Eudicot primary cell wall glucomannan is related in synthesis, structure, and function to xyloglucan.

Hemicellulose polysaccharides influence assembly and properties of the plant primary cell wall (PCW), perhaps by interacting with cellulose to affect the deposition and bundling of cellulose fibrils. However, the functional differences between plant cell wall hemicelluloses such as glucomannan, xylan, and xyloglucan (XyG) remain unclear. As the most abundant hemicellulose, XyG is considered important in eudicot PCWs, but plants devoid of XyG show relatively mild phenotypes. We report here that a patterned ß-galactoglucomannan (ß-GGM) is widespread in eudicot PCWs and shows remarkable similarities to XyG. The sugar linkages forming the backbone and side chains of ß-GGM are analogous to those that make up XyG, and moreover, these linkages are formed by glycosyltransferases from the same CAZy families. Solid-state nuclear magnetic resonance indicated that ß-GGM shows low mobility in the cell wall, consistent with interaction with cellulose. Although Arabidopsis ß-GGM synthesis mutants show no obvious growth defects, genetic crosses between ß-GGM and XyG mutants produce exacerbated phenotypes compared with XyG mutants. These findings demonstrate a related role of these two similar but distinct classes of hemicelluloses in PCWs. This work opens avenues to study the roles of ß-GGM and XyG in PCWs.

Convergent emergence of Glucomannan ß-galactosyltransferase activity in Asterids and Rosids.

ß-Galactoglucomannan (ß-GGM) is a primary cell wall polysaccharide in rosids and asterids. The ß-GGM polymer has a backbone of repeating ß-(1,4)-glucosyl and mannosyl residues, usually with mono- α-(1,6)-galactosyl substitution or ß-(1,2)-galactosyl α-galactosyl disaccharide sidechains on the mannosyl residues. Mannan ß-GalactosylTransferases (MBGTs) are therefore required for ß-GGM synthesis. The single MBGT identified so far, AtMBGT1, lies in glycosyltransferase family 47A subclade VII, and was identified in Arabidopsis. However, despite the presence of ß-GGM, an orthologous gene is absent in tomato (Solanum lycopersicum), a model asterid. In this study, we screened candidate MBGT genes from the tomato genome, functionally tested the activities of encoded proteins, and identified the tomato MBGT (SlMBGT1) in GT47A-III. Interestingly therefore, AtMBGT1 and SlMBGT1 are located in different GT47A subclades. Further, phylogenetic and glucomannan structural analysis from different species raised the possibility that various asterids possess conserved MBGTs in an asterid-specific subclade of GT47A-III, indicating that MBGT activity has been acquired convergently among asterids and rosids. The present study highlights the promiscuous emergence of donor and acceptor preference in GT47A enzymes. The independent acquisition of the activity also suggests an adaptive advantage for eudicots to acquire ß-GGM ß-galactosylation, and hence also suggests the disaccharide side chains are important for ß-GGM function.

Galactoglucomannan structure of Arabidopsis seed-coat mucilage in GDP-mannose synthesis impaired mutants.

Cell-wall polysaccharides are synthesized from nucleotide sugars by glycosyltransferases. However, in what way the level of nucleotide sugars affects the structure of the polysaccharides is not entirely clear. guanosine diphosphate (GDP)-mannose (GDP-Man) is one of the major nucleotide sugars in plants and serves as a substrate in the synthesis of mannan polysaccharides. GDP-Man is synthesized from mannose 1-phosphate and GTP by a GDP-Man pyrophosphorylase, VITAMIN C DEFECTIVE1 (VTC1), which is positively regulated by the interacting protein KONJAC1 (KJC1) in Arabidopsis. Since seed-coat mucilage can serve as a model of the plant cell wall, we examined the influence of vtc1 and kjc1 mutations on the synthesis of mucilage galactoglucomannan. Sugar composition analysis showed that mannose content in adherent mucilage of kjc1 and vtc1 mutants was only 42% and 11% of the wild-type, respectively, indicating a drastic decrease of galactoglucomannan. On the other hand, structural analysis based on specific oligosaccharides released by endo-ß-1,4-mannanase indicated that galactoglucomannan had a patterned glucomannan backbone consisting of alternating residues of glucose and mannose and the frequency of α-galactosyl branches was also similar to the wild type structure. These results suggest that the structure of mucilage galactoglucomannan is mainly determined by properties of glycosyltransferases rather than the availability of nucleotide sugars.

Expression of a fungal exo-ß-1,3-galactanase in Arabidopsis reveals a role of type II arabinogalactans in the regulation of cell shape.

Arabinogalactan-proteins (AGPs) are a family of plant extracellular proteoglycans implicated in many physiological events. AGP is decorated with type II arabinogalactans (AGs) consisting of a ß-1,3-galactan backbone and ß-1,6-galactan side chains, to which other sugars are attached. Based on the fact that a type II AG-specific inhibitor, ß-Yariv reagent, perturbs growth and development, it has been proposed that type II AGs participate in the regulation of cell shape and tissue organization. However, the mechanisms by which type II AGs participate have not yet been established. Here, we describe a novel system that causes specific degradation of type II AGs in Arabidopsis, by which a gene encoding a fungal exo-ß-1,3-galactanase that specifically hydrolyzes ß-1,3-galactan backbones of type II AGs is expressed under the control of a dexamethasone-inducible promoter. Dexamethasone treatment increased the galactanase activity, leading to a decrease in Yariv reagent-reactive AGPs in transgenic Arabidopsis. We detected the typical oligosaccharides released from type II AGs by Il3GAL in the soluble fraction, demonstrating that Il3GAL acted on type II AG in the transgenic plants. Additionally, this resulted in severe tissue disorganization in the hypocotyl and cotyledons, suggesting that the degradation of type II AGs affected the regulation of cell shape.

KONJAC1 and 2 Are Key Factors for GDP-Mannose Generation and Affect l-Ascorbic Acid and Glucomannan Biosynthesis in Arabidopsis.

Humans are unable to synthesize l-ascorbic acid (AsA), yet it is required as a cofactor in many critical biochemical reactions. The majority of human dietary AsA is obtained from plants. In Arabidopsis thaliana, a GDP-mannose pyrophosphorylase (GMPP), VITAMIN C DEFECTIVE1 (VTC1), catalyzes a rate-limiting step in AsA synthesis: the formation of GDP-Man. In this study, we identified two nucleotide sugar pyrophosphorylase-like proteins, KONJAC1 (KJC1) and KJC2, which stimulate the activity of VTC1. The kjc1kjc2 double mutant exhibited severe dwarfism, indicating that KJC proteins are important for growth and development. The kjc1 mutation reduced GMPP activity to 10% of wild-type levels, leading to a 60% reduction in AsA levels. On the contrary, overexpression of KJC1 significantly increased GMPP activity. The kjc1 and kjc1kjc2 mutants also exhibited significantly reduced levels of glucomannan, which is also synthesized from GDP-Man. Recombinant KJC1 and KJC2 enhanced the GMPP activity of recombinant VTC1 in vitro, while KJCs did not show GMPP activity. Yeast two-hybrid assays suggested that the stimulation of GMPP activity occurs via interaction of KJCs with VTC1. These results suggest that KJCs are key factors for the generation of GDP-Man and affect AsA level and glucomannan accumulation through the stimulation of VTC1 GMPP activity.

Heterologous expression and characterization of an Arabidopsis ß-l-arabinopyranosidase and α-d-galactosidases acting on ß-l-arabinopyranosyl residues.

The major plant sugar l-arabinose (l-Ara) has two different ring forms, l-arabinofuranose (l-Araf) and l-arabinopyranose (l-Arap). Although l-Ara mainly appears in the form of α-l-Araf residues in cell wall components, such as pectic α-1,3:1,5-arabinan, arabinoxylan, and arabinogalactan-proteins (AGPs), lesser amounts of it can also be found as ß-l-Arap residues of AGPs. Even though AGPs are known to be rapidly metabolized, the enzymes acting on the ß-l-Arap residues remain to be identified. In the present study, four enzymes, which we call ß-l-ARAPASE (APSE) and α-GALACTOSIDASE 1 (AGAL1), AGAL2, and AGAL3, are identified as those enzymes that are likely to be responsible for the hydrolysis of the ß-l-Arap residues in Arabidopsis thaliana. An Arabidopsis apse-1 mutant showed significant reduction in ß-l-arabinopyranosidase activity, and an apse-1 agal3-1 double-mutant exhibited even less activity. The apse-1 and the double-mutants both had more ß-l-Arap residues in the cell walls than wild-type plants. Recombinant APSE expressed in the yeast Pichia pastoris specifically hydrolyzed ß-l-Arap residues and released l-Ara from gum arabic and larch arabinogalactan. The recombinant AGAL3 also showed weak ß-l-arabinopyranosidase activity beside its strong α-galactosidase activity. It appears that the ß-l-Arap residues of AGPs are hydrolysed mainly by APSE and partially by AGALs in Arabidopsis.

A protease/peptidase from culture medium of Flammulina velutipes that acts on arabinogalactan-protein.

Arabinogalactan-proteins (AGPs) are highly diverse plant proteoglycans found on the plant cell surface. AGPs have large arabinogalactan (AG) moieties attached to a core-protein rich in hydroxyproline (Hyp). The AG undergoes hydrolysis by various glycoside hydrolases, most of which have been identified, whereas the core-proteins is presumably degraded by unknown proteases/peptidases secreted from fungi and bacteria in nature. Although several enzymes hydrolyzing other Hyp-rich proteins are known, the enzymes acting on the core-proteins of AGPs remain to be identified. The present study describes the detection of protease/peptidase activity toward AGP core-proteins in the culture medium of winter mushroom (Flammulina velutipes) and partial purification of the enzyme by several conventional chromatography steps. The enzyme showed higher activity toward Hyp residues than toward proline and alanine residues and acted on core-proteins prepared from gum arabic. Since the activity was inhibited in the presence of Pefabloc SC, the enzyme is probably a serine protease.

Action of an endo-ß-1,3(4)-glucanase on cellobiosyl unit structure in barley ß-1,3:1,4-glucan.

ß-1,3:1,4-Glucan is a major cell wall component accumulating in endosperm and young tissues in grasses. The mixed linkage glucan is a linear polysaccharide mainly consisting of cellotriosyl and cellotetraosyl units linked through single ß-1,3-glucosidic linkages, but it also contains minor structures such as cellobiosyl units. In this study, we examined the action of an endo-ß-1,3(4)-glucanase from Trichoderma sp. on a minor structure in barley ß-1,3:1,4-glucan. To find the minor structure on which the endo-ß-1,3(4)-glucanase acts, we prepared oligosaccharides from barley ß-1,3:1,4-glucan by endo-ß-1,4-glucanase digestion followed by purification by gel permeation and paper chromatography. The endo-ß-1,3(4)-glucanase appeared to hydrolyze an oligosaccharide with degree of polymerization 5, designated C5-b. Based on matrix-assisted laser desorption/ionization (MALDI) time-of-flight (ToF)/ToF-mass spectrometry (MS)/MS analysis, C5-b was identified as ß-Glc-1,3-ß-Glc-1,4-ß-Glc-1,3-ß-Glc-1,4-Glc including a cellobiosyl unit. The results indicate that a type of endo-ß-1,3(4)-glucanase acts on the cellobiosyl units of barley ß-1,3:1,4-glucan in an endo-manner.

ß-galactosyl Yariv reagent binds to the ß-1,3-galactan of arabinogalactan proteins.

Yariv phenylglycosides [1,3,5-tri(p-glycosyloxyphenylazo)-2,4,6-trihydroxybenzene] are a group of chemical compounds that selectively bind to arabinogalactan proteins (AGPs), a type of plant proteoglycan. Yariv phenylglycosides are widely used as cytochemical reagents to perturb the molecular functions of AGPs as well as for the detection, quantification, purification, and staining of AGPs. However, the target structure in AGPs to which Yariv phenylglycosides bind has not been determined. Here, we identify the structural element of AGPs required for the interaction with Yariv phenylglycosides by stepwise trimming of the arabinogalactan moieties using combinations of specific glycoside hydrolases. Whereas the precipitation with Yariv phenylglycosides (Yariv reactivity) of radish (Raphanus sativus) root AGP was not reduced after enzyme treatment to remove α-l-arabinofuranosyl and ß-glucuronosyl residues and ß-1,6-galactan side chains, it was completely lost after degradation of the ß-1,3-galactan main chains. In addition, Yariv reactivity of gum arabic, a commercial product of acacia (Acacia senegal) AGPs, increased rather than decreased during the repeated degradation of ß-1,6-galactan side chains by Smith degradation. Among various oligosaccharides corresponding to partial structures of AGPs, ß-1,3-galactooligosaccharides longer than ß-1,3-galactoheptaose exhibited significant precipitation with Yariv in a radial diffusion assay on agar. A pull-down assay using oligosaccharides cross linked to hydrazine beads detected an interaction of ß-1,3-galactooligosaccharides longer than ß-1,3-galactopentaose with Yariv phenylglycoside. To the contrary, no interaction with Yariv was detected for ß-1,6-galactooligosaccharides of any length. Therefore, we conclude that Yariv phenylglycosides should be considered specific binding reagents for ß-1,3-galactan chains longer than five residues, and seven residues are sufficient for cross linking, leading to precipitation of the Yariv phenylglycosides.

Differing structures of galactoglucomannan in eudicots and non-eudicot angiosperms.

The structures of cell wall mannan hemicelluloses have changed during plant evolution. Recently, a new structure called ß-galactoglucomannan (ß-GGM) was discovered in eudicot plants. This galactoglucomannan has ß-(1,2)-Gal-α-(1,6)-Gal disaccharide branches on some mannosyl residues of the strictly alternating Glc-Man backbone. Studies in Arabidopsis revealed ß-GGM is related in structure, biosynthesis and function to xyloglucan. However, when and how plants acquired ß-GGM remains elusive. Here, we studied mannan structures in many sister groups of eudicots. All glucomannan structures were distinct from ß-GGM. In addition, we searched for candidate mannan ß-galactosyltransferases (MBGT) in non-eudicot angiosperms. Candidate AtMBGT1 orthologues from rice (OsGT47A-VII) and Amborella (AtrGT47A-VII) did not show MBGT activity in vivo. However, the AtMBGT1 orthologue from rice showed MUR3-like xyloglucan galactosyltransferase activity in complementation analysis using Arabidopsis. Further, reverse genetic analysis revealed that the enzyme (OsGT47A-VII) contributes to proper root growth in rice. Together, gene duplication and diversification of GT47A-VII in eudicot evolution may have been involved in the acquisition of mannan ß-galactosyltransferase activity. Our results indicate that ß-GGM is likely to be a eudicot-specific mannan.

In vivo structural modification of type II arabinogalactans with fungal endo-ß-1, 6-galactanase in Arabidopsis.

Arabinogalactan-proteins (AGPs) are mysterious extracellular glycoproteins in plants. Although AGPs are highly conserved, their molecular functions remain obscure. The physiological importance of AGPs has been extensively demonstrated with ß-Yariv reagent, which specifically binds to AGPs and upon introduction into cells, causes various deleterious effects including growth inhibition and programmed cell death. However, structural features of AGPs that determine their functions have not been identified with ß-Yariv reagent. It is known that AGPs are decorated with large type II arabinogalactans (AGs), which are necessary for their functions. Type II AGs consist of a ß-1,3-galactan main chain and ß-1,6-galactan side chains with auxiliary sugar residues such as L-arabinose and 4-O-methyl-glucuronic acid. While most side chains are short, long side chains such as ß-1,6-galactohexaose (ß-1,6-Gal6) also exist in type II AGs. To gain insight into the structures important for AGP functions, in vivo structural modification of ß-1,6-galactan side chains was performed in Arabidopsis. We generated transgenic Arabidopsis plants expressing a fungal endo-ß-1,6-galactanase, Tv6GAL, that degrades long side chains specifically under the control of dexamethasone (Dex). Two of 6 transgenic lines obtained showed more than 40 times activity of endo-ß-1,6-galactanase when treated with Dex. Structural analysis indicated that long side chains such as ß-1,6-Gal5 and ß-1,6-Gal6 were significantly reduced compared to wild-type plants. Tv6GAL induction caused retarded growth of seedlings, which had a reduced amount of cellulose in cell walls. These results suggest that long ß-1,6-galactan side chains are necessary for normal cellulose synthesis and/or deposition as their defect affects cell growth in plants.

Acetylated Xylan Degradation by Glycoside Hydrolase Family 10 and 11 Xylanases from the White-rot Fungus Phanerochaete chrysosporium.

Endo-type xylanases are key enzymes in microbial xylanolytic systems, and xylanases belonging to glycoside hydrolase (GH) families 10 or 11 are the major enzymes degrading xylan in nature. These enzymes have typically been characterized using xylan prepared by alkaline extraction, which removes acetyl sidechains from the substrate, and thus the effect of acetyl groups on xylan degradation remains unclear. Here, we compare the ability of GH10 and 11 xylanases, PcXyn10A and PcXyn11B, from the white-rot basidiomycete Phanerochaete chrysosporium to degrade acetylated and deacetylated xylan from various plants. Product quantification revealed that PcXyn10A effectively degraded both acetylated xylan extracted from Arabidopsis thaliana and the deacetylated xylan obtained by alkaline treatment, generating xylooligosaccharides. In contrast, PcXyn11B showed limited activity towards acetyl xylan, but showed significantly increased activity after deacetylation of the xylan. Polysaccharide analysis using carbohydrate gel electrophoresis showed that PcXyn11B generated a broad range of products from native acetylated xylans extracted from birch wood and rice straw, including large residual xylooligosaccharides, while non-acetylated xylan from Japanese cedar was readily degraded into xylooligosaccharides. These results suggest that the degradability of native xylan by GH11 xylanases is highly dependent on the extent of acetyl group substitution. Analysis of 31 fungal genomes in the Carbohydrate-Active enZymes database indicated that the presence of GH11 xylanases is correlated to that of carbohydrate esterase (CE) family 1 acetyl xylan esterases (AXEs), while this is not the case for GH10 xylanases. These findings may imply co-evolution of GH11 xylanases and CE1 AXEs.

Glycoside hydrolase subfamily GH5_57 features a highly redesigned catalytic interface to process complex hetero-ß-mannans.

Glycoside hydrolase family 5 (GH5) harbors diverse substrate specificities and modes of action, exhibiting notable molecular adaptations to cope with the stereochemical complexity imposed by glycosides and carbohydrates such as cellulose, xyloglucan, mixed-linkage ß-glucan, laminarin, (hetero)xylan, (hetero)mannan, galactan, chitosan, N-glycan, rutin and hesperidin. GH5 has been divided into subfamilies, many with higher functional specificity, several of which have not been characterized to date and some that have yet to be discovered with the exploration of sequence/taxonomic diversity. In this work, the current GH5 subfamily inventory is expanded with the discovery of the GH5_57 subfamily by describing an endo-ß-mannanase (CapGH5_57) from an uncultured Bacteroidales bacterium recovered from the capybara gut microbiota. Biochemical characterization showed that CapGH5_57 is active on glucomannan, releasing oligosaccharides with a degree of polymerization from 2 to 6, indicating it to be an endo-ß-mannanase. The crystal structure, which was solved using single-wavelength anomalous diffraction, revealed a massively redesigned catalytic interface compared with GH5 mannanases. The typical aromatic platforms and the characteristic α-helix-containing ß6-α6 loop in the positive-subsite region of GH5_7 mannanases are absent in CapGH5_57, generating a large and open catalytic interface that might favor the binding of branched substrates. Supporting this, CapGH5_57 contains a tryptophan residue adjacent and perpendicular to the cleavage site, indicative of an anchoring site for a substrate with a substitution at the -1 glycosyl moiety. Taken together, these results suggest that despite presenting endo activity on glucomannan, CapGH5_57 may have a new type of substituted heteromannan as its natural substrate. This work demonstrates the still great potential for discoveries regarding the mechanistic and functional diversity of this large and polyspecific GH family by unveiling a novel catalytic interface sculpted to recognize complex heteromannans, which led to the establishment of the GH5_57 subfamily.

Structural features conserved in subclass of type II arabinogalactan.

Arabinogalactan-proteins (AGPs) are extracellular proteoglycans, which are presumed to participate in the regulation of cell shape, thus contributing to the excellent mechanical properties of plants. AGPs consist of a hydroxyproline-rich core-protein and large arabinogalactan (AG) sugar chains, called type II AGs. These AGs have a ß-1,3-galactan backbone and ß-1,6-galactan side chains, to which other sugars are attached. The structure of type II AG differs depending on source plant, tissue, and age. Type II AGs obtained from woody plants in large quantity as represented by gum arabic and larch AG, here designated gum arabic-subclass, have a ß-1,3;1,6-galactan structure in which the ß-1,3-galactan backbone is highly substituted with short ß-1,6-galactan side chains. On the other hand, it is unclear whether type II AGs found as the glycan part of AGPs from herbaceous plants, here designated AGP-subclass, also have conserved ß-1,3:1,6-galactan structural features. In the present study we explore similarities of type II AG structures in the AGP-subclass. Type II AGs in fractions obtained from spinach, broccoli, bok choy, komatsuna, and cucumber were hydrolyzed into galactose and ß-1,6-galactooligosaccharides by specific enzymes. Based on the proportion of these sugars, the substitution ratio of the ß-1,3-galactan backbone was calculated as 46-58% in the five vegetables, which is consistently lower than what is seen in gum arabic and larch AG. Although most side chains were short, long chains such as ß-1,6-galactohexaose chains were also observed in these vegetables. The results suggest a conserved ß-1,3;1,6-galactan structure in the AGP-subclass that distinguishes it from the gum arabic-subclass.

Properties of arabinogalactan-proteins in European pear (Pyrus communis L.) fruits.

Arabinogalactans (AGs) and arabinogalactan-proteins (AGPs) were partially purified from an extract of fruits of the European pear (Pyrus communis L.) by DEAE-cellulose ion-exchange and Sepharose 6B gel-filtration chromatography. Among 7 AG(P)-containing fractions, a neutral AGP (SE-1) was confirmed to be highly purified (Mr 67,000) and rich in L-Ara and Gal; this fraction included a small amount (2.6%, w/w) of protein and showed the highest reactivity forming precipitate with ß-Glc Yariv reagent among the 7 fractions, the intensity of which was comparable to that of gum arabic, a standard AGP. Another accompanying minor low-Mr neutral AGP (SE-2; Mr approx. 7200) still contained other polysaccharide (starch fragments) and did not show Yariv reactivity. The carbohydrate moieties of SE-1 consisted of consecutive (1 → 3)-linked ß-galactosyl backbone chains substituted with side chains of (1 → 6)-linked ß-galactosyl residues at O-6, to which mainly single α-l-arabinofuranosyl residues were attached through O-3. This structural feature was also observed for SE-2. Successive digestion of SE-1 with α-l-arabinofuranosidase and exo-ß-(1 → 3)-galactanase with the aid of endo-ß-(1 → 3)-galactanase released most (more than 98%, w/w) of the carbohydrate moieties as low-Mr fragments. These consisted of free L-Ara and Gal, and a series of ß-(1 → 6)-galactooligosaccharides with degree of polymerization (dp) up to at least 17, indicative of attachment of (1 → 6)-linked ß-galactosyl side chains of varying length along the (1 → 3)-linked ß-galactosyl backbone chains.

Yariv reactivity of type II arabinogalactan from larch wood.

Larch arabinogalactan (AG) is classified as a plant type II AG. Its basic structure is constituted by a ß-1,3-galactan main chain with ß-1,6-galactan side chains. But its properties are distinct from other type II AGs. Whereas most type II AGs are found as glycan moieties of arabinogalactan-protein (AGP), larch AG lacks a protein moiety. Larch AG itself is also unlike other type II AGs as it lacks Yariv reactivity, the capability of AG to form insoluble precipitate with ß-Yariv reagents, 1,3,5-tri-(p-glycosyloxyphenylazo)-2,4,6-trihydroxybenzene with ß-glucosyl or ß-galactosyl residues at the termini. For the present study, we prepared ß-galactan I, II, and III from larch AG by performing single, double, and triple Smith degradation, which breaks ß-1,6-galactan side chains, and examined Yariv reactivity of the products. Methylation analysis revealed that ß-galactans II and III had lost more than 90% of their ß-1,6-galactan branches. In the radial gel diffusion assay, ß-galactans II and III showed Yariv reactivity, indicating the presence of a Yariv-reactive structure in larch AG, although native larch AG does not have Yariv reactivity. The Yariv reactivity of the ß-galactans was completely lost after treatment with endo-ß-1,3-galactanase. These results confirm that ß-1,3-galactan is necessary for Yariv reactivity of type II AG. The present results also suggest that high substitution of ß-1,3-galactan with ß-1,6-galactan side chains affects Yariv reactivity in larch AG.

Properties of two fungal endo-ß-1,3-galactanases and their synergistic action with an exo-ß-1,3-galactanase in degrading arabinogalactan-proteins.

Arabinogalactan-proteins (AGPs) are plant proteoglycans, which are widely encountered in the plant kingdom, usually localized on the cell surface. The carbohydrate moieties of AGPs consist of ß-1,3-galactan main chains and ß-1,6-galactan side chains, to which other auxiliary sugars are attached. To date, FvEn3GAL isolated from Flammulina velutipes is the sole ß-1,3-galactanase acting on ß-1,3-galactan in an endo-manner. Here we cloned two homologous genes, designated Af3G and NcEn3GAL, possibly encoding endo-ß-1,3-galactanase from Aspergillus flavus and Neurospora crassa, respectively. The recombinant Af3G (rAf3G) and rNcEn3GAL expressed in Pichia pastoris specifically hydrolyzed ß-1,3-galactan in an endo-manner, as did the rFvEn3GAL. Among galactooligosaccharides, ß-1,3-galactotriose was identified as the smallest substrate for these enzymes. These results suggest that enzymatic characteristics are conserved in many endo-ß-1,3-galactanases belonging to the glycoside hydrolase 16 family. On the other hand, rAf3G and rNcEn3GAL generated more ß-1,3-galactobiose from ß-1,3-galactotetraose than did rFvEn3GAL, suggesting that rAf3G and rNcEn3GAL prefer hydrolyzing the central ß-1,3-glycosidic linkage of three in ß-1,3-galactotetraose. Although rAf3G and rNcEn3GAL alone hardly hydrolyze native AGP, they acted synergistically with a fungal exo-ß-1,3-galactanase on the AGP. These endo-ß-1,3-galactanases presumably aid hydrolysis by internally breaking up AGPs, which creates more sites of attack for exo-ß-1,3-galactanase.

L-Fucose-containing arabinogalactan-protein in radish leaves.

The carbohydrate moieties of arabinogalactan-proteins (AGPs) have ß-(1 → 3)-galactan backbones to which side chains of (1 → 6)-linked ß-Gal residues are attached through O-6. Some of these side chains are further substituted with other sugars. We investigated the structure of L-Fuc-containing oligosaccharides released from the carbohydrate moieties of a radish leaf AGP by digestion with α-L-arabinofuranosidase, followed by exo-ß-(1 → 3)-galactanase. We detected a series of neutral ß-(1 → 6)-galactooligosaccharides branching variously at O-3 of the Gal residues, together with corresponding acidic derivatives terminating in 4-O-methyl-GlcA (4-Me-GlcA) or GlcA at the non-reducing terminals. In neutral oligosaccharides with degree of polymerization (dp) mainly higher than 10, L-Fuc groups were attached through L-Ara residues as the sequence, α-L-Fucp-(1 → 2)-α-L-Araf-(1 →. This sequence was verified by isolation of the pentasaccharide α-L-Fuc-(1 → 2)-α-L-Araf-(1 → 3)-ß-Gal-(1 → 6)-ß-Gal-(1 → 6)-Gal upon digestion of the higher oligosaccharides with endo-ß-(1 → 6)-galactanase. By contrast, in lower polymerized (predominantly dp 4) acidic oligosaccharides, L-Fuc groups were attached directly at the non-reducing terminals through α-(1 → 2)-linkages, resulting in the release of the tetrasaccharides, α-L-Fucp-(1 → 2)-ß-GlcA-(1 → 6)-ß-Gal-(1 → 6)-Gal and α-L-Fucp-(1 → 2)-ß-4-Me-GlcA-(1 → 6)-ß-Gal-(1 → 6)-Gal. In long acidic oligosaccharides with dp mainly higher than 13, L-Fuc groups localized on branches were attached to the uronic acids directly and/or L-Ara residues as in the neutral oligosaccharides.