Identification of a cDNA encoding a plant Lewis-type alpha1,4-fucosyltransferase

Crystal structure of mango α1,3/α1,4-fucosyltransferase elucidates unique elements that regulate Lewis A-dominant oligosaccharide assembly

In various organisms, α1,3/α1,4-fucosyltransferases (CAZy GT10 family enzymes) mediate the assembly of type I (Galβ1,3GlcNAc) and/or type II (Galβ1,4GlcNAc)-based Lewis structures that are widely distributed in glycoconjugates. Unlike enzymes of other species, plant orthologues show little fucosyltransferase activity for type II-based glycans and predominantly catalyze the assembly of the Lewis A structure [Galβ1,3(Fucα1,4)GlcNAc] on the type I disaccharide unit of their substrates. However, the structural basis underlying this unique substrate selectivity remains elusive. In this study, we investigated the structure-function relationship of MiFUT13A, a mango α1,3/α1,4-fucosyltransferase. The prepared MiFUT13A displayed distinct α1,4-fucosyltransferase activity. Consistent with the enzymatic properties of this molecule, X-ray crystallography revealed that this enzyme has a typical GT-B fold-type structure containing a set of residues that are responsible for its SN2-like catalysis. Site-directed mutagenesis and molecular docking analyses proposed a rational binding mechanism for type I oligosaccharides. Within the catalytic cleft, the pocket surrounding Trp121 serves as a binding site, anchoring the non-reducing terminal β1,3-galactose that belongs to the type I disaccharide unit. Furthermore, Glu177 was postulated to function as a general base catalyst through its interaction with the 4-hydroxy group of the acceptor N-acetylglucosamine residue. Adjacent residues, specifically Thr120, Thr157 and Asp175 were speculated to assist in binding of the reducing terminal residues. Intriguingly, these structural elements were not fully conserved in mammalian orthologue which also shows predominant α1,4-fucosyltransferase activity. In conclusion, we have proposed that MiFUT13A generates the Lewis A structure on type I glycans through a distinct mechanism, divergent from that of mammalian enzymes.

Lewis A Glycans Are Present on Proteins Involved in Cell Wall Biosynthesis and Appear Evolutionarily Conserved Among Natural Arabidopsis thaliana Accessions

N-glycosylation is a highly abundant protein modification present in all domains of life. Terminal sugar residues on complex-type N-glycans mediate various crucial biological processes in mammals such as cell-cell recognition or protein-ligand interactions. In plants, the Lewis A trisaccharide constitutes the only known outer-chain elongation of complex N-glycans. Lewis A containing complex N-glycans appear evolutionary conserved, having been identified in all plant species analyzed so far. Despite their ubiquitous occurrence, the biological function of this complex N-glycan modification is currently unknown. Here, we report the identification of Lewis A bearing glycoproteins from three different plant species: Arabidopsis thaliana, Nicotiana benthamiana, and Oryza sativa. Affinity purification via the JIM84 antibody, directed against Lewis A structures on complex plant N-glycans, was used to enrich Lewis A bearing glycoproteins, which were subsequently identified via nano-LC-MS. Selected identified proteins were recombinantly expressed and the presence of Lewis A confirmed via immunoblotting and site-specific N-glycan analysis. While the proteins identified in O. sativa are associated with diverse functions, proteins from A. thaliana and N. benthamiana are mainly involved in cell wall biosynthesis. However, a Lewis A-deficient mutant line of A. thaliana showed no change in abundance of cell wall constituents such as cellulose or lignin. Furthermore, we investigated the presence of Lewis A structures in selected accessions from the 1001 genome database containing amino acid variations in the enzymes required for Lewis A biosynthesis. Besides one relict line showing no detectable levels of Lewis A, the modification was present in all other tested accessions. The data provided here comprises the so far first attempt at identifying Lewis A bearing glycoproteins across different species and will help to shed more light on the role of Lewis A structures in plants.

Engineering mammalian cells to produce plant-specific N-glycosylation on proteins

Protein N-glycosylation is an essential and highly conserved posttranslational modification found in all eukaryotic cells. Yeast, plants and mammalian cells, however, produce N-glycans with distinct structural features. These species-specific features not only pose challenges in selecting host cells for production of recombinant therapeutics for human medical use but also provide opportunities to explore and utilize species-specific glycosylation in design of vaccines. Here, we used reverse cross-species engineering to stably introduce plant core α3fucose (α3Fuc) and β2xylose (β2Xyl) N-glycosylation epitopes in the mammalian Chinese hamster ovary (CHO) cell line. We used directed knockin of plant core fucosylation and xylosylation genes (AtFucTA/AtFucTB and AtXylT) and targeted knockout of endogenous genes for core fucosylation (fut8) and elongation (B4galt1), for establishing CHO cells with plant N-glycosylation capacities. The engineering was evaluated through coexpression of two human therapeutic N-glycoproteins, erythropoietin (EPO) and an immunoglobulin G (IgG) antibody. Full conversion to the plant-type α3Fuc/β2Xyl bi-antennary agalactosylated N-glycosylation (G0FX) was demonstrated for the IgG1 produced in CHO cells. These results demonstrate that N-glycosylation in mammalian cells is amenable for extensive cross-kingdom engineering and that engineered CHO cells may be used to produce glycoproteins with plant glycosylation.

Function of N-glycosylation in plants

Protein N-glycosylation is one of the major post-translational modifications in eukaryotic cells. In lower unicellular eukaryotes, the known functions of N-glycans are predominantly in protein folding and quality control within the lumen of the endoplasmic reticulum (ER). In multicellular organisms, complex N-glycans are important for developmental programs and immune responses. However, little is known about the functions of complex N-glycans in plants. Formed in the Golgi apparatus, plant complex N-glycans have structures distinct from their animal counterparts due to a set of glycosyltransferases unique to plants. Severe basal underglycosylation in the ER lumen induces misfolding of newly synthesized proteins, which elicits the unfolded protein response (UPR) and ER protein quality control (ERQC) pathways. The former promotes higher capacity of proper protein folding and the latter degradation of misfolded proteins to clear the ER. Although our knowledge on plant complex N-glycan functions is limited, genetic studies revealed the importance of complex N-glycans in cellulose biosynthesis and growth under stress.

Main Strategies of Plant Expression System Glycoengineering for Producing Humanized Recombinant Pharmaceutical Proteins

Most the pharmaceutical proteins are derived not from their natural sources, rather their recombinant analogs are synthesized in various expression systems. Plant expression systems, unlike mammalian cell cultures, combine simplicity and low cost of procaryotic systems and the ability for posttranslational modifications inherent in eucaryotes. More than 50% of all human proteins and more than 40% of the currently used pharmaceutical proteins are glycosylated, that is, they are glycoproteins, and their biological activity, pharmacodynamics, and immunogenicity depend on the correct glycosylation pattern. This review examines in detail the similarities and differences between N- and O-glycosylation in plant and mammalian cells, as well as the effect of plant glycans on the activity, pharmacokinetics, immunity, and intensity of biosynthesis of pharmaceutical proteins. The main current strategies of glycoengineering of plant expression systems aimed at obtaining fully humanized proteins for pharmaceutical application are summarized.

Molecular cloning and functional expression of Lewis type α1,3/α1,4-fucosyltransferase cDNAs from Mangifera indica L

In higher plants, complex type N-glycans contain characteristic carbohydrate moieties that are not found in mammals. In particular, the attachment of the Lewis a (Le^a) epitope is currently the only known outer chain elongation that is present in plant N-glycans. Such a modification is of great interest in terms of the biological function of complex type N-glycans in plant species. However, little is known regarding the exact molecular basis underlying their Le^a expression. In the present study, we cloned two novel Lewis type fucosyltransferases (MiFUT13) from mango fruit, Mangifera indica L., heterologously expressed the proteins and structurally and functionally characterized them. Using an HPLC-based assay, we demonstrated that the recombinant MiFUT13 proteins mediate the α1,4-fucosylation of acceptor tetrasaccharides with a strict preference for type I-based structure to type II. The results and other findings suggest that MiFUT13s are involved in the biosynthesis of Le^a containing glycoconjugates in mango fruits.

Glycan-binding F-box protein from Arabidopsis thaliana protects plants from Pseudomonas syringae infection

BACKGROUND

A small group of F-box proteins consisting of a conserved F-box domain linked to a domain homologous to the glycan-binding protein has been identified within the genome of Arabidopsis thaliana. Previously, the so-called F-box-Nictaba protein, encoded by the gene At2g02360, was shown to be a functional lectin which binds N-acetyllactosamine structures. Here, we present a detailed qRT-PCR expression analysis of F-box-Nictaba in Arabidopsis plants upon different stresses and hormone treatments.

RESULTS

Expression of the F-box-Nictaba gene was enhanced after plant treatment with salicylic acid and after plant infection with the virulent Pseudomonas syringae pv. tomato strain DC3000 (Pst DC3000). β-glucuronidase histochemical staining of transgenic Arabidopsis plants displayed preferential activity of the At2g02360 promoter in trichomes present on young rosette leaves. qRT-PCR analyses confirmed high expression of F-box-Nictaba in leaf trichomes. A. thaliana plants overexpressing the gene showed less disease symptoms after Pst DC3000 infection with reduced bacterial colonization compared to infected wild type and F-box-Nictaba knock-out plants.

CONCLUSIONS

Our data show that the Arabidopsis F-box-Nictaba gene is a stress-inducible gene responsive to SA, bacterial infection and heat stress, and is involved in salicylic acid related plant defense responses. This knowledge enriched our understanding of the physiological importance of F-box-Nictaba, and can be used to create plants with better performance in changing environmental conditions.

Glyco-engineering for biopharmaceutical production in moss bioreactors

The production of recombinant biopharmaceuticals (pharmaceutical proteins) is a strongly growing area in the pharmaceutical industry. While most products to date are produced in mammalian cell cultures, namely Chinese hamster ovary cells, plant-based production systems gained increasing acceptance over the last years. Different plant systems have been established which are suitable for standardization and precise control of cultivation conditions, thus meeting the criteria for pharmaceutical production. The majority of biopharmaceuticals comprise glycoproteins. Therefore, differences in protein glycosylation between humans and plants have to be taken into account and plant-specific glycosylation has to be eliminated to avoid adverse effects on quality, safety, and efficacy of the products. The basal land plant Physcomitrella patens (moss) has been employed for the recombinant production of high-value therapeutic target proteins (e.g., Vascular Endothelial Growth Factor, Complement Factor H, monoclonal antibodies, Erythropoietin). Being genetically excellently characterized and exceptionally amenable for precise gene targeting via homologous recombination, essential steps for the optimization of moss as a bioreactor for the production of recombinant proteins have been undertaken. Here, we discuss the glyco-engineering approaches to avoid non-human N- and O-glycosylation on target proteins produced in moss bioreactors.

Moss-based production of asialo-erythropoietin devoid of Lewis A and other plant-typical carbohydrate determinants

Protein therapeutics represent one of the most increasing areas in the pharmaceutical industry. Plants gain acceptance as attractive alternatives for high-quality and economical protein production. However, as the majority of biopharmaceuticals are glycoproteins, plant-specific N-glycosylation has to be taken into consideration. In Physcomitrella patens (moss), glyco-engineering is an applicable tool, and the removal of immunogenic core xylose and fucose residues was realized before. Here, we present the identification of the enzymes that are responsible for terminal glycosylation (α1,4 fucosylation and β1,3 galactosylation) on complex-type N-glycans in moss. The terminal trisaccharide consisting of α1,4 fucose and β1,3 galactose linked to N-acetylglucosamine forms the so-called Lewis A epitope. This epitope is rare on moss wild-type proteins, but was shown to be enriched on complex-type N-glycans of moss-produced recombinant human erythropoietin, while unknown from the native human protein. Via gene targeting of moss galactosyltransferase and fucosyltransferase genes, we identified the gene responsible for terminal glycosylation and were able to completely abolish the formation of Lewis A residues on the recombinant biopharmaceutical.

The secreted plant N-glycoproteome and associated secretory pathways

N-Glycosylation is a common form of eukaryotic protein post-translational modification, and one that is particularly prevalent in plant cell wall proteins. Large scale and detailed characterization of N-glycoproteins therefore has considerable potential in better understanding the composition and functions of the cell wall proteome, as well as those proteins that reside in other compartments of the secretory pathway. While there have been numerous studies of mammalian and yeast N-glycoproteins, less is known about the population complexity, biosynthesis, structural variation, and trafficking of their plant counterparts. However, technical developments in the analysis of glycoproteins and the structures the glycans that they bear, as well as valuable comparative analyses with non-plant systems, are providing new insights into features that are common among eukaryotes and those that are specific to plants, some of which may reflect the unique nature of the plant cell wall. In this review we present an overview of the current knowledge of plant N-glycoprotein synthesis and trafficking, with particular reference to those that are cell wall localized.

Deletion of fucose residues in plant N-glycans by repression of the GDP-mannose 4,6-dehydratase gene using virus-induced gene silencing and RNA interference

Production of pharmaceutical glycoproteins in plants has many advantages in terms of safety and reduced costs. However, plant-produced glycoproteins have N-glycans with plant-specific sugar residues (core β-1,2-xylose and α-1,3-fucose) and a Lewis a (Le(a) ) epitope, i.e., Galβ(1-3)[Fucα(1-4)]GlcNAc. Because these sugar residues and glycan structures seemed to be immunogenic, several attempts have been made to delete them by repressing their respective glycosyltransferase genes. However, until date, such deletions have not been successful in completely eliminating the fucose residues. In this study, we simultaneously reduced the plant-specific core α-1,3-fucose and α-1,4-fucose residues in the Le(a) epitopes by repressing the Guanosine 5'-diphosphate (GDP)-D-mannose 4,6-dehydratase (GMD) gene, which is associated with GDP-L-fucose biosynthesis, in Nicotiana benthamiana plants. Repression of GMD was achieved using virus-induced gene silencing (VIGS) and RNA interference (RNAi). The proportion of fucose-free N-glycans found in total soluble protein from GMD gene-repressed plants increased by 80% and 95% following VIGS and RNAi, respectively, compared to wild-type plants. A small amount of putative galactose substitution in N-glycans from the NbGMD gene-repressed plants was observed, similar to what has been previously reported GMD-knockout Arabidopsis mutant. On the other hand, the recombinant mouse granulocyte-macrophage colony-stimulating factor (GM-CSF) with fucose-deleted N-glycans was successfully produced in NbGMD-RNAi transgenic N. benthamiana plants. Thus, repression of the GMD gene is thus very useful for deleting immunogenic total fucose residues and facilitating the production of pharmaceutical glycoproteins in plants.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update covering the period 2001-2002

This review is the second update of the original review on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates that was published in 1999. It covers fundamental aspects of the technique as applied to carbohydrates, fragmentation of carbohydrates, studies of specific carbohydrate types such as those from plant cell walls and those attached to proteins and lipids, studies of glycosyl-transferases and glycosidases, and studies where MALDI has been used to monitor products of chemical synthesis. Use of the technique shows a steady annual increase at the expense of older techniques such as FAB. There is an increasing emphasis on its use for examination of biological systems rather than on studies of fundamental aspects and method development and this is reflected by much of the work on applications appearing in tabular form.

Adaptation of the "in-gel release method" to N-glycome analysis of low-milligram amounts of material

Protein N-glycosylation is a post-translational modification which plays numerous crucial physiological roles. The N-glycan pattern varies depending on the species organs, tissues and even cell types and their respective physiological states. Obtaining enough starting material from a particular cell type or tissue for N-glycan purification by conventional methods can, in certain cases, be very difficult. Previously, a sensitive technique, the "in-gel release method" that allows the determination of N-glycans attached to a protein isolated by SDS-PAGE, has been developed in this and other laboratories. Here, we describe the adaptation of this method to obtain information on the N-glycome from minute amounts of tissue. The starting material, ranging from less than a milligram to a few milligrams of fresh tissue, is directly ground in Laemmli sample buffer and subject briefly to discontinuous Tris-glycine-SDS-PAGE. The Coomassie-stained band containing the majority of the proteins is subject to the "in-gel release method". The developed technique was used to analyze N-glycan patterns of different samples from Caenorhabditis elegans, Drosophila melanogaster, Spodoptera frugiperda, Trichoplusia ni, Nicotiana benthamiana, Arabidopsis thaliana, and Mus musculus. Furthermore, the technique was used to determine the effects of transient small-scale RNAi-mediated knock-down of a glycosylation-related gene in Drosophila Schneider 2 cell line.

A unique beta1,3-galactosyltransferase is indispensable for the biosynthesis of N-glycans containing Lewis a structures in Arabidopsis thaliana

In plants, the only known outer-chain elongation of complex N-glycans is the formation of Lewis a [Fuc alpha1-4(Gal beta1-3)GlcNAc-R] structures. This process involves the sequential attachment of beta1,3-galactose and alpha1,4-fucose residues by beta1,3-galactosyltransferase and alpha1,4-fucosyltransferase. However, the exact mechanism underlying the formation of Lewis a epitopes in plants is poorly understood, largely because one of the involved enzymes, beta1,3-galactosyltransferase, has not yet been identified and characterized. Here, we report the identification of an Arabidopsis thaliana beta1,3-galactosyltransferase involved in the biosynthesis of the Lewis a epitope using an expression cloning strategy. Overexpression of various candidates led to the identification of a single gene (named GALACTOSYLTRANSFERASE1 [GALT1]) that increased the originally very low Lewis a epitope levels in planta. Recombinant GALT1 protein produced in insect cells was capable of transferring beta1,3-linked galactose residues to various N-glycan acceptor substrates, and subsequent treatment of the reaction products with alpha1,4-fucosyltransferase resulted in the generation of Lewis a structures. Furthermore, transgenic Arabidopsis plants lacking a functional GALT1 mRNA did not show any detectable amounts of Lewis a epitopes on endogenous glycoproteins. Taken together, our results demonstrate that GALT1 is both sufficient and essential for the addition of beta1,3-linked galactose residues to N-glycans and thus is required for the biosynthesis of Lewis a structures in Arabidopsis. Moreover, cell biological characterization of a transiently expressed GALT1-fluorescent protein fusion using confocal laser scanning microscopy revealed the exclusive location of GALT1 within the Golgi apparatus, which is in good agreement with the proposed physiological action of the enzyme.

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.

Towards abolition of immunogenic structures in insect cells: characterization of a honey-bee (Apis mellifera) multi-gene family reveals both an allergy-related core alpha1,3-fucosyltransferase and the first insect Lewis-histo-blood-group-related antigen-synthesizing enzyme

Glycoproteins from honey-bee (Apis mellifera), such as phospholipase A2 and hyaluronidase, are well-known major bee-venom allergens. They carry N-linked oligosaccharide structures with two types of alpha1,3-fucosylation: the modification by alpha1,3-fucose of the innermost core GlcNAc, which constitutes an epitope recognized by IgE from some bee-venom-allergic patients, and an antennal Lewis-like GalNAcbeta1,4(Fucalpha1,3)GlcNAc moiety. We now report the cloning and expression of two cDNAs encoding the relevant active alpha1,3-FucTs (alpha1,3-fucosyltransferases). The first sequence, closest to that of fruitfly (Drosophila melanogaster) FucTA, was found to be a core alpha1,3-FucT (EC 2.4.1.214), as judged by several enzyme and biochemical assays. The second cDNA encoded an enzyme, most related to Drosophila FucTC, that was shown to be capable of generating the Le(x) [Galbeta1-4(Fucalpha1-3)GlcNAc] epitope in vitro and is the first Lewis-type alpha1,3-FucT (EC 2.4.1.152) to be described in insects. The transcription levels of these two genes in various tissues were examined: FucTA was found to be predominantly expressed in the brain tissue and venom glands, whereas FucTC transcripts were detected at highest levels in venom and hypopharyngeal glands. Very low expression of a third homologue of unknown function, FucTB, was also observed in various tissues. The characterization of these honey-bee gene products not only accounts for the observed alpha1,3-fucosylation of bee-venom glycoproteins, but is expected to aid the identification and subsequent down-regulation of the FucTs in insect cell lines of biotechnological importance.

Fucosylation in prokaryotes and eukaryotes

Fucosylated carbohydrate structures are involved in a variety of biological and pathological processes in eukaryotic organisms including tissue development, angiogenesis, fertilization, cell adhesion, inflammation, and tumor metastasis. In contrast, fucosylation appears less common in prokaryotic organisms and has been suggested to be involved in molecular mimicry, adhesion, colonization, and modulating the host immune response. Fucosyltransferases (FucTs), present in both eukaryotic and prokaryotic organisms, are the enzymes responsible for the catalysis of fucose transfer from donor guanosine-diphosphate fucose to various acceptor molecules including oligosaccharides, glycoproteins, and glycolipids. To date, several subfamilies of mammalian FucTs have been well characterized; these enzymes are therefore delineated and used as models. Non-mammalian FucTs that possess different domain construction or display distinctive acceptor substrate specificity are highlighted. It is noteworthy that the glycoconjugates from plants and schistosomes contain some unusual fucose linkages, suggesting the presence of novel FucT subfamilies as yet to be characterized. Despite the very low sequence homology, striking functional similarity is exhibited between mammalian and Helicobacter pylori alpha1,3/4 FucTs, implying that these enzymes likely share a conserved mechanistic and structural basis for fucose transfer; such conserved functional features might also exist when comparing other FucT subfamilies from different origins. Fucosyltranferases are promising tools used in synthesis of fucosylated oligosaccharides and glycoconjugates, which show great potential in the treatment of infectious and inflammatory diseases and tumor metastasis.

Genomic characterization and physical mapping of two fucosyltransferase genes inMedicago truncatula

Fucosyltransferases catalyse fucose transfer onto oligosaccharides. Two fucosylated structures have been identified in plants: the α1,4-fucosylated Lewis-a epitope and the α1,3-fucosylated core. Here we report the cloning, genomic characterization, and physical mapping of two genes encoding proteins similar to α1,4-fucosyltransferase (EC 2.4.1.65, MtFUT1) and α1,3-fucosyltransferase (EC 2.4.1.214, MtFUT2) in Medicago truncatula. Analysis of the genomic organization of the fucosyltransferase genes in M. truncatula, revealed the presence of two genomic variants of the MtFUT1 gene coding sequence, one containing a single intron and the other intronless, whereas in MtFUT2, the gene coding region is interrupted by four introns. Using for the first time fluorescence in situ hybridization (FISH) to physically map fucosyltransferase genes in plants, this study reveals a high genomic dispersion of these genes in Medicago. The MtFUT1 genes are mapped on chromosomes 4, 7, and 8, colocalizing on three of the five MtFUT2 loci. Chromosomes 1 and 5 carry the additional MtFUT2 loci. Moreover, the intensity of the FISH signals reveals marked differences in the number of gene copies per locus for both genes. Simultaneous mapping of rRNA genes on chromosome 5 shows that several MTFUT2 gene loci are inserted within the rDNA array. Insertions of coding DNA sequences into the rDNA repeats were never reported to date.Key words: core α1,3-fucosyltransferase gene, α1,4-fucosyltransferase gene, genomic organization, in situ hybridization, Medicago truncatula.

Biochemical characterization of Silene alba α4-fucosyltransferase and Lewis a products

alpha1,4-Fucosylation has been recently detected in Arabidopsis thaliana [Leonard et al. (2002), Glycobiology 12: 299-306], and corresponding enzymes have also been characterized in Beta vulgaris [Bakker et al. (2001), FEBS Lett, 507: 307-312], and Lycopersicum aesculentum [Wilson (2001), Glycoconjugate J., 18: 439-447]. Here we demonstrated fucosyltransferase activity (FucT) in Silene alba cells and tissues. The Fuc linkage to GlcNAc residues of the lactosamine moiety of the Type I acceptor was confirmed by mass spectrometry experiments. Le(a)-glycoconjugates are found in the Golgi apparatus and plasma membrane of plant cells. In planta, the highest levels of activity were detected in seedlings, young roots and male flowers. The enzyme was stable up to 45( composite function)C and the optimum pH of reaction was 8.0. The enzyme required Mg(2+) or Mn(2+) for activity and was inhibited by Zn(2+) and ethylenediaminetetraacetic acid. Chemical modification of the enzyme with group-selective reagents revealed that selective modifications of arginine and lysine residues had no effect on enzyme activity. However the enzyme contains histidine and tryptophan residues that are essential for its activity. In contrast to human FUT3, the S. alba alpha4-FucT was insensitive to N-ethylmaleimide (NEM) treatment. Measurement of enzyme activity in S. alba cell fractions indicated that the enzyme is bound to microsomal membranes, furthermore a soluble isoform of the protein may be present.

Targeted knockouts of Physcomitrella lacking plant-specific immunogenic N-glycans

Using plants as production factories for therapeutic proteins requires modification of their N-glycosylation pattern because of the immunogenicity of plant-specific sugar residues. In an attempt towards such humanization, we disrupted the genes for alpha1,3-fucosyltransferase and beta1,2-xylosyltransferase in Physcomitrella patens by homologous recombination. The single Deltafuc-t and Deltaxyl-t plants, as well as the double knockout, lacked transcripts of the corresponding genes, but did not differ from the wild-type moss in morphology, growth, development, and ability to secrete a recombinant protein, the human vascular endothelial growth factor VEGF(121), into the culture medium. N-Glycan analysis, however, revealed the absence of 1,3-fucosyl and/or 1,2-xylosyl residues, respectively. Therefore, the modifications described here represent the key step towards the generation of moss lines suitable for the production of plant-made glycosylated biopharmaceuticals with nonallergenic N-glycans.

Molecular Basis of Anti-horseradish Peroxidase Staining in Caenorhabditis elegans

Cross-reactivity with anti-horseradish peroxidase antiserum is a feature of many glycoproteins from plants and invertebrates; indeed staining with this reagent has been used to track neurons in Drosophila melanogaster and Caenorhabditis elegans. Although in insects the evidence indicates that the cross-reaction results from the presence of core alpha1,3-fucosylated N-glycans, the molecular basis for anti-horseradish peroxidase staining in nematodes has been unresolved to date. By using Western blots of wild-type and mutant C. elegans extracts in conjunction with specific inhibitors, we show that the cross-reaction is due to core alpha1,3-fucosylation. Of the various mutants examined, one with a deletion of the fut-1 (K08F8.3) gene showed no reaction to anti-horseradish peroxidase; the molecular phenotype was rescued by injection of either the K08F8 cosmid or the fut-1 open reading frame under control of the let-858 promoter. Furthermore, expression of fut-1 cDNA in Pichia and insect cells in conjunction with antibody staining, high pressure liquid chromatography, and matrix-assisted laser desorption ionization time-of-flight mass spectrometry analyses showed that FUT-1 is a core alpha1,3-fucosyltransferase with an unusual substrate specificity. It is the only core fucosyltransferase in plants and animals described to date that does not require the prior action of N-acetylglucosaminyltransferase I.

Alteration of gibberellin response in transgenic tobacco plants which express a human Lewis fucosyltransferase

In plants, Lewisa type N-glycans may be involved in cell-to-cell communication and recognition. N-glycoproteins harboring Lewisa glycotopes are mainly found in plasma membranes and cell walls. Some can be also involved in cell wall synthesis or the loosening process, and subsequently in cell elongation. In order to determine the potential role(s) of the alpha4-fucosylation during vegetative development, transgenic tobacco plants overexpressing a human Lewis fucosyltransferase (hFUT3), which transfers a fucose residue in a alpha(1,4)-linkage on complex glycans, have been developed. The heterologous enzyme hFUT3 was strongly expressed and fully functional in transgenic tobacco. Transgenic plants showed a delay in growth linked to a reduction of internode length. Furthermore, transgenic seedling roots were significantly shorter than wild-type roots and the length of their epidermis cells was reduced. Strikingly, root growth was completely and specifically restored following gibberellin treatment. Etiolated hypocotyls of hFUT3 overexpressors were also more sensitive to exogenous gibberellin. Furthermore, paclobutrazol, an inhibitor of gibberellin synthesis, induced a similar effect on control and transgenic dark-grown hypocotyls suggesting that gibberellin biosynthesis was probably not altered in seedlings overexpressing hFUT3. Thus, alpha4-fucosylation could act as a possible modulator of conformation and/or functioning of N-glycoproteins involved in the gibberellin-dependent elongation process.

A genetic and structural analysis of the N-glycosylation capabilities

The recent draft sequencing of the rice (Oryza sativa) genome has enabled a genetic analysis of the glycosylation capabilities of an agroeconomically important group of plants, the monocotyledons. In this study, we have not only identified genes putatively encoding enzymes involved in N-glycosylation, but have examined by MALDI-TOF MS the structures of the N-glycans of rice and other monocotyledons (maize, wheat and dates; Zea mays, Triticum aestivum and Phoenix dactylifera); these data show that within the plant kingdom the types of N-glycans found are very similar between monocotyledons, dicotyledons and gymnosperms. Subsequently, we constructed expression vectors for the key enzymes forming plant-typical structures in rice, N-acetylglucosaminyltransferase I (GlcNAc-TI; EC 2.4.1.101), core alpha1,3-fucosyltransferase (FucTA; EC 2.4.1.214) and beta1,2-xylosyltransferase (EC 2.4.2.38) and successfully expressed them in Pichia pastoris. Rice GlcNAc-TI, FucTA and xylosyltransferase are therefore the first monocotyledon glycosyltransferases involved in N-glycan biosynthesis to be characterised in a recombinant form.

Expression of eukaryotic glycosyltransferases in the yeast Pichia pastoris

The methylotrophic yeast Pichia pastoris is often used as an organism for the heterologous expression of proteins and has been used already for production of a number of glycosyltransferases involved in the biosynthesis of N- and O-linked oligosaccharides. In our recent studies, we have examined the expression in P. pastoris of Arabidopsis thaliana and Drosophila melanogaster core alpha1,3-fucosyltransferases (EC 2.4.1.214), A. thaliana beta1,2-xylosyltransferase (EC 2.4.2.38), bovine beta1,4-galactosyltransferase I (EC 2.4.1.38), D. melanogaster peptide O-xylosyltransferase (EC 2.4.2.26), D. melanogaster and Caenorhabditis elegans beta1,4-galactosyltransferase VII (SQV-3; EC 2.4.1.133) and tomato Lewis-type alpha1,4-fucosyltransferase (EC 2.4.1.65). Temperature, cell density and medium formulation have varying effects on the amount of activity resulting from expression under the control of either the constitutive glyceraldehyde-3-phosphate dehydrogenase (GAP) or inducible alcohol oxidase (AOX1) promoters. In the case of the A. thaliana xylosyltransferase these effects were most pronounced, since constitutive expression at 16 degrees C resulted in 30-times more activity than inducible expression at 30 degrees C. Also, the exact nature of the constructs had an effect; whereas soluble forms of the A. thaliana xylosyltransferase and fucosyltransferase were active with N-terminal pentahistidine tags (in the former case facilitating purification of the recombinant protein to homogeneity), a C-terminally tagged form of the A. thaliana fucosyltransferase was inactive. In the case of D. melanogaster beta1,4-galactosyltransferase VII, expression with a yeast secretion signal yielded no detectable activity; however, when a full-length form of the enzyme was introduced into P. pastoris, an active secreted form of the protein was produced.