Heterogeneity of the complex N-linked oligosaccharides at specific glycosylation sites of two secreted carrot glycoproteins

Tomato Allergy: The Characterization of the Selected Allergens and Antioxidants of Tomato (Solanum lycopersicum)-A Review

Tomatoes are one of the most broadly produced and consumed crop plants. They are the source of health-promoting nutrients such as antioxidants, including ascorbic acid, polyphenols, or carotenoids. Despite the beneficial role of tomatoes in the daily diet, they have been confirmed as one of the most prevalent allergenic vegetables. Food allergies can cause many clinical symptoms, e.g., in the gastrointestinal tract, skin, and lungs, as well as anaphylactic shock. A huge amount of clinical research has been carried out to improve the understanding of the immunological mechanisms that lead to the lack of tolerance of food antigens, which can result in either immunoglobulin E (IgE)-mediated reactions or non-IgE-mediated reactions. Lifestyle and diet play an important role in triggering food allergies. Allergy to tomatoes is also linked to other allergies, such as grass pollen and latex allergy. Numerous attempts have been made to identify and characterize tomato allergens; however, the data available on the subject are not sufficient.

Functional characterization of a vacuolar invertase from Solanum lycopersicum: post-translational regulation by N-glycosylation and a proteinaceous inhibitor

Plant vacuolar invertases, which belong to family 32 of glycoside hydrolases (GH32), are key enzymes in sugar metabolism. They hydrolyse sucrose into glucose and fructose. The cDNA encoding a vacuolar invertase from Solanum lycopersicum (TIV-1) was cloned and heterologously expressed in Pichia pastoris. The functional role of four N-glycosylation sites in TIV-1 has been investigated by site-directed mutagenesis. Single mutations to Asp of residues Asn52, Asn119 and Asn184, as well as the triple mutant (Asn52, Asn119 and Asn184), lead to enzymes with reduced specific invertase activity and thermostability. Expression of the N516D mutant, as well as of the quadruple mutant (N52D, N119D, N184D and N516D) could not be detected, indicating that these mutations dramatically affected the folding of the protein. Our data indicate that N-glycosylation is important for TIV-1 activity and that glycosylation of N516 is crucial for recombinant enzyme stability. Using a functional genomics approach a new vacuolar invertase inhibitor of S. lycopersicum (SolyVIF) has been identified. SolyVIF cDNA was cloned and heterologously expressed in Escherichia coli. Specific interactions between SolyVIF and TIV-1 were investigated by an enzymatic approach and surface plasmon resonance (SPR). Finally, qRT-PCR analysis of TIV-1 and SolyVIF transcript levels showed a specific tissue and developmental expression. TIV-1 was mainly expressed in flowers and both genes were expressed in senescent leaves.

Glycosylation and functionality of recombinant β-glucocerebrosidase from various production systems

The glycosylation of recombinant β-glucocerebrosidase, and in particular the exposure of mannose residues, has been shown to be a key factor in the success of ERT (enzyme replacement therapy) for the treatment of GD (Gaucher disease). Macrophages, the target cells in GD, internalize β-glucocerebrosidase through MRs (mannose receptors). Three enzymes are commercially available for the treatment of GD by ERT. Taliglucerase alfa, imiglucerase and velaglucerase alfa are each produced in different cell systems and undergo various post-translational or post-production glycosylation modifications to expose their mannose residues. This is the first study in which the glycosylation profiles of the three enzymes are compared, using the same methodology and the effect on functionality and cellular uptake is evaluated. While the major differences in glycosylation profiles reside in the variation of terminal residues and mannose chain length, the enzymatic activity and stability are not affected by these differences. Furthermore, the cellular uptake and in-cell stability in rat and human macrophages are similar. Finally, in vivo studies to evaluate the uptake into target organs also show similar results for all three enzymes. These results indicate that the variations of glycosylation between the three regulatory-approved β-glucocerebrosidase enzymes have no effect on their function or distribution.

Beta-N-acetylhexosaminidases HEXO1 and HEXO3 are responsible for the formation of paucimannosidic N-glycans in Arabidopsis thaliana

Most plant glycoproteins contain substantial amounts of paucimannosidic N-glycans instead of their direct biosynthetic precursors, complex N-glycans with terminal N-acetylglucosamine residues. We now demonstrate that two β-N-acetylhexosaminidases (HEXO1 and HEXO3) residing in different subcellular compartments jointly account for the formation of paucimannosidic N-glycans in Arabidopsis thaliana. Total N-glycan analysis of hexo knock-out plants revealed that HEXO1 and HEXO3 contribute equally to the production of paucimannosidic N-glycans in roots, whereas N-glycan processing in leaves depends more heavily on HEXO3 than on HEXO1. Because hexo1 hexo3 double mutants do not display any obvious phenotype even upon exposure to different forms of abiotic or biotic stress, it should be feasible to improve the quality of glycoprotein therapeutics produced in plants by down-regulation of endogenous β-N-acetylhexosaminidase activities.

Plant glycosidases acting on protein-linked oligosaccharides

Glycosidases have been used as invaluable tools in glycobiology research for decades, and their role in glycoprotein maturation has been amply studied. The molecular biological coverage of this large group of enzymes has only recently reached an appreciable level. In this review, we present an overview of plant glycosidases, whose DNA/protein sequence has been identified and for which recombinant enzymes have been characterized. The physiological role in the maturation of glycoproteins is discussed as well as the biotechnological prospects arising from knowing the enzymes responsible for the removal of terminal N-acetylglucosamine residues. The current knowledge on plant fucosidases and of the first bits of information on glycosidases acting on arabinogalactan proteins is presented.

Enzymatic properties and subcellular localization of Arabidopsis beta-N-acetylhexosaminidases

Plant glycoproteins contain substantial amounts of paucimannosidic N-glycans lacking terminal GlcNAc residues at their nonreducing ends. It has been proposed that this is due to the action of beta-hexosaminidases during late stages of N-glycan processing or in the course of N-glycan turnover. We have now cloned the three putative beta-hexosaminidase sequences present in the Arabidopsis (Arabidopsis thaliana) genome. When heterologously expressed as soluble forms in Spodoptera frugiperda cells, the enzymes (termed HEXO1-3) could all hydrolyze the synthetic substrates p-nitrophenyl-2-acetamido-2-deoxy-beta-d-glucopyranoside, p-nitrophenyl-2-acetamido-2-deoxy-beta-d-galactopyranoside, 4-methylumbelliferyl-2-acetamido-2-deoxy-beta-d-glucopyranoside, and 4-methylumbelliferyl-6-sulfo-2-acetamido-2-deoxy-beta-d-glucopyranoside, albeit to a varying extent. HEXO1 to HEXO3 were further able to degrade pyridylaminated chitotriose, whereas pyridylaminated chitobiose was only cleaved by HEXO1. With N-glycan substrates, HEXO1 displayed a much higher specific activity than HEXO2 and HEXO3. Nevertheless, all three enzymes were capable of removing terminal GlcNAc residues from the alpha1,3- and alpha1,6-mannosyl branches of biantennary N-glycans without any strict branch preference. Subcellular localization studies with HEXO-fluorescent protein fusions transiently expressed in Nicotiana benthamiana plants showed that HEXO1 is a vacuolar protein. In contrast, HEXO2 and HEXO3 are mainly located at the plasma membrane. These results indicate that HEXO1 participates in N-glycan trimming in the vacuole, whereas HEXO2 and/or HEXO3 could be responsible for the processing of N-glycans present on secretory glycoproteins.

A retrospective look at the cationic peanut peroxidase structure

The cationic peanut peroxidase has been studied in detail, not only with regard to its peptide structure, but also to the sites and role of the three moieties linked to it. Peanut peroxidase lends itself well to a close examination as a potential example for other plant peroxidase studies. It was the first plant peroxidase for which a 3-D structure was derived from crystals, with the glycans intact. Subsequent analysis of peroxidases structures from other plants have not shown great differences to that of the peanut peroxidase. As the period of proteomics follows on the era of genomics, the study of glycans has been brought back into focus. With the potential use of peroxidase as a polymerization agent for industry, there are some aspects of the overall structure that should be kept in mind for successful use of this enzyme. A variety of techniques are now available to assay for these structures/moieties and their roles. Peanut peroxidase data are reviewed in that light, as well as defining some true terms for isozymes. Because a high return of the enzyme in a pure form has been obtained from cultured cells in suspension culture, a brief review of this is also offered.

Biological activity of IgE specific for cross-reactive carbohydrate determinants

BACKGROUND

The clinical relevance of IgE specific for cross-reactive carbohydrate determinants (CCDs) has been a matter of controversy. Until now, no convincing experiments have been performed to test the biologic significance of individual multivalent allergens that carry multiple carbohydrate epitopes.

OBJECTIVE

We sought to contribute to the understanding of the role of CCD-specific IgE antibodies and to study whether CCD-specific IgE antibodies are able to activate basophils using different multivalent glycoproteins as antigens.

METHODS

Purified natural tomato beta-fructofuranosidase (nLyc e 2) and rLyc e 2.02 expressed in Escherichia coli were compared by means of histamine release tests. In addition, native and deglycosylated horseradish peroxidase and a neoglycoprotein consisting of a defined glycopeptide (Manalpha1-6[Xylbeta1-2]Manbeta1-4GlcNAcbeta1-4[Fucalpha1-3]GlcNAc) coupled to BSA were used in histamine release assays using stripped basophils from normal donors resensitized with IgE from CCD-reactive patients with food allergy to tomato.

RESULTS

Ten CCD-positive and 2 CCD-negative sera from patients with tomato allergy underwent histamine release testing by using the glycoproteins and nonglycosylated controls as antigens, respectively. All sera induced histamine release with tomato extract (up to 100%), confirming the allergic status of the donors. Four of the CCD-positive sera induced releases ranging from 12% to 82% with all of the glycoproteins but not with the nonglycosylated or monovalent controls. All other sera showed no response or only very weak response to the glycoproteins.

CONCLUSION

Approximately one third of the CCD-positive sera from patients with tomato allergy have biologically relevant CCD-specific IgE antibodies. Therefore the general claim that CCD-specific IgE is clinically irrelevant has to be reconsidered critically. Hence IgE specific for CCDs should be taken into consideration in the diagnosis and therapy of certain allergies. In the subgroup of patients sensitized to CCDs, the use of natural allergens should be preferred over the use of recombinant allergens expressed in prokaryotic organisms.

Molecular characterization and allergenic activity of Lyc e 2 (beta-fructofuranosidase), a glycosylated allergen of tomato

Until now, only a small amount of information is available about tomato allergens. In the present study, a glycosylated allergen of tomato (Lycopersicon esculentum), Lyc e 2, was purified from tomato extract by a two-step FPLC method. The cDNA of two different isoforms of the protein, Lyc e 2.01 and Lyc e 2.02, was cloned into the bacterial expression vector pET100D. The recombinant proteins were purified by electroelution and refolded. The IgE reactivity of both the recombinant and the natural proteins was investigated with sera of patients with adverse reactions to tomato. IgE-binding to natural Lyc e 2 was completely inhibited by the pineapple stem bromelain glycopeptide MUXF (Man alpha 1-6(Xyl beta 1-2)Man beta 1-4GlcNAc beta 1-4(Fuc alpha 1-3)GlcNAc). Accordingly, the nonglycosylated recombinant protein isoforms did not bind IgE of tomato allergic patients. Hence, we concluded that the IgE reactivity of the natural protein mainly depends on the glycan structure. The amino acid sequences of both isoforms of the allergen contain four possible N-glycosylation sites. By application of MALDI-TOF mass spectrometry the predominant glycan structure of the natural allergen was identified as MMXF (Man alpha 1-6(Man alpha 1-3)(Xyl beta 1-2)Man beta 1-4GlcNAc beta 1-4(Fuc alpha 1-3) GlcNAc). Natural Lyc e 2, but not the recombinant protein was able to trigger histamine release from passively sensitized basophils of patients with IgE to carbohydrate determinants, demonstrating that glycan structures can be important for the biological activity of allergens.

Glycoproteins secreted from suspension-cultured tobacco BY2 cells have distinct glycan structures from intracellular glycoproteins

Glycan structures of glycoproteins secreted in the spent medium of tobacco BY2 suspension-cultured cells were analyzed. The N-glycans were liberated by hydrazinolysis and the resulting oligosaccharides were labeled with 2-aminopyridine. The pyridylaminated (PA) glycans were purified by reversed-phase and size-fractionation HPLC. The structures of the PA sugar chains were identified by a combination of the two-dimensional PA sugar chain mapping, MS analysis, and exoglycosidase digestion. The ratio (40:60) of the amount of glycans with high-mannose-type structure to that with plant-complex-type structure of extracellular glycoproteins is significantly different from that (ratio 10:90) previously found in intracellular glycoproteins [Palacpac et al., Biosci. Biotechnol. Biochem. 63 (1999) 35-39]. Extracellular glycoproteins have six distinct N-glycans (marked by *) from intracellular glycoproteins, and the high-mannose-type structures account for nearly 40% (Man5GlcNAc2, 28.8%; Man6GlcNAc2*, 6.4%; and Man7GlcNAc2*, 3.8%), while the plant-complex-type structures account for nearly 60% (GlcNAc2Man3Xyl1GlcNAc2*, 32.1%; GlcNAc1Man3Xyl1GlcNAc2 (containing two isomers)*, 6.2%; GlcNAc2Man3GlcNAc2*, 4.9%; Man3Xyl1Fuc1GlcNAc2, 8.3%; and Man3Xyl1GlcNAc2, 3.7%).

Small complex-type N-linked glycans are attached to cell-wall bound exo-beta-glucanases of both mung bean and barley seedlings

N-linked glycans of wall-bound exo-beta-glucanases from mung bean and barley seedlings, namely Mung-ExoI and Barley-ExoII, were characterized. The N-linked glycans of Mung-ExoI and Barley-ExoII were liberated by gas-phase hydrazinolysis followed by re-N-acetylation. Their structures were determined by two-dimensional sugar-mapping analysis and MALDI-TOF mass spectrometry. N-glycans from both glucanases were of paucimannosidic-type (small complex-type) structures, Manalpha1-6(+/-Manalpha1-3)(Xylbeta1-2)Manbeta1-4GlcNAcbeta1-4(+/-Fucalpha1-3) GlcNAc, which are known as typical vacuole-type N-glycans. The results suggest that N-glycans of cell-wall glucanase were produced by partial trimming of complex-type N-glycans by exoglycosidases during its transport from Golgi apparatus to cell walls or in the cell walls.

N- and O-linked oligosaccharides of allergenic glycoproteins

Cross-linking of cell-bound IgE on mast cells or basophils by polyvalent antigens causes the release of histamine and other mediators of the allergic response which then lead to the development of allergic symptoms. In this event not only peptide epitopes, but also carbohydrates can act as cross-linking elements. Since peptide epitopes of allergens are subject of most published studies, this review is focused on glycosidic epitopes. The current knowledge of the structures and possible epitopes of oligosaccharides linked to allergenic glycoproteins is briefly reviewed, showing that complex plant N-glycans containing alpha1,3 fucose and beta1,2 xylose are most frequently involved in the structures of IgE epitopes. In own studies a prevalence of up to 29% anti-glycan IgE was determined among pollen-allergic patients. The clinical relevance of these carbohydrate specific IgE antibodies is still a matter of controversial discussions.

Analysis of Asn-linked glycans from vegetable foodstuffs: widespread occurrence of Lewis a, core 1,3-linked fucose and xylose substitutions

The N-glycans from 27 "plant" foodstuffs, including one from a gymnospermic plant and one from a fungus, were prepared by a new procedure and examined by means of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). For several samples, glycan structures were additionally investigated by size-fractionation and reverse-phase high-performance liquid chromatography in conjunction with exoglycosidase digests and finally also (1)H-nuclear magnetic resonance spectroscopy. The glycans found ranged from the typical vacuolar "horseradish peroxidase" type and oligomannose to complex Le(a)-carrying structures. Though the common mushroom exclusively contained N-glycans of the oligomannosidic type, all plant foods contained mixtures of the above-mentioned types. Apple, asparagus, avocado, banana, carrot, celery, hazelnut, kiwi, onion, orange, pear, pignoli, strawberry, and walnut were particularly rich in Le(a)-carrying N-glycans. Although traces of Le(a)-containing structures were also present in almond, pistachio, potato, and tomato, no such glycans could be found in cauliflower. Coconut exhibited almost exclusively N-glycans containing only xylose but no fucose. Oligomannosidic N-glycans dominated in buckwheat and especially in the legume seeds mung bean, pea, peanut, and soybean. Papaya presented a unique set of hybrid type structures partially containing the Le(a) determinant. These results are not only compatible with the hypothesis that the carbohydrate structures are another potential source of immunological cross-reaction between different plant allergens, but they also demonstrate that the Le(a)-type structure is very widespread among plants.

A monoclonal antibody specific for a carbohydrate epitope recognizes an IgE-binding determinant shared by taxonomically unrelated allergenic pollens

Carbohydrate epitopes are capable of binding human IgE from allergic subjects and these epitopes play a role in the cross-reactivity between allergens from unrelated sources. A monoclonal antibody (5E6), specific for a carbohydrate epitope detectable on components of Cupressus arizonica pollen extract, has been produced and characterized. To study the relationship between the epitopes recognized by the monoclonal antibody and by IgE from allergic subjects. To investigate the presence of such carbohydrate IgE determinant in extracts from 21 pollen species belonging to 16 taxonomically related and unrelated families, by means of the monoclonal antibody. IgG-depleted fraction from protein G-purified human allergic serum was obtained. The monoclonal antibody and the IgE from the purified fraction were tested on two glycoproteins, polyamine oxidase and ascorbate oxidase, adsorbed on the ELISA plates. The relationship between the monoclonal- and the IgE-recognized epitopes was investigated by ELISA-competition experiments. Analysis of the distribution of this carbohydrate epitope was performed by direct binding of the monoclonal antibody onto the various extracts. The monoclonal antibody and the IgE were able to bind carbohydrate epitopes on the two plant glycoproteins, ascorbate oxidase and polyamine oxidase. Polyamine oxidase shows only one N-glycosilation site whose carbohydrate moiety seems to be composed of a branched chain of seven ordered sugars, i.e. two N-acetyl-D-glucosamine-, three mannose-, one fucose- and one xylose-residues. This structure bears the epitope recognized by mAb 5E6. Human IgE from the IgG-depleted fraction were found capable of inhibiting the monoclonal antibody binding. The allergenic epitope identified was shared by a large number of extracts with different levels of reactivity (OD490 ranging from 0.110 to 2.060). Our data support the finding that a monoclonal antibody specific for a carbohydrate epitope of Cupressus arizonica pollen extract detects an epitope which is also recognized by IgE from allergic subjects. This characterized reagent could be a useful tool for studying distribution of cross-reactive carbohydrate determinants in allergenic pollen extracts and their components.

Synthesis of allyl 2-O-(alpha-L-arabinofuranosyl)-6-O-(alpha-D-mannopyranosyl)-beta-D-mannopyranoside, a unique plant N-glycan motif containing arabinose

The synthesis of the trisaccharide allyl 2-O-(alpha-L-arabinofuranosyl)-6-O-(alpha-D-mannopyranosyl)-beta-D-mannopyra-noside is reported. Stereoselective glycosylation at C-6 of a non-protected allyl beta-mannoside with the acetylated alpha-D-mannosyl bromide gave the alpha-(1 --> 6)-disaccharide as the main product and the crystalline 3,6-branched trisaccharide as minor compound. Further glycosylation of the 2,3 diol (1 --> 6) disaccharide with L-arabinofuranosyl bromide furnished a mixture of 3-O- and 2-O-alpha-L-Ara-trisaccharides from which the title compound was isolated.

Structural analysis of N-glycans from yellow lupin (Lupinus luteus) seed diphosphonucleotide phosphatase/phosphodiesterase

N-linked oligosaccharide chains released by hydrazinolysis from yellow lupin seed diphosphonucleotide phosphatase/phosphodiesterase were fluorescence labeled and separated by high performance liquid chromatography (GlycoSep N and GlycoSep H columns). Exoglycosidase sequencing elucidated the structures of 24 separated N-glycans. Thirty percent of isolated glycans were found to be of high-mannose type (three to eight mannosyl residues), 42% were complex type and 26% belonged to paucimannosidic type. Among complex type glycans, structures with Lewis(a) epitope were identified. It is very unusual to find all types of plant N-glycans in one protein. Possible reasons for such a broad spectrum of N-glycan structures are discussed.

Amino-acid sequence and glycan structures of cysteine proteases with proline specificity from ginger rhizome Zingiber officinale

The ginger proteases (GP-I and GP-II), isolated from the ginger rhizome Zingiber officinale, have an unusual substrate specificity preference for cleaving peptides with a proline residue at the P2 position. The complete amino-acid sequence of GP-II, a glycoprotein containing 221 amino acids, and about 98% that of GP-I have been determined. Both proteases, which are 82% similar, have cysteine residues at positions 27 and histidines at position 161, corresponding to the essential cysteine-histidine diads found in the papain family of cysteine proteases, and six corresponding cysteine residues that form the three invariant disulfide linkages seen in this family of proteins. The sequence homology with other members (papain, bromelain, actinidin, protease omega, etc.) of this family is approximately 50%. GP-II has two predicted glycosylation sites at Asn99 and Asn156. Analyisis by electrospray and collision-induced dissociation MS showed that both sites were occupied by the glycans (Man)3(Xyl)1(Fuc)1(GlcNAc)2 and (Man)3(Xyl)1(Fuc)1(GlcNAc)3, in a ratio of approximately 7 : 1. Both glycans are xylose containing biantennary complex types that share the common core structural unit, Man1-->6(Man1-->3) (Xyl1-->2)Man1-->4GlcNAc1-->4(Fuc1-->3)GlcNAc for the major form, with an additional N-acetylglucosamine residue being linked, in the minor form, to one of the terminal mannose units of the core structure.

Sequence specific analysis of the heterogeneous glycan chain from peanut peroxidase by 1H-NMR spectroscopy

The cationic peanut peroxidase is a complex enzyme consisting of a heme group, two calcium ions and three complex carbohydrate chains at positions Asn60, 144 and 185. Details of the heme and calcium ligation, necessary for oxidation, have recently been revealed from the three-dimensional structure of the peroxidase. However, the three glycans that may be important for the stability of the enzyme as well as its activity were not resolved. In order to determine the configuration of one of these glycans, PNGase A was used to cleave the glycan from the enzyme at Asn-144. This glycan was studied by two dimensional 1H-NMR spectroscopy to identify the sugar linkages. The results indicated a glycan structure comprising a Man alpha1-6(Xyl beta1-2)Man beta1-4GlcNAc beta1-4(Fuc alpha1-3)GlcNAc beta core but with an additional Man alpha1-3 appendage linked to Man3. The glycan also appeared to be heterogeneous as was noted from a single terminating galactose being linked to approximately 20-25% glycan.

Structure, properties, and tissue localization of apoplastic alpha-glucosidase in crucifers

Apoplastic alpha-glucosidases occur widely in plants but their function is unknown because appropriate substrates in the apoplast have not been identified. Arabidopsis contains at least three alpha-glucosidase genes; Aglu-1 and Aglu-3 are sequenced and Aglu-2 is known from six expressed sequence tags. Antibodies raised to a portion of Aglu-1 expressed in Escherichia coli recognize two proteins of 96 and 81 kD, respectively, in vegetative tissues of Arabidopsis, broccoli (Brassica oleracea L.), and mustard (Brassica napus L.). The acidic alpha-glucosidase activity from broccoli flower buds was purified using concanavalin A and ion-exchange chromatography. Two active fractions were resolved and both contained a 96-kD immunoreactive polypeptide. The N-terminal sequence from the 96-kD broccoli alpha-glucosidase indicated that it corresponds to the Arabidopsis Aglu-2 gene and that approximately 15 kD of the predicted N terminus was cleaved. The 81-kD protein was more abundant than the 96-kD protein, but it was not active with 4-methylumbelliferyl-alpha-D-glucopyranoside as the substrate and it did not bind to concanavalin A. In situ activity staining using 5-bromo-4-chloro-3-indolyl-alpha-D-glucopyranoside revealed that the acidic alpha-glucosidase activity is predominantly located in the outer cortex of broccoli stems and in vascular tissue, especially in leaf traces.

Structures of N-linked oligosaccharides of glycoproteins from tobacco BY2 suspension cultured cells

The structures of N-linked sugar chains of glycoproteins expressed in tobacco BY2 cultured cells are reported. Five pyridylaminated (PA-) N-linked sugar chains were derived and purified from hydrazinolysates of the glycoproteins by reversed-phase HPLC and size-fractionation HPLC. The structures of the PA-sugar chains purified were identified by two-dimensional PA-sugar chain mapping, ion-spray MS/MS analysis, and exoglycosidase digestions. The five structures fell into two categories; the major class (92.5% as molar ratio) was a xylose containing-type (Man3Fuc1 Xyl1GlcNAc2 (41.0%), GlcNAc2Man3Fuc1Xyl1GlcNAc2 (26.5%), GlcNAc1Man3Fuc1Xyl1GlcNAc2 (21.7%), Man3 Xyl1GlcNAc2 (3.3%)), and the minor class was a high-mannose type (Man5GlcNAc2 (7.5%)). This is the first report to show that alpha(1-->3) fucosylation of N-glycans does occur but beta(1-->4) galactosylation of the sugar chains does not in the tobacco cultured cells.

Oligosaccharide and polypeptide homology of lupin (Lupinus luteus L.) acid phosphatase subunits

Peptide mapping of lupin acid phosphatase clearly demonstrated the homology between its two subunits. Sequenced tryptic peptides also showed 78% identity (92% similarity) to the red bean acid phosphatase. Peptides exclusive for the 50-kDa subunit are homologous to N-terminally located sequences in red bean acid phosphatase, leading to the assumption that the shorter subunit of lupin acid phosphatase is generated by the deletion of the N-terminal part of the longer subunit. Carbohydrate moiety was found to be identical in both subunits. Oligosaccharide chains released by hydrazinolysis from the both subunits were fluorescently labeled and separated by HPLC. The structure of oligosaccharides was elucidated by exoglycosidase sequencing. Seventeen percent of isolated glycans were found to be of the high-mannose type, while the rest belonged to plant complex-type structures. Most of the complex glycans were fucosylated and xylosylated; some were fucosylated or xylosylated only.

IgE antibodies specific for carbohydrates in a patient allergic to gum arabic (Acacia senegal)

The present study deals with the detailed investigation of the IgE antibody response of a gum arabic-allergic patient. The patient showed multiple serologic and skin test sensitizations to a range of pollen, other inhalants and foods, and bee venom, and to the recombinant allergens Bet v 1 and Bet v 2. Moreover, the patient's serum reacted strongly to gum-arabic extract. The NaIO4-treated and thus deglycosylated extract showed no binding to IgE. In contrast, removal of the protein backbone by basic hydrolysis did not deplete the IgE reactivity. Therefore, it is concluded that the gum arabic-specific IgE antibodies of this patient were mainly directed against the carbohydrate fraction of this material. In IgE-inhibition assays, cross-reactions occurred in the range of 60% between gum arabic and known immunogenic N-glycans containing alpha1-3-linked fucose. Since the inhibition graphs were not parallel and the inhibition was not complete with heterologue antigens, the cross-reacting epitopes of gum arabic appeared to be different from the latter well-known cross-reactive carbohydrate determinants (CCD). Inhibition may have been caused by a partial immunologic identity of the investigated carbohydrate moieties. A strong IgE response to the fucose-containing glycan from bromelain was measured in a glycan ELISA that utilizes purified glycopeptides at the solid phase. This response, which may explain the multiple sensitizations without clinical significance diagnosed in the patient, could originate from inhalation of pollen, which is known to contain similar glycans, or from occupational sensitization during work as a baker and confectioner. Since the gum-arabic protein showed only very weak participation in the IgE reactivity, the clinical symptoms of the patient caused by gum arabic may be attributed to carbohydrate epitopes. Due to the repetitive polysaccharide sequence of gum arabic, several epitopes for the cross-linking of IgE should exist.

N-glycoprotein biosynthesis in plants: recent developments and future trends

N-glycosylation is a major modification of proteins in plant cells. This process starts in the endoplasmic reticulum by the co-translational transfer of a precursor oligosaccharide to specific asparagine residues of the nascent polypeptide chain. Processing of this oligosaccharide into high-mannose-type, paucimannosidic-type, hybrid-type or complex-type N-glycans occurs in the secretory pathway as the glycoprotein moves from the endoplasmic reticulum to its final destination. At the end of their maturation, some plant N-glycans have typical structures that differ from those found in their mammalian counterpart by the absence of sialic acid and the presence of beta(1,2)-xylose and alpha( 1,3)-fucose residues. Glycosidases and glycosyltransferases that respectively catalyse the stepwise trimming and addition of sugar residues are generally considered as working in a co-ordinated and highly ordered fashion to form mature N-glycans. On the basis of this assembly line concept, fast progress is currently made by using N-linked glycan structures as milestones of the intracellular transport of proteins along the plant secretory pathway. Further developments of this approach will need to more precisely define the topological distribution of glycosyltransferases within a plant Golgi stack. In contrast with their acknowledged role in the targeting of lysosomal hydrolases in mammalian cells, N-glycans have no specific function in the transport of glycoproteins into the plant vacuole. However, the presence of N-glycans, regardless of their structures, is necessary for an efficient secretion of plant glycoproteins. In the biotechnology field, transgenic plants are rapidly emerging as an important system for the production of recombinant glycoproteins intended for therapeutic purposes, which is a strong motivation to speed up research in plant glycobiology. In this regard, the potential and limits of plant cells as a factory for the production of mammalian glycoproteins will be illustrated.

Core alpha1,3-fucose is a key part of the epitope recognized by antibodies reacting against plant N-linked oligosaccharides and is present in a wide variety of plant extracts

Carbohydrates have been suggested to account for some IgE cross-reactions between various plant, insect, and mollusk extracts, while some IgG antibodies have been successfully raised against plant glycoproteins. A rat monoclonal antibody raised against elderberry abscission tissue (YZ1/2.23) and rabbit polyclonal antiserum against horseradish peroxidase were screened for reactivity in enzyme-linked immunosorbent assay against a range of plant glycoproteins and extracts as well as neoglycoproteins, bee venom phospholipase, and several animal glycoproteins. Of the oligosaccharides tested, Man3XylFucGlcNAc2(MMXF3) derived from horseradish peroxidase was the most potent inhibitor of the reactivity of both YZ1/2.23 and anti-horseradish peroxidase to native horseradish peroxidase glycoprotein. The reactivity of YZ1/2. 23 and anti-horseradish peroxidase against Sophora japonica lectin was most inhibited by a neoglycoconjugate of bromelain glycopeptide cross-linked to bovine serum albumin, while the defucosylated form of this conjugate was inactive as an inhibitor. A wide range of plant extracts was found to react against YZ1/2.23 and anti-horseradish peroxidase, with particularly high reactivities recorded for grass pollen and nut extracts. All these reactivities were inhibitable with the bromelain glycopeptide/bovine serum albumin conjugate. Bee venom phospholipase and whole bee venom reacted weakly with YZ1/2.23 but more strongly with anti-horseradish peroxidase in a manner inhibitable with the bromelain glycopeptide/bovine serum albumin conjugate, while hemocyanin from Helix pomatia reacted poorly with YZ1/2.23 but did react with anti-horseradish peroxidase. It is concluded that the alpha1, 3-fucose residue linked to the chitobiose core of plant glycoproteins is the most important residue in the epitope recognized by the two antibodies studied, but that the polyclonal anti-horseradish peroxidase antiserum also contains antibody populations that recognize the xylose linked to the core mannose of many plant and gastropod N-linked oligosaccharides.

Glycans of higher plant peroxidases: recent observations and future speculations

Plant peroxidases are composed of a peptide and associated heme, calcium and glycans. The 3D structure of the major cationic peanut peroxidase has revealed the sites of the heme and calcium. But the diffraction of the glycans was not sufficient to show their structure. This review presents research that has been executed to obtain putative glycans and their binding sites, and to gain an indirect insight into these glycans. It also offers approaches that will be used to determine the function of the glycans on the peanut peroxidase. Some comparisons are made with other plant glycoproteins including peroxidases from plants other than peanut.

Purification and characterization of acid phosphatase from yellow lupin (Lupinus luteus) seeds

Acid phosphatase (EC 3.1.3.2) from yellow lupin (Lupinus luteus) seeds was purified to homogeneity by ammonium sulphate fractionation, affinity chromatography, cation-exchange chromatography, gel filtration or reverse-phase HPLC. The enzyme is a dimer with the 50 kD and 44 kD subunits and contains 7.3% of carbohydrate, forming at least four oligosaccharide chains. The optimum pH for the enzyme is 5.4. The apparent Km for p-nitrophenyl phosphate was estimated to be 0.28 mM and Vmax = 1780 IU/mg of protein. The purified phosphatase has the highest specific activities reported for any plant acid phosphatases measured for any native or synthetic substrate. The enzyme has broad specificity; however, cyclic nucleotides, pyrophosphate or phytate are not cleaved. It is inhibited by molybdate, fluoride and phosphate. There is no change in the enzyme activity in the presence of EDTA, phenanthroline and tartrate.

Cross-reactivity between the major allergen from olive pollen and unrelated glycoproteins: evidence of an epitope in the glycan moiety of the allergen

Ole e 1, the major allergen from olive pollen, is a glycoprotein containing a single Asn-linked glycan moiety. Rabbit antiserum against this protein has been obtained; and its immunologic cross-reactivities in Western blotting with ascorbate oxidase, horseradish peroxidase, bromelain, ovalbumin, and honeybee venom phospholipase A2 have been studied. Ascorbate oxidase, peroxidase, and bromelain are recognized by the Ole e 1 antiserum. When these three proteins are deglycosylated by periodate treatment, such an immunologic reaction does not occur. The relative affinities of these proteins have been analyzed by direct and inhibition ELISA experiments. A commercially available antibody against horseradish peroxidase has also been considered in these studies. This antibody reacts with Ole e 1 but not with the periodate-deglycosylated allergen. Horseradish peroxidase, bromelain, and ascorbate oxidase are recognized by the IgE of sera from patients who are hypersensitive to olive tree pollen. This binding is also abolished by periodate treatment. The results are interpreted in terms of the presence of an epitope in the carbohydrate moiety of Ole e 1, which would contain a xylose involved in recognition by both IgE and IgG antibodies.

Structural organization and differential expression of carrot beta-fructofuranosidase genes: identification of a gene coding for a flower bud-specific isozyme

Three genomic clones (Inv*Dc1, Inv*Dc2 and Inv*Dc3) were isolated by using the cDNA for carrot cell wall beta-fructofuranosidase as a probe. The expression patterns of the three genes differed markedly. High levels of Inv*Dc1 transcripts were found in leaves and roots of young carrot, whereas in plants with developing tap roots no transcripts were detected. A high level of mRNA of Inv*Dc1 was also present in suspension-cultured cells. In developing reproductive organs, only low levels of transcripts of Inv*Dc1 were found in flower buds and flowers and none at later stages of development. In contrast, Inv*Dc2 and Inv*Dc3 were not expressed in vegetative plant organs. Invb1*Dc1 was exclusively and strongly expressed in flower buds, and Inv*Dc3 at a very low level in suspension-cultured cells.

Immunogenicity of the N-glycans of peanut peroxidase

The three tryptic glycopeptides of cationic peanut peroxidase (C. PRX) and the sole one of anionic peanut peroxidase (A. PRX) were individually coupled to bovine serum albumin to raise antisera. The three categories of antibodies directed towards three N-glycans of C. PRX (anti-GLa, anti-GLb and anti-GLc) were isolated from antisera with glycan-conjugated ECH Sepharose 4B affinity columns and the distribution of epitopes on the N-glycans was investigated. The reactivity of anti-GLa, anti-GLb and anti-GLc is inhibited 25-40% by 1 M fucose, compared with a slight inhibition by N-acetylglycosamine and xylose. Mannose and galactose showed no inhibition to anti-GLa and only a slight inhibition to anti-GLb and anti-GLc. All of anti-GLa, anti-GLb and anti-GLc recognize A. PRX and horseradish peroxidase but do not recognize fetuin. Also, their reactivity is inhibited by bromelain by more than 70%. The three categories of antibodies present high homogeneity and appear to be directed mainly towards the core structure [Xyl] (Man)3 [Fuc] (GlcNAc)2. An effective and simple method to screen antibodies with carbohydrate specificities is described herein.

The oligosaccharides of the Fe(III)-Zn(II) purple acid phosphatase of the red kidney bean. Determination of the structure by a combination of matrix-assisted laser desorption/ionization mass spectrometry and selective enzymic degradation

Purple acid phosphatase of the common bean Phaseolus vulgaris (KBPase), a dimeric 110-kDa glycoprotein related to the mammalian purple acid phosphatases with a two-metal cluster at the active site contains five oligosaccharide side chains/monomer. The N-linked glycan structures were characterized by selective enzymic degradation in combination with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The purified protein was cleaved by cyanogen bromide. One 30-kDa large methionine-free fragment required a further tryptic digest. The peptides were separated by HPLC and the glycosylated species were identified both by their heterogeneous mass spectra and by an immunoassay. None of the glycopeptides proved to have more than one glycosylation site. The composition of the carbohydrate moieties were calculated by comparing the mass spectra of the glycopeptides before and after enzymic deglycosylation. These results were complemented by data from a carbohydrate composition analysis. In four of the five peptides an alpha 1-3 fucose attached to the asparagine-linked N-acetylglucosamine prevented removal of the glycan by peptide N-glycosidase F; peptide N-glycosidase A removed all carbohydrates from the peptides. To reveal the sequence of the carbohydrate moiety including the linkage positions between the different saccharides, one of the glycopeptides was degraded by specific exoglycosidases. The enzymic degradations by these hydrolases were monitored by mass spectrometry of small aliquots taken at intervals during the reaction. The detailed structure of this one glycan in conjunction with the respective mass spectra and the composition analysis were used to infer the structure of the other four glycans. All glycans of the KBPase have a complex-type xylose-containing structure with four of the five having an additional fucose.

Purification and characterization of a soluble beta-fructofuranosidase from Daucus carota

Soluble beta-fructofuranosidase with an intracellular location and an isoelectric point of 3.8 (isoenzyme I) was purified and characterized from dry seeds and seedlings of carrot (Daucus carota). The enzyme hydrolyzed sucrose with a Km of 5 mM and a broad pH optimum around 5.0. The purified protein, which was N-glycosylated with high-mannose-containing and high-xylose-containing complex glycans, eluted as a monomeric polypeptide with a molecular mass of 68,000 from a gel-filtration column. On SDS/PAGE, the protein separated in the presence of SDS and 2-mercaptoethanol into three polypeptides with molecular masses of 68, 43 and 25 kDa. The amount of the 68-kDa polypeptide was highest in dry seeds and decreased with increasing age of carrot seedlings. Amino acid sequence analysis and immunological studies showed that the 43-kDa and 25-kDa polypeptides were N-terminal and C-terminal proteolytic fragments of the 68-kDa polypeptide. A comparison of partial amino acid sequences of the soluble beta-fructofuranosidase with the complete sequence of carrot cell-wall beta-fructofuranosidase showed that their N-terminal sequences were different, whereas some of the internal tryptic peptide sequences were up to 70% identical.

1H-NMR structural determination of the N-linked carbohydrate chains on glycopeptides obtained from the bean lectin phytohemagglutinin

Phytohemagglutinin, the lectin of the common bean Phaseolus vulgaris, is a N-linked glycoprotein with one high-mannose-type and one xylose-containing oligosaccharide side chain per polypeptide. The high-mannose-type glycan is attached to Asn12 and the complex-type glycan to Asn60 [Sturm, A. & Chrispeels, M. J. (1986) Plant Physiol. 81, 320-322]. The structures of the oligosaccharides were elucidated from two glycopeptides obtained from the lectin by Pronase digestion, affinity chromatography on concanavalin-A--Sepharose and gel-filtration chromatography on a column of BioGel P-4. The N-linked glycan structures were investigated by 500-MHz 1H-NMR spectroscopy and were established to be: [formula; see text]

Xylose-specific antibodies as markers of subcompartmentation of terminal glycosylation in the Golgi apparatus of sycamore cells

Antibodies specific for xylose-containing plant complex N-linked glycans are used for indirect immunolocalization of xylosyltransferase in sycamore cells. The use of high pressure freezing and freeze substitution for sample preparation resulted in very good morphological preservation of the different Golgi cisternae. Xylosyltransferase shows a diffuse distribution all over the Golgi stacks and xylosylation appears to be an early processing event that is initiated in the cis Golgi compartment.