Endo-xyloglucan transferase, a novel class of glycosyltransferase that catalyzes transfer of a segment of xyloglucan molecule to another xyloglucan molecule.

Xyloglucans are the major component of plant cell walls and bind tightly to the surface of individual cellulose microfibrils, thereby cross-linking them into a complex polysaccharide network of the cell wall. The cleavage and reconnection of xyloglucan cross-links are considered to play the leading role during chemical processes essential for wall expansion and, therefore, cell growth and differentiation. Although it is hypothesized that some transglycosylation is involved in these chemical processes, the enzyme responsible for the reaction was not identified. We have now purified a novel class of endo-type glycosyltransferase to apparent homogeneity from the extracellular space or the cell wall of the epicotyls of Vigna angularis, a bean plant. The enzyme is a glycoprotein with a molecular mass of about 33 kDa. The enzyme catalyzes both 1) endo-type splitting of a xyloglucan molecule and 2) linking of a newly generated reducing end of the xyloglucan to the nonreducing end of another xyloglucan molecule, thereby mediating the transfer of a large segment of the xyloglucan to another xyloglucan molecule. The transferase exhibits no glycosidase or glycanase activity. Substrate specificity of the enzyme was investigated using several polysaccharides with different glycosidic linkages as donor substrates and pyridylamino oligosaccharides as acceptor substrates, in which the reducing end of the carbohydrate was tagged with a fluorescent group. The enzyme required a basic xyloglucan structure, i.e. a beta-(1-->4)-glucosyl backbone with xylosyl side chains, for both acceptor and donor activity. Galactosyl or fucosyl side chains on the main chain were not required for the acceptor activity. The enzyme exhibited higher reaction rates when xyloglucans with higher M(r) were used as donor substrates. Xyloglucans smaller than 10 kDa were no longer the donor substrate. On the other hand, pyridylamino heptasaccharide acted as a good acceptor as did xyloglucan polymers. Based on these results we propose to designate this novel enzyme a xyloglucan: xyloglucano-transferase, to be abbreviated endo-xyloglucan transferase (EXT) or xyloglucan recombinase. This enzyme is the first enzyme identified that mediates the transfer of a high M(r) segment between polysaccharide molecules to generate chimeric polymers. We conclude that endo-xyloglucan transferase functions as a reconnecting enzyme for xyloglucans and is involved in the interweaving or reconstruction of cell wall matrix, which is responsible for chemical creepage that leads to morphological changes in the cell wall.     

Role of transglycosylation in the integration of xyloglucan into the growing plant cell wall in vivo

Abstract

Xyloglucan endotransglycosylase (XET) activity is widespread in plant cell walls, but its action on xyloglucan in vivo has been difficult to prove because the reaction products are not expected to differ chemically from the reactants. By feeding of cultured Rosa cells with [¹³C]glucose and [³H]arabinose followed by [¹²-C]glucose, and isopyenic centrifugation of the extracted xyloglucan in caesium trifluoroacetate, we have obtained evidence for the annealing of segments of newly-secreted xyloglucan to xyloglucan chains that were already present in the cell wall. This is the first evidence for interpolymeric transglycosylation of xyloglucan in vivo.     

Bilberry xyloglucan--novel building blocks containing beta-xylose within a complex structure

Bilberries are known to have one of the most complex xyloglucan structures described in the plant kingdom until now. To characterise this structure, xyloglucans were enzymatically degraded and the oligosaccharides obtained were analysed. More than 20 different building blocks were found to make up the xyloglucan polymer. Bilberry xyloglucan was of XXXG-type, but some XXG-type oligomers were present, as well. The building blocks contain galactose-xylose (L) and fucose-galactose-xylose (F) side chains. In both side chains, the galactose units can be acetylated. In addition, beta-xylose-alpha-xylose (U) side chains were shown. This U chain was present in three building blocks described before (XUXG, XLUG and XUFG) and four novel blocks that have not been described in the literature previously: XUG, XUUG, XLUG and XXUG.     

Assessment of in vitro binding of isolated pectic domains to cellulose by adsorption isotherms, electron microscopy, and X-ray diffraction methods

Isolated pectic domains representative of the pectic backbone and the neutral sugar side chains were tested for their ability to interact with cellulose in comparison to the well-known binding of xyloglucan. Pectic side chains displayed a significant in vitro binding capacity to cellulose, whereas pectic backbone domains exhibited only slight adsorption to cellulose microfibrils. To support the binding results, electron microscopy and X-ray diffraction were applied. Celluloses from bacteria and sugar beet cell walls were used as substrates for the precipitation of isolated pectic domains or xyloglucan by acetone vapor diffusion. Pectic side chains grew attached to the cellulose surfaces, whereas pectic backbone domains were observed separately from cellulose microfibrils. Xyloglucan seeded with cellulose provoked a decrease of microfibrils entanglement, but no clear cross-links between neighboring microfibrils were observed. These results led to the elucidation of the pectic domains responsible for binding with cellulose microfibrils.     

Xyloglucan in the primary cell wall: assessment by FESEM,selective enzyme digestions and nanogold affinity tags

Xyloglucan has been hypothesized to bind extensively to cellulose microfibril surfaces and to tether microfibrils into a load‐bearing network, thereby playing a central role in wall mechanics and growth, but this view is challenged by newer results. Here we combined high‐resolution imaging by field emission scanning electron microscopy (FESEM) with nanogold affinity tags and selective endoglucanase treatments to assess the spatial location and conformation of xyloglucan in onion cell walls. FESEM imaging of xyloglucanase‐digested cell walls revealed an altered microfibril organization but did not yield clear evidence of xyloglucan conformations. Backscattered electron detection provided excellent detection of nanogold affinity tags in the context of wall fibrillar organization. Labelling with xyloglucan‐specific CBM76 conjugated with nanogold showed that xyloglucans were associated with fibril surfaces in both extended and coiled conformations, but tethered configurations were not observed. Labelling with nanogold‐conjugated CBM3, which binds the hydrophobic surface of crystalline cellulose, was infrequent until the wall was predigested with xyloglucanase, whereupon microfibril labelling was extensive. When tamarind xyloglucan was allowed to bind to xyloglucan‐depleted onion walls, CBM76 labelling gave positive evidence for xyloglucans in both extended and coiled conformations, yet xyloglucan chains were not directly visible by FESEM. These results indicate that an appreciable, but still small, surface of cellulose microfibrils in the onion wall is tightly bound with extended xyloglucan chains and that some of the xyloglucan has a coiled conformation.     

Endo-xyloglucan transferase,a new class of transferase involved in cell wall construction

Cell shape in plants is constrained by cell walls, which are thick yet dynamic structures composed of crystalline cellulose microfibrils and matrix polymers. Xyloglucans are the principal component of the matrix polymers and bind tightly to the surface of cellulose microfibrils and thereby cross-link them to form an interwoven xyloglucan-cellulose network structure. Thus, cleavage and reconnection of the cross-links between xyloglucan molecules are required for the rearrangement of the cell wall architecture, the process essential for both cell wall expansion and the wall deposition occurring during cell growth and differentiation. Endoxyloglucan transferase (EXT) is a newly identified class of transferase that catalyzes molecular grafting between xyloglucan molecules. This enzyme catalyzes both endo-type splitting of a xyloglucan molecule and reconnection of a newly generated reducing terminus of the xyloglucan to the non-reducing terminus of another xyloglucan molecule, thereby mediating molecular grafting between xyloglucan cross-links in plant cell walls. Molecular cloning and sequencing of EXT-cDNAs derived from five different plant species includingA. thaliana andV. angularis has revealed that the amino acid sequence of the mature protein is extensively conserved in the five different plant species, indicating that EXT protein is ubiquitous among higher plants. This structural study has also disclosed the presence of a group of xyloglucan related proteins (XRPs) with transferase activity in higher plants. Current data strongly suggest that these proteins are involved in a wide spectrum of physiological activities including cell wall expansion and deposition in growing cell walls. Recipient of the Botanical Sociaty Award of Young Scientists, 1993.     

Synergistic interaction of xyloglucan and xanthan investigated by rheology, differential scanning calorimetry, and NMR

A new synergistic interaction between tamarind seed xyloglucan and xanthan was found and investigated by rheology, differential scanning calorimetry (DSC), and NMR. The effect of the acetyl and pyruvate groups in the side chain in xanthan on the synergistic interaction was also examined. The shear moduli G' and G' ' of the mixture solution of xyloglucan and native (or acetate-free) xanthan increased steeply at around 22 degrees C upon cooling. An exothermic DSC peak appeared at the same temperature. A drastic decrease in the of the acetyl and pyruvate groups of the xanthan side chain was observed from 1H NMR spectra only in the mixture at low temperatures (<25 degrees C). It was found that the pyruvate group is more restricted in the mixture solution compared with the acetyl group. The mixture of xyloglucan and pyruvate-free xanthan showed no synergistic interaction. We concluded that this synergistic interaction is caused by the intermolecular binding between xyloglucan and xanthan, and, in the heterotypic junction zones, the xanthan side chain becomes a new state that is different from both the coil and helix states.     

Modification of cellulose fiber surfaces by use of a lipase and a xyloglucan endotransglycosylase

A strategy for the modification of cellulose fiber surfaces was developed that used the ability of Candida antarctica lipase B (CALB) to acylate carbohydrates with high regioselectivity, combined with the transglycosylating activity of the Populus tremula x P. tremuloides xyloglucan endotransglycosylase 16A (PttXET16A). Xyloglucan oligosaccharides (XGOs) prepared from tamarind xyloglucan were acylated with CALB as a catalyst and vinyl stearate or gamma-thiobutyrolactone as acyl donors to produce carbohydrate molecules with hydrophobic alkyl chains or reactive sulfhydryl groups, respectively. The modified XGOs were shown to act as glycosyl acceptors in the transglycosylation reaction catalyzed by PttXET16A and could therefore be incorporated into high M(r) xyloglucan chains. The resulting xyloglucan molecules exhibited a high affinity for cellulose surfaces, which enabled the essentially irreversible introduction of fatty acid esters or thiol groups to cellulose fibers.     

Xyloglucan is recognized by carbohydrate-binding modules that interact with beta-glucan chains

Enzyme systems that attack the plant cell wall contain noncatalytic carbohydrate-binding modules (CBMs) that mediate attachment to this composite structure and play a pivotal role in maximizing the hydrolytic process. Although xyloglucan, which includes a backbone of beta-1,4-glucan decorated primarily with xylose residues, is a key component of the plant cell wall, CBMs that bind to this polymer have not been identified. Here we showed that the C-terminal domain of the modular Clostridium thermocellum enzyme CtCel9D-Cel44A (formerly known as CelJ) comprises a novel CBM (designated CBM44) that binds with equal affinity to cellulose and xyloglucan. We also showed that accommodation of xyloglucan side chains is a general feature of CBMs that bind to single cellulose chains. The crystal structures of CBM44 and the other CBM (CBM30) in CtCel9D-Cel44A display a beta-sandwich fold. The concave face of both CBMs contains a hydrophobic platform comprising three tryptophan residues that can accommodate up to five glucose residues. The orientation of these aromatic residues is such that the bound ligand would adopt the twisted conformation displayed by cello-oligosaccharides in solution. Mutagenesis studies confirmed that the hydrophobic platform located on the concave face of both CBMs mediates ligand recognition. In contrast to other CBMs that bind to single polysaccharide chains, the polar residues in the binding cleft of CBM44 play only a minor role in ligand recognition. The mechanism by which these proteins are able to recognize linear and decorated beta-1,4-glucans is discussed based on the structures of CBM44 and the other CBMs that bind single cellulose chains.     

Crystallographic insight into the evolutionary origins of xyloglucan endotransglycosylases and endohydrolases

The xyloglucan endotransglycosylase/hydrolase (XTH) gene family encodes enzymes of central importance to plant cell wall remodeling. The evolutionary history of plant XTH gene products is incompletely understood vis‐à‐vis the larger body of bacterial endoglycanases in Glycoside Hydrolase Family 16 (GH16). To provide molecular insight into this issue, high‐resolution X‐ray crystal structures and detailed enzyme kinetics of an extant transitional plant endoglucanase (EG) were determined. Functionally intermediate between plant XTH gene products and bacterial licheninases of GH16, Vitis vinifera EG16 (VvEG16) effectively catalyzes the hydrolysis of the backbones of two dominant plant cell wall matrix glycans, xyloglucan (XyG) and β(1,3)/β(1,4)‐mixed‐linkage glucan (MLG). Crystallographic complexes with extended oligosaccharide substrates reveal the structural basis for the accommodation of both unbranched, mixed‐linked (MLG) and highly decorated, linear (XyG) polysaccharide chains in a broad, extended active‐site cleft. Structural comparison with representative bacterial licheninases, a xyloglucan endotranglycosylase (XET), and a xyloglucan endohydrolase (XEH) outline the functional ramifications of key sequence deletions and insertions across the phylogenetic landscape of GH16. Although the biological role(s) of EG16 orthologs remains to be fully resolved, the present biochemical and tertiary structural characterization provides key insight into plant cell wall enzyme evolution, which will continue to inform genomic analyses and functional studies across species.     

The structure of plant cell walls: v. On the binding of xyloglucan to cellulose fibers

Cell wall strength is decreased by both auxin treatment and low pH. In a recently proposed model of the plant cell wall, xyloglucan polymers are hydrogen-bonded to cellulose fibrils, forming the only noncovalent link in the network of polymers which cross-link the cellulose fibers. The decreased strength of the cell wall seen upon lowering the pH might be due to an effect of hydrogen ions on the rate of xyloglucan creep along cellulose fibers. This paper investigates binding of xyloglucan fragments to cellulose. At equilibrium, the per cent of nine- and seven-sugar xyloglucan fragments which are bound to cellulose is sensitive to both temperature and the concentration of nonaqueous solvents. However, neither the per cent of xyloglucan fragments bound to cellulose at equilibrium, nor the rate at which the xyloglucan fragments bind to cellulose, is sensitive to changes in hydrogen ion concentration. These results support the hypothesis that, within the cell wall, xyloglucan chains are connected to cellulose fibers by hydrogen bonds, but these results suggest that this interconnection between xyloglucan and cellulose is unlikely to be the point within the wall which regulates the rate of cell elongation.     

Polysaccharides in germination. Xyloglucans (`amyloids'') from the cotyledons of white mustard

Two xyloglucan fractions have been isolated from the cotyledons of resting white-mustard seeds, the first by extraction with hot EDTA, and the second by subsequent extraction with alkali or lithium thiocyanate. Although both appear to have the ;amyloid' type of structure in which chains of (1-->4)-linked beta-d-glucopyranose residues carry d-xylose-rich side chains through position 6, these side chains are rather different in structure in the two polysaccharide fractions, and the second or ;insoluble' xyloglucan has fewer of them. The side chains in both polysaccharides are also different from those in other seed amyloids, especially in having xylose linked through positions 3 and 4 (instead of through position 2 as usual) and in containing fucose residues. Both polysaccharides show the characteristic blue ;amyloid' colour with iodine in the presence of sodium sulphate, and it is suggested that this arises by the interaction of iodine molecules and possibly iodide ions within the interstices between aggregated xyloglucan chains. ;Soluble' xyloglucan is metabolized during germination and is presumed to have a reserve function. ;Insoluble' xyloglucan is metabolized less completely over the period studied but its lack of turnover during cell-wall differentiation indicates that it also is a reserve. These and other beta-(1-->4)-linked reserve polysaccharides of seeds might also have a structural function which is of particular value for the survival of the dormant seed.     

Conformational folding of xyloglucan side chains in aqueous solution from molecular dynamics simulation

Molecular dynamics simulation was carried out on xyloglucan with explicit water molecules to investigate the folding mechanism of side chains onto a main chain in aqueous solution. The model xyloglucan was composed of 12 beta-D-glucopyranoses as a main chain substituted with six galactoses and three xyloses as side chains. Two conditions were set for the ribbon-like main chain; one is restricted to be 'flat' and the other is without restriction. The free main chain of xyloglucan has a 'twisted' conformation as the major one. Conformational folding of side chains onto the main chain was analyzed with dihedral angles at each glycosidic linkage. In a 5-ns calculation, the xyloglucan has a tendency to contract in both the restricted and the free systems, but the mode of contraction is different. Side chains tend to stick onto the flat surface of the main chain in the restricted system, while they do not tightly do so in the free one; instead the main chain takes a twisted and sometimes embowed conformation. This result indicates that the main chain has greater attractive forces to bind side chains when it is flat, while it loses the ability as it is twisted.     

Fucosylation of xyloglucan: localization of the transferase in dictyosomes of pea stem cells

Microsomal membranes from elongating regions of etiolated Pisum sativum stems were separated by rate-zonal centrifugation on Renografin gradients. The transfer of labeled fucose and xylose from GDP-[¹⁴C] fucose and UDP-[¹⁴C]xylose to xyloglucan occurred mainly in dictyosomeenriched fractions. No transferase activity was detected in secretory vesicle fractions. Pulse-chase experiments using pea stem slices incubated with [³H]fucose suggest that xyloglucan chains are fucosylated and their structure completed within the dictyosomes, before being transported to the cell wall by secretory vesicles.     

In vitro assembly of cellulose/xyloglucan networks: ultrastructural and molecular aspects

Features of the interaction between cellulose and xyloglucan have been studied using the cellulose-producing bacterium Acetobacter aceti ssp. xylinum (ATCC 53524) and tamarind seed xyloglucan. Direct microscopic evidence is provided for the generation of cross-bridges between cellulose ribbons produced in the presence of xyloglucan but not carboxymethyl-cellulose. Cross-bridge lengths are very similar to those observed for de-pectinated onion cell walls. Similar cross-bridge lengths are observed following mixing of isolated A. xylinum cellulose and xyloglucan, showing that network formation can be an abiotic process. The level of incorporation of xyloglucan in an actively growing system (ca. 38% of cellulose) is an order of magnitude higher than that observed in mixtures of isolated polymers and is comparable with cell wall levels. NMR spectroscopy suggests that 80–85% of incorporated xyloglucan is segmentally rigid with the backbone adopting an extended ‘cellulosic’ conformation and probably aligned with cellulose chains. The remaining xyloglucan is more mobile and is assigned to cross-bridges with, on average, a twisted backbone conformation. No evidence for specific involvement of side-chain residues in binding is found, and the observation of cross-bridges with a non-fucosylated xyloglucan shows that fucose residues are not essential for network formation. Xyloglucan causes cellulose ribbons to become more amorphous and to have a decreased 1α/1β crystallite ratio without any significant alteration in ribbon diameter. Based on the findings that levels of xyloglucan incorporation, the presence and lengths of cross-bridges, and the modification of cellulosic molecular organization are all similar to those found in plant cell walls, we suggest that A. aceti ssp. xylinum is a more useful model for primary plant cell walls and their assembly than has previously been appreciated.     

Glyco-array technology for efficient monitoring of plant cell wall glycosyltransferase activities

The plant cell wall is a complex network of polysaccharides. The diversity in the linkage types connecting all monosaccharides within these polysaccharides would need a large set of glycosyltransferases to catalyze their formation. Development of a methodology that would allow monitoring of glycosyltransferase activities in an easy and high-throughput manner would help assign biochemical functions, and understand their roles in building this complex network. A microarray-based method was optimized for testing glycosyltransferases involved in plant wall biosynthesis using an α(1,2)fucosyltransferase involved in xyloglucan biosynthesis. The method is simple, sensitive, and easy to implement in any lab. Tamarind xyloglucan polymer and trimer, and a series of cello-oligosaccharides were immobilized on a thin-coated photo-activable glass slide. The slide with the attached sugars was then used to estimate the incorporation of [¹⁴C]Fuc onto xyloglucan polymer and trimer. [¹⁴C]-radiolabel incorporation is revealed with a standard phosphoimager scanner, after exposure of the glycochip to a phosphor screen and detection. The method proved to be sensitive enough to detect as low as 45 cpm/spot. Oriented anchoring of small oligosaccharides (trimer) was required for optimal transferase activities. The glycochip was also used to monitor and estimate xyloglucan fucosyltransferase activity in detergent-solubilized crude extracts from pea microsomes that are known to contain this enzyme activity. Our data indicate that the methodology can be used for efficient and rapid monitoring of glycosyltransferase activities involved in plant wall polysaccharides biosynthesis. Matthew Shipp and Ramya Nadella contributed equally to this work.     

An investigation of the substrate specificity of the xyloglucanase Cel74A from Hypocrea jecorina

The substrate specificity of the xyloglucanase Cel74A from Hypocrea jecorina (Trichoderma reesei) was examined using several polysaccharides and oligosaccharides. Our results revealed that xyloglucan chains are hydrolyzed at substituted Glc residues, in contrast to the action of all known xyloglucan endoglucanases (EC 3.2.1.151). The building block of xyloglucan, XXXG (where X is a substituted Glc residue, and G is an unsubstituted Glc residue), was rapidly degraded to XX and XG (k(cat) = 7.2 s(-1) and Km = 120 microM at 37 degrees C and pH 5), which has only been observed before with the oligoxyloglucan-reducing-end-specific cellobiohydrolase from Geotrichum (EC 3.2.1.150). However, the cellobiohydrolase can only release XG from XXXGXXXG, whereas Cel74A hydrolyzed this substrate at both chain ends, resulting in XGXX. Differences in the length of a specific loop at subsite + 2 are discussed as being the basis for the divergent specificity of these xyloglucanases.     

Tensile properties of Arabidopsis cell walls depend on both a xyloglucan cross-linked microfibrillar network and rhamnogalacturonan II-borate complexes

The mechanical properties of plant organs depend upon anatomical structure, cell-cell adhesion, cell turgidity, and the mechanical properties of their cell walls. By testing the mechanical responses of Arabidopsis mutants, it is possible to deduce the contribution that polymers of the cell wall make to organ strength. We developed a method to measure the tensile parameters of the expanded regions of turgid or plasmolyzed dark-grown Arabidopsis hypocotyls and applied it to the fucose biosynthesis mutant mur1, the xyloglucan glycosyltransferase mutants mur2 and mur3, and the katanin mutant bot1. Hypocotyls from plants grown in the presence of increasing concentrations of dichlorobenzonitrile, an inhibitor of cellulose synthesis, were considerably weakened, indicating the validity of our approach. In order of decreasing strength, the hypocotyls of mur2 > bot1 and mur1 > mur3 were each found to have reduced strength and a proportionate reduction in modulus compared with wild type. The tensile properties of the hypocotyls and of the inflorescence stems of mur1 were rescued by growth in the presence of high concentrations of borate, which is known to cross-link the pectic component rhamnogalacturonan II. From comparison of the mechanical responses of mur2 and mur3, we deduce that galactose-containing side chains of xyloglucan make a major contribution to overall wall strength, whereas xyloglucan fucosylation plays a comparatively minor role. We conclude that borate-complexed rhamnogalacturonan II and galactosylated xyloglucan contribute to the tensile strength of cell walls.