Substrate subsite recognition of the xyloglucan endo-transglycosylase or xyloglucan-specific endo-(1-->4)-beta-D-glucanase from the cotyledons of germinated nasturtium (Tropaeolum majus L.) seeds

Carbohydrate Gel Electrophoresis

Polysaccharide analysis using carbohydrate gel electrophoresis (PACE) relies on derivatization of reducing ends of sugars with a fluorophore, followed by electrophoresis under optimized conditions in polyacrylamide gels. PACE is a sensitive and simple tool for studying polysaccharide structure or quantity and also has applications in the investigation of enzyme specificity.

Functional and chemical characterization of XAF: a heat-stable plant polymer that activates xyloglucan endotransglucosylase/hydrolase (XTH)

BACKGROUND AND AIMS

Xyloglucan endotransglucosylase/hydrolase (XTH) proteins that possess xyloglucan endotransglucosylase (XET) activity contribute to cell-wall assembly and remodelling, orchestrating plant growth and development. Little is known about in-vivo XET regulation, other than at the XTH transcriptional level. Plants contain 'cold-water-extractable, heat-stable polymers' (CHPs) which are XTH-activating factors (XAFs) that desorb and thereby activate wall-bound XTHs. Because XAFs may control cell-wall modification in vivo, we have further explored their nature.

METHODS

Material was cold-water-extracted from 25 plant species; proteins were precipitated by heat-denaturation, then CHP was ethanol-precipitated. For XAF assays, CHP (or sub-fractions thereof) was applied to washed Arabidopsis thaliana cell walls, and the enzymes thus solubilized were assayed radiochemically for XET activity. In some experiments, the CHP was pre-treated with trifluoroacetic acid (TFA), alkali (NaOH) or glycanases.

KEY RESULTS

CHP specifically desorbed wall-bound XTHs, but not β-glucosidases, phosphatases or peroxidases. CHP preparations from 25 angiosperms all possessed XAF activity but had no consistent monosaccharide composition. Of 11 individual plant polymers tested, only gum arabic and tamarind xyloglucan were XAF-active, albeit less so than CHP. On gel-permeation chromatography, XAF-active cauliflower CHP eluted with a molecular weight of ~7000-140 000, although no specific sugar residue(s) co-eluted exactly with XAF activity. Cauliflower XAF activity survived cold alkali and warm dilute TFA (which break ester and glycofuranosyl linkages, respectively), but was inactivated by hot 2 m TFA (which breaks glycopyranosyl linkages). Cauliflower XAF activity was remarkably stable to diverse glycanases and glycosidases.

CONCLUSIONS

XAFs are naturally occurring heat-stable polymers that specifically desorb (thereby activating) wall-bound XTHs. Their XAF activity considerably exceeds that of gum arabic and tamarind xyloglucan, and they were not identifiable as any major plant polysaccharide. We propose that XAF is a specific, minor, plant polymer that regulates xyloglucan transglycosylation in vivo, and thus wall assembly and restructuring.

Engineering the acceptor substrate specificity in the xyloglucan endotransglycosylase TmXET6.3 from nasturtium seeds (Tropaeolum majus L.)

The knowledge of substrate specificity of XET enzymes is important for the general understanding of metabolic pathways to challenge the established notion that these enzymes operate uniquely on cellulose-xyloglucan networks. Xyloglucan xyloglucosyl transferases (XETs) (EC 2.4.1.207) play a central role in loosening and re-arranging the cellulose-xyloglucan network, which is assumed to be the primary load-bearing structural component of plant cell walls. The sequence of mature TmXET6.3 from Tropaeolum majus (280 residues) was deduced by the nucleotide sequence analysis of complete cDNA by Rapid Amplification of cDNA Ends, based on tryptic and chymotryptic peptide sequences. Partly purified TmXET6.3, expressed in Pichia occurred in N-glycosylated and unglycosylated forms. The quantification of hetero-transglycosylation activities of TmXET6.3 revealed that (1,3;1,4)-, (1,6)- and (1,4)-β-D-glucooligosaccharides were the preferred acceptor substrates, while (1,4)-β-D-xylooligosaccharides, and arabinoxylo- and glucomanno-oligosaccharides were less preferred. The 3D model of TmXET6.3, and bioinformatics analyses of identified and putative plant xyloglucan endotransglycosylases (XETs)/hydrolases (XEHs) of the GH16 family revealed that H94, A104, Q108, K234 and K237 were the key residues that underpinned the acceptor substrate specificity of TmXET6.3. Compared to the wild-type enzyme, the single Q108R and K237T, and double-K234T/K237T and triple-H94Q/A104D/Q108R variants exhibited enhanced hetero-transglycosylation activities with xyloglucan and (1,4)-β-D-glucooligosaccharides, while those with (1,3;1,4)- and (1,6)-β-D-glucooligosaccharides were suppressed; the incorporation of xyloglucan to (1,4)-β-D-glucooligosaccharides by the H94Q variant was influenced most extensively. Structural and biochemical data of non-specific TmXET6.3 presented here extend the classic XET reaction mechanism by which these enzymes operate in plant cell walls. The evaluations of TmXET6.3 transglycosylation activities and the incidence of investigated residues in other members of the GH16 family suggest that a broad acceptor substrate specificity in plant XET enzymes could be more widespread than previously anticipated.

Analyzing Xyloglucan Endotransglycosylases by Incorporating Synthetic Oligosaccharides into Plant Cell Walls

The plant cell wall is a cellular exoskeleton consisting predominantly of a complex polysaccharide network that defines the shape of cells. During growth, this network can be loosened through the action of xyloglucan endotransglycosylases (XETs), glycoside hydrolases that "cut and paste" xyloglucan polysaccharides through a transglycosylation process. We have analyzed cohorts of XETs in different plant species to evaluate the substrate specificities of xyloglucan acceptors by using a set of synthetic oligosaccharides obtained by automated glycan assembly. The ability of XETs to incorporate the oligosaccharides into polysaccharides printed as microarrays and into stem sections of Arabidopsis thaliana, beans, and peas was assessed. We found that single xylose substitutions are sufficient for transfer, and xylosylation of the terminal glucose residue is not required by XETs, independent of plant species. To obtain information on the potential xylosylation pattern of the natural acceptor of XETs, that is, the nonreducing end of xyloglucan, we further tested the activity of xyloglucan xylosyl transferase (XXT) 2 on the synthetic xyloglucan oligosaccharides. These data shed light on inconsistencies between previous studies towards determining the acceptor substrate specificities of XETs and have important implications for further understanding plant cell wall polysaccharide synthesis and remodeling.

Bonds broken and formed during the mixed-linkage glucan : xyloglucan endotransglucosylase reaction catalysed by Equisetum hetero-trans-β-glucanase

Mixed-linkage glucan∶xyloglucan endotransglucosylase (MXE) is one of the three activities of the recently characterised hetero-trans-β-glucanase (HTG), which among land plants is known only from Equisetum species. The biochemical details of the MXE reaction were incompletely understood - details that would promote understanding of MXE's role in vivo and enable its full technological exploitation. We investigated HTG's site of attack on one of its donor substrates, mixed-linkage (1→3),(1→4)-β-d-glucan (MLG), with radioactive oligosaccharides of xyloglucan as the acceptor substrate. Comparing three different MLG preparations, we showed that the enzyme favours those with a high content of cellotetraose blocks. The reaction products were analysed by enzymic digestion, thin-layer chromatography (TLC), high-pressure liquid chromatography (HPLC) and gel-permeation chromatography (GPC). Equisetum HTG consistently cleaved the MLG at the third consecutive β-(1→4)-bond following (towards the reducing terminus) a β-(1→3)-bond. It then formed a β-(1→4)-bond between the MLG and the non-reducing terminal glucose residue of the xyloglucan oligosaccharide, consistent with its xyloglucan endotransglucosylase/hydrolase subfamily membership. Using size-homogeneous barley MLG as the donor substrate, we showed that HTG does not favour any particular region of the MLG chain relative to the polysaccharide's reducing and non-reducing termini; rather, it selects its target cellotetraosyl unit stochastically along the MLG molecule. This work improves our understanding of how enzymes can exhibit promiscuous substrate specificities and provides the foundations to explore strategies for engineering novel substrate specificities into transglycanases.

Biochemistry and physiological roles of enzymes that 'cut and paste' plant cell-wall polysaccharides

The plant cell-wall matrix is equipped with more than 20 glycosylhydrolase activities, including both glycosidases and glycanases (exo- and endo-hydrolases, respectively), which between them are in principle capable of hydrolysing most of the major glycosidic bonds in wall polysaccharides. Some of these enzymes also participate in the 'cutting and pasting' (transglycosylation) of sugar residues-enzyme activities known as transglycosidases and transglycanases. Their action and biological functions differ from those of the UDP-dependent glycosyltransferases (polysaccharide synthases) that catalyse irreversible glycosyl transfer. Based on the nature of the substrates, two types of reaction can be distinguished: homo-transglycosylation (occurring between chemically similar polymers) and hetero-transglycosylation (between chemically different polymers). This review focuses on plant cell-wall-localized glycosylhydrolases and the transglycosylase activities exhibited by some of these enzymes and considers the physiological need for wall polysaccharide modification in vivo. It describes the mechanism of transglycosylase action and the classification and phylogenetic variation of the enzymes. It discusses the modulation of their expression in plants at the transcriptional and translational levels, and methods for their detection. It also critically evaluates the evidence that the enzyme proteins under consideration exhibit their predicted activity in vitro and their predicted action in vivo. Finally, this review suggests that wall-localized glycosylhydrolases with transglycosidase and transglycanase abilities are widespread in plants and play important roles in the mechanism and control of plant cell expansion, differentiation, maturation, and wall repair.

Trans-α-xylosidase and trans-β-galactosidase activities, widespread in plants, modify and stabilize xyloglucan structures

Cell-wall components are hydrolysed by numerous plant glycosidase and glycanase activities. We investigated whether plant enzymes also modify xyloglucan structures by transglycosidase activities. Diverse angiosperm extracts exhibited transglycosidase activities that progressively transferred single sugar residues between xyloglucan heptasaccharide (XXXG or its reduced form, XXXGol) molecules, at 16 μM and above, creating octa- to decasaccharides plus smaller products. We measured remarkably high transglycosylation:hydrolysis ratios under optimized conditions. To identify the transferred monosaccharide(s), we devised a dual-labelling strategy in which a neutral radiolabelled oligosaccharide (donor substrate) reacted with an amino-labelled non-radioactive oligosaccharide (acceptor substrate), generating radioactive cationic products. For example, 37 μM [Xyl-³H]XXXG plus 1 mM XXLG-NH₂ generated ³H-labelled cations, demonstrating xylosyl transfer, which exceeded xylosyl hydrolysis 1.6- to 7.3-fold, implying the presence of enzymes that favour transglycosylation. The transferred xylose residues remained α-linked but were relatively resistant to hydrolysis by plant enzymes. Driselase digestion of the products released a trisaccharide (α-[³H]xylosyl-isoprimeverose), indicating that a new xyloglucan repeat unit had been formed. In similar assays, [Gal-³H]XXLG and [Gal-³H]XLLG (but not [Fuc-³H]XXFG) yielded radioactive cations. Thus plants exhibit trans-α-xylosidase and trans-β-galactosidase (but not trans-α-fucosidase) activities that graft sugar residues from one xyloglucan oligosaccharide to another. Reconstructing xyloglucan oligosaccharides in this way may alter oligosaccharin activities or increase their longevity in vivo. Trans-α-xylosidase activity also transferred xylose residues from xyloglucan oligosaccharides to long-chain hemicelluloses (xyloglucan, water-soluble cellulose acetate, mixed-linkage β-glucan, glucomannan and arabinoxylan). With xyloglucan as acceptor substrate, such an activity potentially affects the polysaccharide's suitability as a substrate for xyloglucan endotransglucosylase action and thereby modulates cell expansion. We conclude that certain proteins annotated as glycosidases can function as transglycosidases.

AtBGAL10 is the main xyloglucan β-galactosidase in Arabidopsis, and its absence results in unusual xyloglucan subunits and growth defects

In growing cells, xyloglucan is thought to connect cellulose microfibrils and regulate their separation during wall extension. In Arabidopsis (Arabidopsis thaliana), a significant proportion of xyloglucan side chains contain β-galactose linked to α-xylose at O2. In this work, we identified AtBGAL10 (At5g63810) as the gene responsible for the majority of β-galactosidase activity against xyloglucan. Xyloglucan from bgal10 insertional mutants was found to contain a large proportion of unusual subunits, such as GLG and GLLG. These subunits were not detected in a bgal10 xyl1 double mutant, deficient in both β-galactosidase and α-xylosidase. Xyloglucan from bgal10 xyl1 plants was enriched instead in XXLG/XLXG and XLLG subunits. In both cases, changes in xyloglucan composition were larger in the endoglucanase-accessible fraction. These results suggest that glycosidases acting on nonreducing ends digest large amounts of xyloglucan in wild-type plants, while plants deficient in any of these activities accumulate partly digested subunits. In both bgal10 and bgal10 xyl1, siliques and sepals were shorter, a phenotype that could be explained by an excess of nonreducing ends leading to a reinforced xyloglucan network. Additionally, AtBGAL10 expression was examined with a promoter-reporter construct. Expression was high in many cell types undergoing wall extension or remodeling, such as young stems, abscission zones, or developing vasculature, showing good correlation with α-xylosidase expression.

Phylogenetic variation in glycosidases and glycanases acting on plant cell wall polysaccharides, and the detection of transglycosidase and trans-β-xylanase activities

Wall polysaccharide chemistry varies phylogenetically, suggesting a need for variation in wall enzymes. Although plants possess the genes for numerous putative enzymes acting on wall carbohydrates, the activities of the encoded proteins often remain conjectural. To explore phylogenetic differences in demonstrable enzyme activities, we extracted proteins from 57 rapidly growing plant organs with three extractants, and assayed their ability to act on six oligosaccharides 'modelling' selected cell-wall polysaccharides. Based on reaction products, we successfully distinguished exo- and endo-hydrolases and found high taxonomic variation in all hydrolases screened: β-D-xylosidase, endo-(1→4)-β-D-xylanase, β-D-mannosidase, endo-(1→4)-β-D-mannanase, α-D-xylosidase, β-D-galactosidase, α-L-arabinosidase and α-L-fucosidase. The results, as GHATAbase, a searchable compendium in Excel format, also provide a compilation for selecting rich sources of enzymes acting on wall carbohydrates. Four of the hydrolases were accompanied, sometimes exceeded, by transglycosylase activities, generating products larger than the substrate. For example, during β-xylosidase assays on (1→4)-β-D-xylohexaose (Xyl₆), Marchantia, Selaginella and Equisetum extracts gave negligible free xylose but approximately equimolar Xyl₅ and Xyl₇, indicating trans-β-xylosidase activity, also found in onion, cereals, legumes and rape. The yield of Xyl₉ often exceeded that of Xyl₇₋₈, indicating that β-xylanase was accompanied by an endotransglycosylase activity, here called trans-β-xylanase, catalysing the reaction 2Xyl₆ → Xyl₃ + Xyl₉. Similar evidence also revealed trans-α-xylosidase, trans-α-arabinosidase and trans-α-arabinanase activities acting on xyloglucan oligosaccharides and (1→5)-α-L-arabino-oligosaccharides. In conclusion, diverse plants differ dramatically in extractable enzymes acting on wall carbohydrate, reflecting differences in wall polysaccharide composition. Besides glycosidase and glycanase activities, five new transglycosylase activities were detected. We propose that such activities function in the assembly and re-structuring of the wall matrix.

Carbohydrate gel electrophoresis

Polysaccharide analysis using carbohydrate gel electrophoresis (PACE) relies on derivatization of the reducing ends of sugars with a fluorophore, followed by electrophoresis under optimized conditions in polyacrylamide gels. PACE is a sensitive and simple tool for studying polysaccharide structure or quantity and also has applications in the investigation of enzyme specificity.

Lack of α-xylosidase activity in Arabidopsis alters xyloglucan composition and results in growth defects

Xyloglucan is the main hemicellulose in the primary cell walls of most seed plants and is thought to play a role in regulating the separation of cellulose microfibrils during growth. Xylose side chains block the degradation of the backbone, and α-xylosidase activity is necessary to remove them. Two Arabidopsis (Arabidopsis thaliana) mutant lines with insertions in the α-xylosidase gene AtXYL1 were characterized in this work. Both lines showed a reduction to undetectable levels of α-xylosidase activity against xyloglucan oligosaccharides. This reduction resulted in the accumulation of XXXG and XXLG in the liquid growth medium of Atxyl1 seedlings. The presence of XXLG suggests that it is a poor substrate for xyloglucan β-galactosidase. In addition, the polymeric xyloglucan of Atxyl1 lines was found to be enriched in XXLG subunits, with a concomitant decrease in XXFG and XLFG. This change can be explained by extensive exoglycosidase activity at the nonreducing ends of xyloglucan chains. These enzymes could thus have a larger role than previously thought in the metabolism of xyloglucan. Finally, Atxyl1 lines showed a reduced ability to control the anisotropic growth pattern of different organs, pointing to the importance of xyloglucan in this process. The promoter of AtXYL1 was shown to direct expression to many different organs and cell types undergoing cell wall modifications, including trichomes, vasculature, stomata, and elongating anther filaments.

Structural basis for the action of xyloglucan endotransglycosylases/hydrolases: insights from homology modeling

Xyloglucan endotransglycosylase/hydrolases (XTH) are believed to play an important role in modifying the cell wall structure through two different, but related actions: the cleavage of a cross-linking xyloglucan polymer (xyloglucan endohydrolase (XEH) activity) and transfer of a newly generated end to another sugar polymer (xyloglucan endotransglycosylase (XET) activity). These enzymes normally show predominantly either XET or XEH. Question remains concerning the origin of the XET or XEH activity of poplar (Populus trichocarpa) XTH proteins as well as the 3-dimensional structural features that might contribute to the different activities for different phylogenetic groups. Previous investigations have demonstrated that the key structural features controlling the activity may involve the loop 1 and 2 regions of the XTH enzymes. Here we report homology models for 40 poplar (Populus trichocarpa) XTH proteins and analyze their loop 1 and 2 regions. These analyses allow us to generate some hypotheses concerning the possible activities of these enzymes. The results show that the protein models for subfamilies (SFs) I/II/IV match well with that of the known XET enzyme (Ptt-XET16-34 from a Populus hybrid), suggesting that they might mainly function as the XET. It is found that the SF III-A models match well with that of the known XEH TmXTH1. Therefore, they may function as XEH as well. Although the SF III-B members are found to have a truncated loop 2, comparison of the homology models with the existing TmXTH1 structure seems to suggest that they might have relatively high possibility to function as XEH instead of XET.

Xyloglucan endotransglycosylases (XETs) from germinating nasturtium (Tropaeolum majus) seeds: isolation and characterization of the major form

Five forms of xyloglucan endotransglycosylase/hydrolase (XTH) differing in their isoelectric points (pI) were detected in crude extracts from germinating nasturtium seeds. Without further fractionation, all five forms behaved as typical endotransglycosylases since they exhibited only transglycosylating (XET) activity and no xyloglucan-hydrolysing (XEH) activity. They all were glycoproteins with identical molecular mass, and deglycosylation led to a decrease in molecular mass from approximately 29 to 26.5 kDa. The major enzyme form having pI 6.3, temporarily designated as TmXET(6.3), was isolated and characterized. Molecular and biochemical properties of TmXET(6.3) confirmed its distinction from the XTHs described previously from nasturtium. The enzyme exhibited broad substrate specificity by transferring xyloglucan or hydroxyethylcellulose fragments not only to oligoxyloglucosides and cello-oligosaccharides but also to oligosaccharides derived from beta-(1,4)-d-glucuronoxylan, beta-(1,6)-d-glucan, mixed-linkage beta-(1,3; 1,4)-d-glucan and at a relatively low rate also to beta-(1,3)-gluco-oligosaccharides. The transglycosylating activity with xyloglucan as donor and cello-oligosaccharides as acceptors represented 4.6%, with laminarioligosaccharides 0.23%, with mixed-linkage beta-(1,3; 1,4)-d-gluco-oligosaccharides 2.06%, with beta-(1,4)-d-glucuronoxylo-oligosaccharides 0.31% and with beta-(1,6)-d-gluco-oligosaccharides 0.69% of that determined with xyloglucan oligosaccharides as acceptors. Based on the sequence homology of tryptic fragments with the sequences of known XTHs, the TmXET(6.3) was classified into group II of the XTH phylogeny of glycoside hydrolase family GH16.

Analysis of nasturtium TmNXG1 complexes by crystallography and molecular dynamics provides detailed insight into substrate recognition by family GH16 xyloglucan endo-transglycosylases and endo-hydrolases

Reorganization and degradation of the wall crosslinking and seed storage polysaccharide xyloglucan by glycoside hydrolase family 16 (GH16) endo-transglycosylases and hydrolases are crucial to the growth of the majority of land plants, affecting processes as diverse as germination, morphogenesis, and fruit ripening. A high-resolution, three-dimensional structure of a nasturtium (Tropaeolum majus) endo-xyloglucanase loop mutant, TmNXG1-DeltaYNIIG, with an oligosaccharide product bound in the negative active-site subsites, has been solved by X-ray crystallography. Comparison of this novel complex to that of the strict xyloglucan endo-transglycosylase PttXET16-34 from hybrid aspen (Populus tremula x tremuloides), previously solved with a xylogluco-oligosaccharide bound in the positive subsites, highlighted key protein structures that affect the disparate catalytic activities displayed by these closely related enzymes. Combination of these "partial" active-site complexes through molecular dynamics simulations in water allowed modeling of wild-type TmNXG1, TmNXG1-DeltaYNIIG, and wild-type PttXET16-34 in complex with a xyloglucan octadecasaccharide spanning the entire catalytic cleft. A comprehensive analysis of these full-length complexes underscored the importance of various loops lining the active site. Subtle differences leading to a tighter hydrogen bonding pattern on the negative (glycosyl donor) binding subsites, together with loop flexibility on the positive (glycosyl acceptor) binding subsites appear to favor hydrolysis over transglycosylation in GH16 xyloglucan-active enzymes.

Anionic derivatives of xyloglucan function as acceptor but not donor substrates for xyloglucan endotransglucosylase activity

Tamarind xyloglucan was oxidised by reaction with sodium hypochlorite in the presence of 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO). Galactose residues and non-xylosylated glucose residues were thus converted into galacturonic and glucuronic acid residues, respectively, producing an anionic polysaccharide. Acid hydrolysis of oxidised xyloglucan yielded two aldobiouronic acids, deduced to be beta-D: -GalpA-(1-->2)-D-Xyl and beta-D: -GlcpA-(1-->4)-D-Glc. Anionic xyloglucan had a decreased ability to hydrogen-bond to cellulose and to complex with iodine. It was almost totally resistant to digestion by cellulase [endo-(1-->4)-beta-glucanase] and did not serve as a donor substrate for xyloglucan endotransglucosylase (XET) activity. Like several other anionic polysaccharides, it promoted XET activity when unmodified (non-ionic) xyloglucan was used as donor substrate. Anionic xyloglucan may mimic polyanions whose presence in the plant cell wall promotes the action of endogenous XTH proteins. NaOCl with TEMPO oxidised the heptasaccharide, XXXG, to form XXX-glucarate, which did serve as an acceptor substrate although at a rate approximately fourfold less than XXXG itself. Anionic derivatives of xyloglucan, acting as acceptor but not donor substrates, may be valuable tools for exploring the biological roles of XTHs in the integration versus the re-structuring of xyloglucan in the plant cell wall.

Synthesis of a Library of Xylogluco-Oligosaccharides for Active-Site Mapping of Xyloglucan endo-Transglycosylase

Complex oligosaccharides containing alpha-D-xylosyl-(1-->6)-beta-D-glucosyl residues and unsubstituted beta-(1-->4)-linked D-glucosyl units were readily synthesized using enzymatic coupling catalyzed by the Cel7B E197A glycosynthase from Humicola insolens. Constituting this library required four key steps: (1) preparing unprotected building blocks by chemical synthesis or enzymatic degradation of xyloglucan polymers; (2) generating the donor synthon in the enzymatic coupling by temporarily introducing a lactosyl motif on the 4-OH of the terminal glucosyl units of the xylogluco-oligosaccharides; (3) synthesizing the corresponding alpha-fluorides, followed by their de-O-acetylation and the glycosynthase-catalyzed condensation of these donors onto various acceptors; and (4) enzymatically releasing lactose or galactose from the reaction product, affording the target molecules in good overall yields. These complex oligosaccharides proved useful for mapping the active site of a key enzyme in plant cell wall biosynthesis and modification: the xyloglucan endo-transglycosylase (XET). We also report some preliminary enzymatic results regarding the efficiency of these compounds.

Kinetic analysis using low-molecular mass xyloglucan oligosaccharides defines the catalytic mechanism of a Populus xyloglucan endotransglycosylase

Plant XETs [XG (xyloglucan) endotransglycosylases] catalyse the transglycosylation from a XG donor to a XG or low-molecular-mass XG fragment as the acceptor, and are thought to be important enzymes in the formation and remodelling of the cellulose-XG three-dimensional network in the primary plant cell wall. Current methods to assay XET activity use the XG polysaccharide as the donor substrate, and present limitations for kinetic and mechanistic studies of XET action due to the polymeric and polydisperse nature of the substrate. A novel activity assay based on HPCE (high performance capillary electrophoresis), in conjunction with a defined low-molecular-mass XGO {XG oligosaccharide; (XXXGXXXG, where G=Glcbeta1,4- and X=[Xylalpha1,6]Glcbeta1,4-)} as the glycosyl donor and a heptasaccharide derivatized with ANTS [8-aminonaphthalene-1,3,6-trisulphonic acid; (XXXG-ANTS)] as the acceptor substrate was developed and validated. The recombinant enzyme PttXET16A from Populus tremula x tremuloides (hybrid aspen) was characterized using the donor/acceptor pair indicated above, for which preparative scale syntheses have been optimized. The low-molecular-mass donor underwent a single transglycosylation reaction to the acceptor substrate under initial-rate conditions, with a pH optimum at 5.0 and maximal activity between 30 and 40 degrees C. Kinetic data are best explained by a ping-pong bi-bi mechanism with substrate inhibition by both donor and acceptor. This is the first assay for XETs using a donor substrate other than polymeric XG, enabling quantitative kinetic analysis of different XGO donors for specificity, and subsite mapping studies of XET enzymes.

Apoplastic Glycosidases Active Against Xyloglucan Oligosaccharides of Arabidopsis thaliana

All four glycanases necessary for the degradation of xyloglucan oligosaccharides (alpha-fucosidase, alpha-xylosidase, beta-galactosidase and beta-glucosidase) were found in the apoplastic fluid of Arabidopsis thaliana. These activities acted cooperatively on xyloglucan oligosaccharides (XLFG), leading to the sequential formation of XXFG, XXLG, XXXG, GXXG and XXG, as identified by matrix-assisted laser desorption ionization time of flight (MALDI-TOF). AtFXG1 (At1g67830) and AtXYL1 (At1g68560) had been previously identified as the Arabidopsis genes coding for alpha-fucosidase and alpha-xylosidase, respectively. As for the genes coding for beta-galactosidase activity, we identified in phylogenetic trees 12 candidates from family 35 of glycoside hydrolases. Similarly, four genes from family 3 were selected as possible beta-glucosidases active on xyloglucan. The expression level of all the selected genes was studied in different plant regions (young and mature rosette leaves, apical and basal region of the inflorescence stem, roots, flower and siliques) using quantitative real-time reverse transcription-PCR. The expression patterns were very diverse as well as their relationship with growth rates, showing a very complex situation. This could lead to highly varying proportions of the different xyloglucan oligosaccharides in different plant regions and developmental stages.

Primary cell wall metabolism: tracking the careers of wall polymers in living plant cells

Numerous examples have been presented of enzyme activities, assayed in vitro, that appear relevant to the synthesis of structural polysaccharides, and to their assembly and subsequent degradation in the primary cell walls (PCWs) of higher plants. The accumulation of the corresponding mRNAs, and of the (immunologically recognized) proteins, has often also (or instead) been reported. However, the presence of these mRNAs, antigens and enzymic activities has rarely been shown to correspond to enzyme action in the living plant cell. In some cases, apparent enzymic action is observed in vivo for which no enzyme activity can be detected in in-vitro assays; the converse also occurs. Methods are reviewed by which reactions involving structural wall polysaccharides can be tracked in vivo. Special attention is given to xyloglucan endotransglucosylase (XET), one of the two enzymic activities exhibited in vitro by xyloglucan endotransglucosylase/hydrolase (XTH) proteins, because of its probable importance in the construction and restructuring of the PCW's major hemicellulose. Attention is also given to the possibility that some reactions observed in the PCW in vivo are not directly enzymic, possibly involving the action of hydroxyl radicals. It is concluded that some proposed wall enzymes, for example XTHs, do act in vivo, but that for other enzymes this is not proven. Contents I. Primary cell walls: composition, deposition and roles 642 II. Reactions that have been proposed to occur in primary cell walls 645 III. Tracking the careers of wall components in vivo: evidence for action of enzymes in the walls of living plant cells 656 IV. Evidence for the occurrence of nonenzymic polymer scission in vivo? 666 VI. Conclusion 667 References 667.

Multiple forms of endo-1,4-beta-glucanases in the endosperm of Euphorbia heterophylla L

Germinating seeds of Euphorbia heterophylla L. contain endo-1,4-beta-glucanases which degrade carboxymethylcellulose (CMC). The activity decreased approximately 66% in extracts of endosperm containing isopropanol or ethanol. The endoglucanases were isolated from endosperm extracts using ammonium sulphate fractionation followed by Sephacryl S-100-HR chromatography resulting in two main peaks: I and II. Peak I endoglucanase was further purified about 15-fold on DEAE-Sephadex A50 and then by affinity chromatography (CF11-cellulose). Peak II endoglucanases were further purified 10-fold on CM-cellulose chromatography. The results indicated the occurrence of a 66 kDa endoglucanase (fractionated by SDS-PAGE and visualized by activity staining using Congo Red). Several acidic (pI 3.0 to 5.7) and basic (pI 8.5 to 10.0) forms from both peaks which differed in their capacities for degrading CMC or xyloglucans from Copaifera langsdorffii or Hymenaea courbaril were detected.

The XTH family of enzymes involved in xyloglucan endotransglucosylation and endohydrolysis: current perspectives and a new unifying nomenclature

The polysaccharide xyloglucan is thought to play an important structural role in the primary cell wall of dicotyledons. Accordingly, there is considerable interest in understanding the biochemical basis and regulation of xyloglucan metabolism, and research over the last 16 years has identified a large family of cell wall proteins that specifically catalyze xyloglucan endohydrolysis and/or endotransglucosylation. However, a confusing and contradictory series of nomenclatures has emerged in the literature, of which xyloglucan endotransglycosylases (XETs) and endoxyloglucan transferases (EXGTs) are just two examples, to describe members of essentially the same class of genes/proteins. The completion of the first plant genome sequencing projects has revealed the full extent of this gene family and so this is an opportune time to resolve the many discrepancies in the database that include different names being assigned to the same gene. Following consultation with members of the scientific community involved in plant cell wall research, we propose a new unifying nomenclature that conveys an accurate description of the spectrum of biochemical activities that cumulative research has shown are catalyzed by these enzymes. Thus, a member of this class of genes/proteins will be referred to as a xyloglucan endotransglucosylase/hydrolase (XTH). The two known activities of XTH proteins are referred to enzymologically as xyloglucan endotransglucosylase (XET, which is hereby re-defined) activity and xyloglucan endohydrolase (XEH) activity. This review provides a summary of the biochemical and functional diversity of XTHs, including an overview of the structure and organization of the Arabidopsis XTH gene family, and highlights the potentially important roles that XTHs appear to play in numerous examples of plant growth and development.

Ten isoenzymes of xyloglucan endotransglycosylase from plant cell walls select and cleave the donor substrate stochastically

To map the preferred cleavage sites of xyloglucan endotransglycosylases (XETs; EC 2.4.1.207) along the donor substrate chain, we incubated the enzymes with tamarind (Tamarindus indica) xyloglucan (donor substrate; approximately 205 kDa; 21 microM) plus the nonasaccharide [(3)H]XLLGol (Gal(2).Xyl(3).Glc(3). [(3)H]glucitol; acceptor substrate; 0.6 microM). After short incubation times, to minimize multiple cleavages, the size of the (3)H-labelled transglycosylation products (determined by gel-permeation chromatography) indicated the positions of the cleavage sites relative to the non-reducing terminus of the donor. There was very little difference between the size profiles of the products formed by any of ten XETs tested [one native XET purified from cauliflower (Brassica oleracea) florets, four native XET isoenzymes purified from etiolated mung-bean (Phaseolus aureus) shoots, native XETs purified from lentil (Lens culinaris) and nasturtium (Tropaeolum majus) seeds, and three insect-cell-produced thale-cress (Arabidopsis thaliana) XETs (EXGT, TCH4 and MERI-5)]. All such product profiles showed a good fit to a model in which the enzyme chooses its donor substrate independently of size and attacks it, once only, at a randomly selected cleavage site. The results therefore do not support the hypothesis that different XET isoenzymes are adapted to produce longer or shorter products such as might favour either the efficient integration of new xyloglucan into the cell wall or the re-structuring of old xyloglucan within an expanding wall.

Fucosyltransferase and the biosynthesis of storage and structural xyloglucan in developing nasturtium fruits

Young, developing fruits of nasturtium (Tropaeolum majus L.) accumulate large deposits of nonfucosylated xyloglucan (XG) in periplasmic spaces of cotyledon cells. This "storage" XG can be fucosylated by a nasturtium transferase in vitro, but this does not happen in vivo, even as a transitory signal for secretion. The only XG that is clearly fucosylated in these fruits is the structural fraction (approximately 1% total) that is bound to cellulose in growing primary walls. The two fucosylated subunits that are formed in vitro are identical to those found in structural XG in vivo. The yield of XG-fucosyltransferase activity from membrane fractions is highest per unit fresh weight in the youngest fruits, especially in dissected cotyledons, but declines when storage XG is forming. A block appears to develop in the secretory machinery of young cotyledon cells between sites that galactosylate and those that fucosylate nascent XG. After extensive galactosylation, XG traffic is diverted to the periplasm without fucosylation. The primary walls buried beneath accretions of storage XG eventually swell and lose cohesion, probably because they continue to extend without incorporating components such as fucosylated XG that are needed to maintain wall integrity.

A xyloglucan oligosaccharide-active, transglycosylating beta-D-glucosidase from the cotyledons of nasturtium (Tropaeolum majus L) seedlings--purification, properties and characterization of a cDNA clone

A beta-D-glucosidase has been purified to apparent homogeneity from the cotyledons of germinated nasturtium (Tropaeolum majus L.) seedlings during the mobilization of the xyloglucan stored in the cotyledonary cell walls. The purified protein (Mr 76, 000; a glycoprotein; pl > 9.5; apparent pH optimum 4.5; temperature optimum 30 degrees C) catalysed the hydrolysis of p-nitrophenyl-beta-D-glucopyranoside, cello-oligosaccharides, beta-linked glucose disaccharides, and certain xyloglucan oligosaccharides. Glucose disaccharides with different linkages were hydrolysed at different rates [(1-->3) > (1-->4) > (1-->2) > (1-->6)] with significant transglycosylation occurring in the early stages of the reaction. Cello-oligosaccharide hydrolysis was also accompanied by extensive transglycosylation to give transitory accumulations of higher oligosaccharides. At least some of the glycosyl linkages formed during transglycosylation were (1-->6)-beta. Xyloglucan oligosaccharides xylose-substituted at the non-reducing terminal glucose residue (XXXG, XXLG, XLXG and XLLG, where G is an unsubstituted glucose residue, X is a xylose-substituted glucose residue, and L is a galactosylxylose-substituted glucose residue) were not hydrolysed. Some xyloglucan oligosaccharides with an unsubstituted non-reducing terminal glucose residue (GXXG, GXLG and GXG) were hydrolysed, but others (GLXG and GLLG) were not. This indicated steric hindrance by L but not X substitution at the glucose residue next to the one at the non-reducing end of the oligosaccharide. Hydrolysis of xyloglucan oligosaccharides was not accompanied by transglycosylation. Natural xyloglucan subunit oligosaccharides (XXXG, XXLG, XLXG, XLLG) were totally degraded to their monosaccharide components when treated with nasturtium beta-D-galactosidase. (Edwards et al (1988) J. Biol. Chem. 263, 4333-4337), followed by alternations of nasturtium xyloglucan-specific alpha-xylosidase (Fanutti et al (1991) Planta 184, 137-147) and this enzyme. Several extensively overlapping cDNA clones were obtained by RT-PCR and by screening cDNA libraries. A composite, full-length DNA had an open reading frame of 1962 bp, encoding a polypeptide of 654 amino acids, including all N-terminal and internal sequences obtained from the purified beta-glucosidase protein, and a motif resembling plant signal sequences thought to direct proteins to the cell wall. Database searches revealed homology with beta-glucosidases from several sources (plant, bacteria, yeast), notably with glycosylhydrolases of 'Family 3', according to the classification of Henrissat (Henrissat (1991) Biochem. J. 280, 309-316). There was strong sequence homology with a beta-glucan exo-hydrolase from barley (Hrmova et al. (1996) J. Biol. Chem. 271, 5277-5286). The nasturtium beta-glucosidase is ascribed a role in xyloglucan mobilization, and its interaction with the alpha-xylosidase and the beta-galactosidase is modelled.

Dimers of a GFG hexasaccharide occur in apple fruit xyloglucan

Apple fruit xyloglucan is predominantly built up from XXXG, XXFG, and XLFG units (G = beta-D-Glcp-, X = alpha-D-Xylp-(1-->6)-beta-D-Glcp-, L = beta-D-Galp-(1-->2)-alpha-D-Xylp-(1-->6)-beta-D-Glcp-, F = alpha-L-Fucp-(1-->2)-beta-D-Galp-(1-->2)-alpha-D-Xyl p-(1-->6)-beta-D-Glcp-). However, small amounts of oligosaccharides with a less heavily branched glucan backbone also occur. Structural analysis of two such oligosaccharides, isolated from a xyloglucan preparation digested with endoglucanase i.v., using a combination of FAB mass spectrometry and 1H NMR spectroscopy, afforded the identification of GFG and a dimer of GFG. The finding of the dodecasaccharide GFGGFG as a structural element of apple fruit xyloglucan is most unusual.