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

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.

β1,3-galactosyltransferase on chromosome 6 is essential for the formation of Lewis a structure on N-glycan in Oryza sativa

β1,3-galactose is the component of outer-chain elongation of complex N-glycans that, together with α1,4-fucose, forms Lewis a structures in plants. Previous studies have revealed that N-glycan maturation is mediated by sequential attachment of β1,3-galactose and α1,4-fucose by individual β1,3-galactosyltransferase (GalT) and α1,4-fucosyltransferase (1,4-FucT), respectively. Although GalT from several species has been studied, little information about GalT from rice is available. I therefore characterized three GalT candidate genes on different chromosomes in Oryza sativa. Seeds of rice lines that had T-DNA insertions in regions corresponding to individual putative GalT genes were obtained from a Rice Functional Genomic Express Database and plants grown until maturity. Homozygotes were selected from the next generation by genotyping PCR, and used for callus induction. Callus extracts of two independent T-DNA mutant rice which have T-DNA insertions at the same gene on chromosome 6 but in different exons showed highly reduced band intensity on a western blots using an anti-Lewis a antibody. Cell extracts and cultured media from suspension culture of the one of these mutant rice were further analysed by N-glycan profiling using matrix-associated laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF). Identified N-glycan species containing β1,3-galactose from both cell extracts and cultured media of knock-out mutant were less than 0.5% of total N-glycans while that of WT cells were 9.8% and 49.1%, respectively. This suggests that GalT located on rice chromosome 6 plays a major role in N-glycan galactosylation, and mutations within it lead to blockage of Lewis a epitope formation.

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.

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.

H. pylori α1-3/4-fucosyltransferase (Hp3/4FT)-catalyzed one-pot multienzyme (OPME) synthesis of Lewis antigens and human milk fucosides

Helicobacter pylori α1-3/4-fucosyltransferase (Hp3/4FT) was expressed in Escherichia coli at a level of 30 mg L^-1 culture and used as a diverse catalyst in a one-pot multienzyme (OPME) system for high-yield production of l-fucose-containing carbohydrates including Lewis antigens such as Lewis a, b, and x, O-sulfated Lewis x, and sialyl Lewis x and human milk fucosides such as 3-fucosyllactose (3-FL), lacto-N-fucopentaose (LNFP) III, and lacto-N-difuco-hexaose (LNDFH) II and III. Noticeably, while difucosylation of tetrasaccharides was readily achieved using an excess amount of donor, the synthesis of LNFP III was achieved by Hp3/4FT-catalyzed selective fucosylation of the N-acetyllactosamine (LacNAc) component in lacto-N-neotetraose (LNnT).

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.

Distantly related plant and nematode core α1,3-fucosyltransferases display similar trends in structure-function relationships

Here, we present a comparative structure-function study of a nematode and a plant core α1,3-fucosyltransferase based on deletion and point mutations of the coding regions of Caenorhabditis elegans FUT-1 and Arabidopsis thaliana FucTA (FUT11). In particular, our results reveal a novel "first cluster motif" shared by both core and Lewis-type α1,3-fucosyltransferases of the GT10 family. To evaluate the role of the conserved serine within this motif, this residue was replaced with alanine in FucTA (S218) and FUT-1 (S243). The S218A replacement completely abolished the enzyme activity of FucTA, while the S243A mutant of FUT-1 retained 20% of the "wild-type" activity. Based on the results of homology modeling of FucTA, other residues potentially involved in the donor substrate binding were examined, and mutations of N219 and R226 dramatically affected enzymatic activity. Finally, as both FucTA and FUT-1 were shown to be N-glycosylated, we examined the putative N-glycosylation sites. While alanine replacements at single potential N-glycosylation sites of FucTA resulted in a loss of up to 80% of the activity, a triple glycosylation site mutant still retained 5%, as compared to the control. In summary, our data indicate similar trends in structure-function relationships of distantly related enzymes which perform similar biochemical reactions and form the basis for future work aimed at understanding the structure of α1,3-fucosyltransferases in general.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for the period 2005-2006

This review is the fourth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2006. The review covers fundamental studies, fragmentation of carbohydrate ions, method developments, and applications of the technique to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, glycated proteins, glycolipids from bacteria, glycosides, and various other natural products. There is a short section on the use of MALDI-TOF mass spectrometry for the study of enzymes involved in glycan processing, a section on industrial processes, particularly the development of biopharmaceuticals and a section on the use of MALDI-MS to monitor products of chemical synthesis of carbohydrates. Large carbohydrate-protein complexes and glycodendrimers are highlighted in this final section.

UDP-glucose:(6-methoxy)podophyllotoxin 7-O-glucosyltransferase from suspension cultures of Linum nodiflorum

Cell cultures of Linum species store 6-methoxypodophyllotoxin (MPTOX), podophyllotoxin (PTOX) and related lignans as O-glucosides. UDP-glucose:(M)PTOX 7-O-glucosyltransferase has been detected and characterised in protein preparations of suspension-cultured cells of Linum nodiflorum L. (Linaceae). The maximal lignan glucoside contents in the cells are preceded by a rapid increase of the specific glucosyltransferase activity on day six of the culture period. MPTOX glucoside is the major lignan with up to 1.18 mg g(-1) of the cell dry wt which is more than 30-fold of the PTOX glucoside content. Of the three aryltetralin lignans tested as substrates, PTOX and MPTOX display comparable apparent K(m) values of 4.7 and 5.4 microM, respectively. 5'-Demethoxy-6-methoxypodophyllotoxin is converted with the highest velocity of 25.2 pkat mg(-1) while also possessing a higher K(m) of 14.7 microM. Two-substrate test series indicate that all three compounds compete for the active site of a single protein. The structurally similar lignan beta-peltatin acts as competitive inhibitor as well. However, the 6-O-glucosidation is most likely catalysed by a separate enzyme. The (M)PTOX 7-O-glucosyltransferase works best at a pH around 9 and a temperature around 35 degrees C. A 15-30% increase of the reaction rate is effected by the addition of 0.9 mM Mn(2+).

Production of a monoclonal antibody in plants with a humanized N-glycosylation pattern

In recent years, plants have become an attractive alternative for the production of recombinant proteins. However, their inability to perform authentic mammalian N-glycosylation may cause limitations for the production of therapeutics. A major concern is the presence of beta1,2-xylose and core alpha1,3-fucose residues on complex N-linked glycans, as these N-glycan epitopes are immunogenic in mammals. In our attempts towards the humanization of plant N-glycans, we have generated an Arabidopsis thaliana knockout line that synthesizes complex N-glycans lacking immunogenic xylose and fucose epitopes. Here, we report the expression of a monoclonal antibody in these glycan-engineered plants that carry a homogeneous mammalian-like complex N-glycan pattern without beta1,2-xylose and core alpha1,3-fucose. Plant and Chinese hamster ovary (CHO)-derived immunoglobulins (IgGs) exhibited no differences in electrophoretic mobility and enzyme-linked immunosorbent specificity assays. Our results demonstrate the feasibility of a knockout strategy for N-glycan engineering of plants towards mammalian-like structures, thus providing a significant improvement in the use of plants as an expression platform.

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.

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.