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

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.

The Land-Sea Connection: Insights Into the Plant Lineage from a Green Algal Perspective

The colonization of land by plants generated opportunities for the rise of new heterotrophic life forms, including humankind. A unique event underpinned this massive change to earth ecosystems-the advent of eukaryotic green algae. Today, an abundant marine green algal group, the prasinophytes, alongside prasinodermophytes and nonmarine chlorophyte algae, is facilitating insights into plant developments. Genome-level data allow identification of conserved proteins and protein families with extensive modifications, losses, or gains and expansion patterns that connect to niche specialization and diversification. Here, we contextualize attributes according to Viridiplantae evolutionary relationships, starting with orthologous protein families, and then focusing on key elements with marked differentiation, resulting in patchy distributions across green algae and plants. We place attention on peptidoglycan biosynthesis, important for plastid division and walls; phytochrome photosensors that are master regulators in plants; and carbohydrate-active enzymes, essential to all manner of carbohydratebiotransformations. Together with advances in algal model systems, these areas are ripe for discovering molecular roles and innovations within and across plant and algal lineages.

Cracking the "Sugar Code": A Snapshot of N- and O-Glycosylation Pathways and Functions in Plants Cells

Glycosylation is a fundamental co-translational and/or post-translational modification process where an attachment of sugars onto either proteins or lipids can alter their biological function, subcellular location and modulate the development and physiology of an organism. Glycosylation is not a template driven process and as such produces a vastly larger array of glycan structures through combinatorial use of enzymes and of repeated common scaffolds and as a consequence it provides a huge expansion of both the proteome and lipidome. While the essential role of N- and O-glycan modifications on mammalian glycoproteins is already well documented, we are just starting to decode their biological functions in plants. Although significant advances have been made in plant glycobiology in the last decades, there are still key challenges impeding progress in the field and, as such, holistic modern high throughput approaches may help to address these conceptual gaps. In this snapshot, we present an update of the most common O- and N-glycan structures present on plant glycoproteins as well as (1) the plant glycosyltransferases (GTs) and glycosyl hydrolases (GHs) responsible for their biosynthesis; (2) a summary of microorganism-derived GHs characterized to cleave specific glycosidic linkages; (3) a summary of the available tools ranging from monoclonal antibodies (mAbs), lectins to chemical probes for the detection of specific sugar moieties within these complex macromolecules; (4) selected examples of N- and O-glycoproteins as well as in their related GTs to illustrate the complexity on their mode of action in plant cell growth and stress responses processes, and finally (5) we present the carbohydrate microarray approach that could revolutionize the way in which unknown plant GTs and GHs are identified and their specificities characterized.

A cotton α1,3-/4-fucosyltransferase-encoding gene, FucT4, plays an important role in cell elongation and is significantly associated with fiber quality

Fucosylation, one of the key posttranslational modifications, plays an important role in plants. It is involved in the development, signal transduction, reproduction, and disease resistance. α1,3-/4-Fucosyltransferase is responsible for transferring L-fucose from GDP-L-fucose to the N-glycan to exert fucosylational functions. However, the roles of the fucosyltransferase gene in cotton remain unknown. This study provided a comprehensive investigation of its possible functions. A genome-wide analysis identified four, four, eight, and eight FucT genes presented in the four sequenced cotton species, diploid Gossypium raimondii, G. arboreum, tetraploid G. hirsutum acc. TM-1, and G. barbadense cv. H7124, respectively. These FucTs were classified into two groups, with FucT4 homologs alone as a group. We isolated FucT4 in TM-1 and H7124, and named it GhFucT4 and GbFucT4, respectively. Quantitative RT-PCR and transcriptome data demonstrated that GhFucT4 had the highest expression levels in fibers among all GhFucT genes. Association studies and QTL co-localization supported the possible involvement of GhFucT4 in cotton fiber development. GhFucT4 and GbFucT4 shared high sequence identities, and FucT4 had higher expression in H7124 fiber tissues compared with TM-1. Furthermore, ectopic expression of FucT4 in transgenic Arabidopsis promoted root cell elongation, upregulated expression of genes related to cell wall loosening, and led to longer primary root. These results collectively indicate that FucT4 plays an important role in promoting cell elongation and modulating fiber development, which could be utilized to improve fiber quality traits in cotton breeding.

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.

An important role of l-fucose biosynthesis and protein fucosylation genes in Arabidopsis immunity

Plants mount coordinated immune responses to defend themselves against pathogens. However, the cellular components required for plant immunity are not fully understood. The jasmonate-mimicking coronatine (COR) toxin produced by Pseudomonas syringae pv. tomato (Pst) DC3000 functions to overcome plant immunity. We previously isolated eight Arabidopsis (scord) mutants that exhibit increased susceptibility to a COR-deficient mutant of PstDC3000. Among them, the scord6 mutant exhibits defects both in stomatal closure response and in restricting bacterial multiplication inside the apoplast. However, the identity of SCORD6 remained elusive. In this study, we aim to identify the SCORD6 gene. We identified SCORD6 via next-generation sequencing and found it to be MURUS1 (MUR1), which is involved in the biosynthesis of GDP-l-fucose. Discovery of SCORD6 as MUR1 led to a series of experiments that revealed a multi-faceted role of l-fucose biosynthesis in stomatal and apoplastic defenses as well as in pattern-triggered immunity and effector-triggered immunity, including glycosylation of pattern-recognition receptors. Furthermore, compromised stomatal and/or apoplastic defenses were observed in mutants of several fucosyltransferases with specific substrates (e.g. O-glycan, N-glycan or the DELLA transcriptional repressors). Collectively, these results uncover a novel and broad role of l-fucose and protein fucosylation in plant immunity.

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.

How Fucose of Blood Group Glycotopes Programs Human Gut Microbiota

Formation of appropriate gut microbiota is essential for human health. The first two years of life is the critical period for this process. Selection of mutualistic microorganisms of the intestinal microbiota is controlled by the FUT2 and FUT3 genes that encode fucosyltransferases, enzymes responsible for the synthesis of fucosylated glycan structures of mucins and milk oligosaccharides. In this review, the mechanisms of the selection and maintenance of intestinal microorganisms that involve fucosylated oligosaccharides of breast milk and mucins of the newborn's intestine are described. Possible reasons for the use of fucose, and not sialic acid, as the major biological signal for the selection are also discussed.

Cell wall O-glycoproteins and N-glycoproteins: aspects of biosynthesis and function

Cell wall O-glycoproteins and N-glycoproteins are two types of glycomolecules whose glycans are structurally complex. They are both assembled and modified within the endomembrane system, i.e., the endoplasmic reticulum (ER) and the Golgi apparatus, before their transport to their final locations within or outside the cell. In contrast to extensins (EXTs), the O-glycan chains of arabinogalactan proteins (AGPs) are highly heterogeneous consisting mostly of (i) a short oligo-arabinoside chain of three to four residues, and (ii) a larger β-1,3-linked galactan backbone with β-1,6-linked side chains containing galactose, arabinose and, often, fucose, rhamnose, or glucuronic acid. The fine structure of arabinogalactan chains varies between, and within plant species, and is important for the functional activities of the glycoproteins. With regards to N-glycans, ER-synthesizing events are highly conserved in all eukaryotes studied so far since they are essential for efficient protein folding. In contrast, evolutionary adaptation of N-glycan processing in the Golgi apparatus has given rise to a variety of organism-specific complex structures. Therefore, plant complex-type N-glycans contain specific glyco-epitopes such as core β,2-xylose, core α1,3-fucose residues, and Lewis(a) substitutions on the terminal position of the antenna. Like O-glycans, N-glycans of proteins are essential for their stability and function. Mutants affected in the glycan metabolic pathways have provided valuable information on the role of N-/O-glycoproteins in the control of growth, morphogenesis and adaptation to biotic and abiotic stresses. With regards to O-glycoproteins, only EXTs and AGPs are considered herein. The biosynthesis of these glycoproteins and functional aspects are presented and discussed in this review.

Classification, naming and evolutionary history of glycosyltransferases from sequenced green and red algal genomes

The Archaeplastida consists of three lineages, Rhodophyta, Virideplantae and Glaucophyta. The extracellular matrix of most members of the Rhodophyta and Viridiplantae consists of carbohydrate-based or a highly glycosylated protein-based cell wall while the Glaucophyte covering is poorly resolved. In order to elucidate possible evolutionary links between the three advanced lineages in Archaeplastida, a genomic analysis was initiated. Fully sequenced genomes from the Rhodophyta and Virideplantae and the well-defined CAZy database on glycosyltransferases were included in the analysis. The number of glycosyltransferases found in the Rhodophyta and Chlorophyta are generally much lower then in land plants (Embryophyta). Three specific features exhibited by land plants increase the number of glycosyltransferases in their genomes: (1) cell wall biosynthesis, the more complex land plant cell walls require a larger number of glycosyltransferases for biosynthesis, (2) a richer set of protein glycosylation, and (3) glycosylation of secondary metabolites, demonstrated by a large proportion of family GT1 being involved in secondary metabolite biosynthesis. In a comparative analysis of polysaccharide biosynthesis amongst the taxa of this study, clear distinctions or similarities were observed in (1) N-linked protein glycosylation, i.e., Chlorophyta has different mannosylation and glucosylation patterns, (2) GPI anchor biosynthesis, which is apparently missing in the Rhodophyta and truncated in the Chlorophyta, (3) cell wall biosynthesis, where the land plants have unique cell wall related polymers not found in green and red algae, and (4) O-linked glycosylation where comprehensive orthology was observed in glycosylation between the Chlorophyta and land plants but not between the target proteins.

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.

N-glycans of Phaeodactylum tricornutum diatom and functional characterization of its N-acetylglucosaminyltransferase I enzyme

N-glycosylation, a major co- and post-translational event in the synthesis of proteins in eukaryotes, is unknown in aquatic photosynthetic microalgae. In this paper, we describe the N-glycosylation pathway in the diatom Phaeodactylum tricornutum. Bio-informatic analysis of its genome revealed the presence of a complete set of sequences potentially encoding for proteins involved in the synthesis of the lipid-linked Glc(3)Man(9)GlcNAc(2)-PP-dolichol N-glycan, some subunits of the oligosaccharyltransferase complex, as well as endoplasmic reticulum glucosidases and chaperones required for protein quality control and, finally, the α-mannosidase I involved in the trimming of the N-glycan precursor into Man-5 N-glycan. Moreover, one N-acetylglucosaminyltransferase I, a Golgi glycosyltransferase that initiates the synthesis of complex type N-glycans, was predicted in the P. tricornutum genome. We demonstrated that this gene encodes for an active N-acetylglucosaminyltransferase I, which is able to restore complex type N-glycans maturation in the Chinese hamster ovary Lec1 mutant, defective in its endogeneous N-acetylglucosaminyltransferase I. Consistent with these data, the structural analyses of N-linked glycans demonstrated that P. tricornutum proteins carry mainly high mannose type N-glycans ranging from Man-5 to Man-9. Although representing a minor glycan population, paucimannose N-glycans were also detected, suggesting the occurrence of an N-acetylglucosaminyltransferase I-dependent maturation of N-glycans in this diatom.

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.

Synthesis of Lewis X epitopes on plant N-glycans

Glycoproteins from tobacco line xFxG1, in which expression of a hybrid beta-(1-->4)-galactosyltransferase (GalT) and a hybrid alpha-(1-->3)-fucosyltransferase IXa (FUT9a) is combined, contained an abundance of hybrid N-glycans with Lewis X (Le(X)) epitopes. A comparison with N-glycan profiles from plants expressing only the hybrid beta-(1-->4)-galactosyltransferase suggested that the fucosylation of the LacNAc residues in line xFxG1 protected galactosylated N-glycans from endogenous plant beta-galactosidase activity.

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.

Arginine/lysine residues in the cytoplasmic tail promote ER export of plant glycosylation enzymes

Plant N-glycan processing enzymes are arranged along the early secretory pathway, forming an assembly line to facilitate the step-by-step modification of oligosaccharides on glycoproteins. Thus, these enzymes provide excellent tools to study signals and mechanisms, promoting their localization and retention in the endoplasmic reticulum (ER) and Golgi apparatus. Herein, we focused on a detailed investigation of amino acid sequence motifs present in their short cytoplasmic tails in respect to ER export. Using site-directed mutagenesis, we determined that single arginine/lysine residues within the cytoplasmic tail are sufficient to promote rapid Golgi targeting of Golgi-resident N-acetylglucosaminyltransferase I (GnTI) and alpha-mannosidase II (GMII). Furthermore, we reveal that an intact ER export motif is essential for proper in vivo function of GnTI. Coexpression studies with Sar1p provided evidence for COPII-dependent transport of GnTI to the Golgi. Our data provide evidence that efficient ER export of Golgi-resident plant N-glycan processing enzymes occurs through a selective mechanism based on recognition of single basic amino acids present in their cytoplasmic tails.

Down-regulated expression of plant-specific glycoepitopes in alfalfa

SUMMARY

Compared with other plant expression systems used for pharmaceutical protein production, alfalfa offers the advantage of very homogeneous N-glycosylation. Therefore, this plant was selected for further attempts at glycoengineering. Two main approaches were developed in order to humanize N-glycosylation in alfalfa. The first was a knock-down of two plant-specific N-glycan maturation enzymes, beta1,2-xylosyltransferase and alpha1,3-fucosyltransferases, using sense, antisense and RNA interference strategies. In a second approach, with the ultimate goal of rebuilding the whole human sialylation pathway, human beta1,4-galactosyltransferase was expressed in alfalfa in a native form or in fusion with a targeting domain from N-acetylglucosaminyltransferase I, a glycosyltransferase located in the early Golgi apparatus in Nicotiana tabacum. Both knock-down and knock-in strategies strongly, but not completely, inhibited the biosynthesis of alpha1,3-fucose- and beta1,2-xylose-containing glycoepitopes in transgenic alfalfa. However, recombinant human beta1,4-galactosyltransferase activity in transgenic alfalfa completely prevented the accumulation of the Lewis a glycoepitope on complex N-glycans.

Expression of bioactive single-chain murine IL-12 in transgenic plants

Interleukin-12 (IL-12), an important immunomodulator for cell-mediated immunity, shows significant potential as a vaccine adjuvant and anticancer therapeutic. However, its clinical application is limited in part by lack of an effective bioproduction system for this complex heterodimeric glycoprotein. Transgenic plants show promise as scalable bioproduction platforms for challenging biopharmaceutical proteins. To test the potential of plants to effectively produce bioactive IL-12, we developed transgenic tobacco plant lines and derived root cultures yielding high levels of mouse IL-12 (MuIL-12). Functional IL-12 is a heterodimer consisting of two disulfide-linked subunits, p35 and p40. To ensure the stoichiometric expression and assembly of p35 and p40, we expressed a single-chain version of MuIL-12. Plant-derived single-chain MuIL-12 was characterized and purified for in vitro bioactivity assays. Our results demonstrated precise cleavage of the endogenous mouse p40 signal peptide in plants as well as addition of N-linked glycans. Plant-derived MuIL-12 triggered induction of interferon-gamma (IFN-gamma) secretion from mouse splenocytes and stimulated splenocyte proliferation with comparable activities to those observed for commercially available animal cell-derived MuIL-12. These studies indicate that plants produce fully functional MuIL-12 at levels compatible with commercial production and may serve as an effective bioproduction platform for bioactive IL-12s from other species for human or veterinary vaccine and therapeutic applications.

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.

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.

Specificity of DC-SIGN for mannose- and fucose-containing glycans

The dendritic cell specific C-type lectin dendritic cell specific ICAM-3 grabbing non-integrin (DC-SIGN) binds to "self" glycan ligands found on human cells and to "foreign" glycans of bacterial or parasitic pathogens. Here, we investigated the binding properties of DC-SIGN to a large array of potential ligands in a glycan array format. Our data indicate that DC-SIGN binds with K(d)<2muM to a neoglycoconjugate in which Galbeta1-4(Fucalpha1-3)GlcNAc (Le(x)) trisaccharides are expressed multivalently. A lower selective binding was observed to oligomannose-type N-glycans, diantennary N-glycans expressing Le(x) and GalNAcbeta1-4(Fucalpha1-3)GlcNAc (LacdiNAc-fucose), whereas no binding was observed to N-glycans expressing core-fucose linked either alpha1-6 or alpha1-3 to the Asn-linked GlcNAc of N-glycans. These results demonstrate that DC-SIGN is selective in its recognition of specific types of fucosylated glycans and subsets of oligomannose- and complex-type N-glycans.

Molecular cloning and heterologous expression of beta1,2-xylosyltransferase and core alpha1,3-fucosyltransferase from maize

Maize is considered a promising alternative production system for pharmaceutically relevant proteins. However, like in all other plant species asparagine-linked oligosaccharides of maize glycoproteins are modified with beta1,2-xylose and core alpha1,3-fucose sugar residues, which are considered to be immunogenic in mammals. This altered N-glycosylation when compared to mammalian cells may reduce the potential of maize as a production system for heterologous glycoproteins. Here we report the cloning and characterization of the cDNA sequences coding for the maize enzymes beta1,2-xylosyltransferase (XylT) and core alpha1,3-fucosyltransferase (FucT). The cloned XylT and FucT cDNAs were shown to encode enzymatically active proteins, which were independently able to convert a mammalian acceptor glycoprotein into an antigen binding anti-plant N-glycan antibodies. The complete sequence of the XylT gene was determined. Evidence for the presence of at least three XylT and FucT gene loci in the maize genome was obtained. The identification of the two enzymes and their genes will allow the targeted downregulation or even elimination of beta1,2-xylose and core alpha1,3-fucose addition to recombinant glycoproteins produced in maize.

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.

Localization of Organelle Proteins by Isotope Tagging (LOPIT)

We describe a proteomics method for determining the subcellular localization of membrane proteins. Organelles are partially separated using centrifugation through self-generating density gradients. Proteins from each organelle co-fractionate and therefore exhibit similar distributions in the gradient. Protein distributions can be determined through a series of pair-wise comparisons of gradient fractions, using cleavable ICAT to enable relative quantitation of protein levels by MS. The localization of novel proteins is determined using multivariate data analysis techniques to match their distributions to those of proteins that are known to reside in specific organelles. Using this approach, we have simultaneously demonstrated the localization of membrane proteins in both the endoplasmic reticulum and the Golgi apparatus in Arabidopsis. Localization of organelle proteins by isotope tagging is a new tool for high-throughput protein localization, which is applicable to a wide range of research areas such as the study of organelle function and protein trafficking.

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.

Generation of Arabidopsis thaliana plants with complex N-glycans lacking beta1,2-linked xylose and core alpha1,3-linked fucose

The plant glycosyltransferases, beta1,2-xylosyltransferase (XylT) and core alpha1,3-fucosyltransferase (FucT), are responsible for the transfer of beta1,2-linked xylose and core alpha1,3-linked fucose residues to glycoprotein N-glycans. These glycan epitopes are not present in humans and thus may cause immunological responses, which represent a limitation for the therapeutic use of recombinant mammalian glycoproteins produced in transgenic plants. Here we report the genetic modification of the N-glycosylation pathway in Arabidopsis thaliana plants. Knockout plants were generated with complete deficiency of XylT and FucT. These plants lack antigenic protein-bound N-glycans and instead synthesise predominantly structures with two terminal betaN-acetylglucosamine residues (GlcNAc(2)Man(3)GlcNAc(2)).

C-terminal amino acids of Helicobacter pylori alpha1,3/4 fucosyltransferases determine type I and type II transfer

The alpha1,3/4 fucosyltransferase (FucT) enzyme from Helicobacter pylori catalyzes fucose transfer from donor GDP-beta-l-fucose to the GlcNAc group of two series of acceptor substrates in H. pylori lipopolysaccharide: betaGal1,3betaGlcNAc (Type I) or betaGal1,4betaGlcNAc (Type II). Fucose is added either in alpha1,3 linkage of Type II acceptor to produce Lewis X or in alpha1,4 linkage of Type I acceptor to produce Lewis A, respectively. H. pylori FucTs from different strains have distinct Type I or Type II substrate specificities. FucT in H. pylori strain NCTC11639 has an exclusive alpha1,3 activity because it recognizes only Type II substrates, whereas FucT in H. pylori strain UA948 can utilize both Type II and Type I acceptors; thus it has both alpha1,3 and alpha1,4 activity, respectively. To identify elements conferring substrate specificity, 12 chimeric FucTs were constructed by domain swapping between 11639FucT and UA948FucT and characterized for their ability to transfer fucose to Type I and Type II acceptors. Our results indicate that the C-terminal region of H. pylori FucTs controls Type I and Type II acceptor specificity. In particular, the highly divergent C-terminal portion, seven amino acids DNPFIFC at positions 347-353 in 11639FucT, and the corresponding 10 amino acids CNDAHYSALH at positions 345-354 in UA948FucT, controls the Type I and Type II acceptor recognition. This is the opposite of mammalian FucTs where acceptor preference is determined primarily by the N-terminal residues in the hypervariable stem domain.

Structural requirements for Arabidopsis beta1,2-xylosyltransferase activity and targeting to the Golgi

Characterization of a beta1,2-xylosyltransferase from Arabidopsis thaliana (AtXylT) was carried out by expression in Sf9 insect cells using a baculovirus vector system. Serial deletions at both the N- and C-terminal ends proved that integrity of a large domain located between amino acid 31 and the C-terminal lumenal region is required for AtXylT activity expression. The influence of N-glycosylation on AtXylT activity has been evaluated using either tunicamycin or mutagenesis of potential N-glycosylation sites. AtXylT is glycosylated on two of its three potential N-glycosylation sites (Asn51, Asn301, Asn478) and the occupancy of at least one of these two sites (Asn51 and Asn301) is necessary for AtXylT stability and activity. Contribution of the N-terminal part of AtXylT in targeting and intracellular distribution of this protein was studied by expression of variably truncated, GFP-tagged AtXylT forms in tobacco cells using confocal and electron microscopy. These studies have shown that the transmembrane domain of AtXylT and its short flanking amino acid sequences are sufficient to specifically localize a reporter protein to the medial Golgi cisternae in tobacco cells. This study is the first detailed characterization of a plant glycosyltransferase at the molecular level.

Alpha1,4-fucosyltransferase activity: a significant function in the primate lineage has appeared twice independently

In the animal kingdom the enzymes that catalyze the formation of alpha1,4 fucosylated-glycoconjugates are known only in apes (chimpanzee) and humans. They are encoded by FUT3 and FUT5 genes, two members of the Lewis FUT5-FUT3-FUT6 gene cluster, which had originated by duplications of an alpha3 ancestor gene. In order to explore more precisely the emergence of the alpha1,4 fucosylation, new Lewis-like fucosyltransferase genes were studied in species belonging to the three main primate groups. Two Lewis-like genes were found in brown and ruffed lemurs (prosimians) as well as in squirrel monkey (New World monkey). In the latter, one gene encodes an enzyme which transfers fucose only in alpha1,3 linkage, whereas the other is a pseudogene. Three genes homologous to chimpanzee and human Lewis genes were identified in rhesus macaque (Old World monkey), and only one encodes an alpha3/4-fucosyltransferase. The ability of new primate enzymes to transfer fucose in alpha1,3 or alpha1,3/4 linkage confirms that the amino acid R or W in the acceptor-binding motif "HH(R/W)(D/E)" is required for the type 1/type 2 acceptor specificity. Expression of rhesus macaque genes proved that fucose transfer in alpha1,4 linkage is not restricted to the hominoid family and may be extended to other Old World monkeys. Moreover, the presence of only one enzyme supporting the alpha1,4 fucosylation in rhesus macaque versus two enzymes in hominoids suggests that this function occurred twice independently during primate evolution.

The presence of Lewis a epitopes in Arabidopsis thaliana glycoconjugates depends on an active alpha4-fucosyltransferase gene

The presence of an alpha4-fucosyltransferase in plants was first deduced from the characterization of Lewis-a glycoepitopes in some N-glycans. The first plant gene encoding an alpha4-fucosyltransferase was recently cloned in Beta vulgaris. In the present paper we provide evidence for the presence of an alpha4-fucosyltransferase in A. thaliana by measurement of this glycosyltransferase activity from a purified microsomal preparation and by immunolocalization of Le(a) epitopes on glycans N-linked to glycoproteins located to the Golgi apparatus and on the cell surface. The corresponding gene AtFT4 (AY026941) was characterized. A unique copy of this gene was found in A. thaliana genome, and a single AtFT4 transcript was revealed in leaves, in roots, and at a lower extent in flowers. The coding sequence of AtFT4 gene is interrupted by two introns spanning 465 bp and 84 bp, respectively. The putative 393-amino-acid protein (44 kDa, pI: 6.59) contains an N-terminal hydrophobic region and one potential N-glycosylation site, but AtFT4 has poor homology (less than 30%) to the other alpha3/4-fucosyltransferases except for motif II. When expressed in COS 7 cells the protein is able to transfer Fuc from GDP-Fuc to a type 1 acceptor substrate, but this transferase activity is detected only in the culture medium of transfected cells

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

Recently, it has been found that plants, including tomato (Lycopersicon esculentum), express the Lewis-a epitope, Galbeta1,3(Fucalpha1,4)GlcNAc, on some N-glycans. By searching the EST database, it was possible to identify a tomato cDNA encoding a protein, designated FucTC, of 413 amino acids with homology to plant and mammalian alpha1,3/4-fucosyltransferases. The cDNA was expressed in Pichia pastoris and the recombinant enzyme was found to transfer fucose from GDP-Fuc (K(m) 16 microM) to lacto-N-tetraose (Galbeta1,3GlcNAcbeta1,3Galbeta1,4Glc; K(m) 80 microM) as well as to beta1,3- and beta1,4-galactosylated N-glycans. It is concluded that FucTC is responsible for the biosynthesis of Lewis-a on N-glycans in tomato.