaccharide acceptor, but instead utilized a LacdiNAc acceptor to generate GalNAcbeta1-4(Fucalpha1-3)GlcNAcbeta1-3Galbeta1-4Glc, which is a novel activity. CEFT-3 utilized a LacNAc acceptor to generate Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-3Galbeta1-4Glc without requiring cations. CEFT-4 was similar to CEFT-3, but its activity was enhanced by some divalent cations. Recombinant CEFT-5 was well expressed, but did not act on available acceptors. Each CEFT was optimally active at room temperature and rapidly lost activity at 37 degrees C. Promoter analysis showed that CEFT-1 is expressed in C. elegans eggs and adults, but its expression was restricted to a few neuronal cells at the head and tail. We prepared deletion mutants for each enzyme for phenotypic analysis. While loss of CEFT-1 correlated with loss of pentasaccharide core activity and core alpha1,3-fucosylated glycans in worms, loss of other enzymes did not correlate with any phenotypic changes. These results suggest that each of the alpha1,3FTs in C. elegans has unique specificity and expression patterns.

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.

The role of protein glycosylation in allergy

The asparagine-linked carbohydrate moieties of plant and insect glycoproteins are the most abundant environmental immune determinants. They are the structural basis of what is known as cross-reactive carbohydrate determinants (CCDs). Despite some structural variation, the two main motifs are the xylose and the core-3-linked fucose, which form the essential part of two independent epitopes. Plants contain both epitopes, insect glycoproteins only fucose. These epitopes and other fucosylated determinants are also found in helminth parasites where they exert remarkable immunomodulatory effects. About 20% or more of allergic patients generate specific anti-glycan IgE, which is often accompanied by IgG. Even though antibody-binding glycoproteins are widespread in pollens, foods and insect venoms, CCDs do not appear to cause clinical symptoms in most, if not all patients. When IgE binding is solely due to CCDs, a glycoprotein allergen thus can be rated as clinical irrelevant allergen. Low binding affinity between IgE and plant N-glycans now drops out as a plausible explanation for the benign nature of CCDs. This rather may result from blocking antibodies induced by an incidental 'immune therapy' ('glyco-specific immune therapy') exerted by everyday contact with plant materials, e.g. fruits or vegetables. The need to detect and suppress anti-CCD IgE without interference from peptide epitopes can be best met by artificial glycoprotein allergens. Hydroxyproline-linked arabinose (single beta-arabinofuranosyl residues) has been identified as a new IgE-binding carbohydrate epitope in the major mugwort allergen. However, currently the occurrence of this O-glycan determinant appears to be rather restricted.

An antibody produced in tobacco expressing a hybrid beta-1,4-galactosyltransferase is essentially devoid of plant carbohydrate epitopes

N-glycosylation of a mAb may have a major impact on its therapeutic merits. Here, we demonstrate that expression of a hybrid enzyme (called xylGalT), consisting of the N-terminal domain of Arabidopsis thaliana xylosyltransferase and the catalytic domain of human beta-1,4-galactosyltransferase I (GalT), in tobacco causes a sharp reduction of N-glycans with potentially immunogenic core-bound xylose (Xyl) and fucose (Fuc) residues as shown by Western blot and MALDI-TOF MS analysis. A radioallergosorbent test inhibition assay with proteins purified from leaves of WT and these transgenic tobacco plants using sera from allergic patients suggests a significant reduction of potential immunogenicity of xylGalT proteins. A mAb purified from leaves of plants expressing xylGalT displayed an N-glycan profile that featured high levels of galactose, undetectable xylose, and a trace of fucose. Hence, a transgenic plant expressing the hybrid GalT might yield more effective and safer monoclonals for therapeutic purposes than WT plants and even transgenic plants expressing the unchanged GalT.

Neutral N-glycans of the gastropod Arion lusitanicus

The neutral N-glycan structures of Arion lusitanicus (gastropod) skin, viscera and egg glycoproteins were examined after proteolytic digestion, release of the glycans from the peptides, fluorescent labelling with 2-aminopyridine and fractionation by charge, size and hydrophobicity to obtain pure glycan structures. The positions and linkages of the sugars in the glycan were analysed by two dimensional HPLC (size and hydrophobicity) and MALDI-TOF mass spectrometry before and after digestion with specific exoglycosidases. The most striking feature in the adult tissues was the high amount of oligomannosidic and small paucimannosidic glycans terminated with 3-O-methylated mannoses. The truncated structures often contained modifications of the inner core by beta1,2-linked xylose to the beta-mannose residue and/or an alpha-fucosylation (mainly alpha1,6-) of the innermost GlcNAc residue. Skin and viscera showed predominantly the same glycans, however, in different amounts. Traces of large structures carrying 3-O-methylated galactoses were also detected. The egg glycans contained mainly (approximately 75%) oligomannosidic structures and some paucimannosidic structures modified by xylose or alpha1,6-fucose, but in this case no methylation of any monosaccharide was detected. Thus, gastropods seem to be capable of producing many types of structures ranging from those typical in human to structures similar to those found in nematodes, and therefore will be a valuable model to understand the regulation of glycosylation. Furthermore, this opens the way for using this organism as a host for the production of recombinant proteins. The detailed knowledge on glycosylation also may help to identify targets for pest control.

Plant members of the alpha1-->3/4-fucosyltransferase gene family encode an alpha1-->4-fucosyltransferase, potentially involved in Lewis(a) biosynthesis, and two core alpha1-->3-fucosyltransferases

Three putative alpha1-->3/4-fucosyltransferase (alpha1-->3/4-FucT) genes have been detected in the Arabidopsis thaliana genome. The products of two of these genes have been identified in vivo as core alpha1-->3-FucTs involved in N-glycosylation. An orthologue of the third gene was isolated from a Beta vulgaris cDNA library. The encoded enzyme efficiently fucosylates Galbeta1-->3GlcNAcbeta1-->3Galbeta1-->4Glc. Analysis of the product by 400 MHz (1)H-nuclear magnetic resonance spectroscopy showed that the product is alpha1-->4-fucosylated at the N-acetylglucosamine residue. In vitro, the recombinant B. vulgaris alpha1-->4-FucT acts efficiently only on neutral type 1 chain-based glycan structures. In plants the enzyme is expected to be involved in Lewis(a) formation on N-linked glycans.

N-Glycan analysis by matrix-assisted laser desorption/ionization mass spectrometry of electrophoretically separated nonmammalian proteins: application to peanut allergen Ara h 1 and olive pollen allergen Ole e 1

A method has been developed which allows the analysis of glycoproteins separated by SDS-PAGE. The procedure, though applicable to N-glycosylated glycoproteins of any origin, is particularly devised for glycoproteins potentially containing fucose in alpha1,3-linkage to the reducing GlcNAc as may be found in plants and invertebrates, e.g., insects and parasitic helminths. Starting with an established procedure for mass spectrometric peptide mapping, the analysis of N-glycans by matrix-assisted laser desorption/ionization mass spectrometry involved the use of peptide:N-glycosidase A, a triphasic microcolumn for sample cleanup, and a new matrix mixture consisting of 2,5-dihyhydroxybenzoic acid, 1-hydroxyisoquinoline, and arabinosazone. The method was tested on proteins with N-glycans of known structure, i.e., as horseradish peroxidase, zucchini ascorbate oxidase, soybean agglutinin, honeybee venom hyaluronidase, bovine ribonuclease B, and bovine fetuin. An electrophoretic band corresponding to 4 microg of glycoprotein was generally sufficient to allow detection of the major N-glycan species. As an additional benefit, a peptide mass map is generated which serves to identify the analyzed protein. The method was applied to glycoprotein allergens whose glycan structures were unknown. Ara h 1 and Ole e 1, major allergens from peanut and olive pollen, respectively, contained mainly xylosylated N-glycans with the composition Man(3(-4))XylGlcNAc(2) in the case of Ara h 1 and GlcNAc(1-2)Man(3)XylGlcNAc(2) in the case of Ole e 1 where also some GlcNAc(0-2)Man(3)XylFucGlcNAc(2) was found.