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First published online May 12, 2006; 10.1104/pp.106.080309 Plant Physiology 141:1098-1105 (2006) © 2006 American Society of Plant Biologists
Identification of a Mung Bean Arabinofuranosyltransferase That Transfers Arabinofuranosyl Residues onto (1, 5)-Linked
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ABSTRACT |
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 TOP ABSTRACT RESULTSDISCUSSIONMATERIALS AND METHODSLITERATURE CITED |
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-L-arabinofuranose to exogenous (1, 5)-linked 
-L-arabino-oligosaccharides labeled at their reducing ends with 2-aminobenzamide. The transferred residue was shown, using 1H-nuclear magnetic resonance spectroscopy and -L-arabinofuranosidase treatment, to be -L-Araf and to be linked to O-5 of the nonreducing terminal Araf residue of the acceptor oligosaccharide. The enzyme was nonprocessive because only a single Araf residue was added to the acceptor molecule. Arabino-oligosaccharides with a degree of polymerization between 3 and 8 were acceptor substrates. The 2-aminobenzamide-labeled arabino-tetra- and pentasaccharides were the most effective acceptor substrates analyzed. The enzyme has a pH optimum between 6.5 and 7.0 and its activity is stimulated by Mn2+ and Co2+ ions. The apparent Km and Vmax values of the arabinofuranosyltransferase for UDP-arabinofuranose are 243 µM and 243 pmol min–1 mg protein–1, respectively. The highest enzyme activity was detected in the elongating regions of mung bean hypocotyls. The data show that UDP-arabinofuranose is the donor molecule for the generation of arabino-oligosaccharides composed of Araf residues.
; O'Neill and York, 2003), whereas xylogalacturonan is present only in specialized tissues of some plants (O'Neill and York, 2003) and apiogalacturonan has only been detected in duckweed (Lemna minor; O'Neill and York, 2003).
RG-I has a repeating disaccharide backbone of [-4--D-GalpA-(1, 2)--L-Rhap-1]n (O'Neill et al., 1990; Carpita and Gibeaut, 1993; O'Neill and York, 2003). Between 20% and 80% of the rhamnosyl residues in RG-I are substituted at O-4 with arabinan, galactan, and arabinogalactan side chains (Ridley et al., 2001; Schols and Voragen, 2002). Arabinan consists of linear (1, 5)-linked -L-arabinofuranosyl (Araf) residues that are substituted at O-3 or O-2 with Araf residues.
The biosynthesis of pectic polysaccharides is believed to require at least 58 distinct glycosyl-, methyl-, and acetyltransferases based on the one-linkage/one-enzyme hypothesis (Mohnen, 2002). However, very few enzymes involved in pectin biosynthesis have been identified and characterized biochemically and genetically (Sterling et al., 2006). Arabinosyltransferase (AraT) activity has been detected in vitro by measuring the incorporation of [14C]Ara from UDP-[14C]arabinopyranosyl (Arap) onto endogenous and exogenous acceptors (Odzuck and Kauss, 1972; Bolwell and Northcote, 1981; Rodgers and Bolwell, 1992). However, the acceptor substrates and the enzymatically formed products were poorly characterized in these studies. Nunan and Scheller (2003) reported that mung bean (Vigna radiata) hypocotyls contain AraT activity that transferred Arap from UDP-Arap onto exogenous arabino-oligosaccharides. Subsequently, we showed that the transferred Arap residue was -linked to O-3 of the nonreducing terminal Araf residue (Ishii et al., 2005a). Such a result suggests that UDP-Arap is not the donor for the biosynthesis of -linked arabinan. As far as we are aware, there have been no reports describing AraT activity using UDP--L-Araf (UDP-Araf) as a donor substrate for pectin biosynthetic enzymes.
Here we describe the use of UDP-Araf and 2-aminobenzamide (2AB)-labeled (1, 5)--L-arabino-oligosaccharides as acceptors to demonstrate the presence of arabinofuranosyltransferase (ArafT) activity in Golgi membrane fractions from mung bean hypocotyls. Our data are consistent with the presence of ArafT activity, which catalyzes the transfer of the Araf residue from UDP-Araf onto exogenous acceptors to form (1, 5)-linked -L-arabino-oligosaccharides.
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RESULTS |
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TOPABSTRACTRESULTSDISCUSSIONMATERIALS AND METHODSLITERATURE CITED |
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The assay for ArafT activity was performed by reacting UDP-Araf (2 mM) as a donor substrate and 2AB-labeled arabino-oligosaccharides (10 µM) as exogenous acceptors in the presence of Golgi membrane fractions isolated from etiolated mung bean hypocotyls. The enzymatically formed products were analyzed by HPLC with fluorescence detection (Fig. 1A
). Products corresponding to 2AB-labeled octa- (Ara8-2AB), nona- (Ara9-2AB), and deca- (Ara10-2AB) saccharides were formed when the 2AB-labeled hepta-saccharide (Ara7-2AB) was used as the acceptor. Such a result indicates that the Ara residue is transferred from UDP-Araf onto Ara7-2AB, thereby elongating the acceptor oligosaccharides by at least one residue (Fig. 1A). About 25% of Ara7-2AB was converted into Ara8-2AB when the reaction was allowed to proceed for 4 h. 2AB-labeled arabino-oligosaccharides with a degree of polymerization (DP) up to 10 formed when the reaction was extended to 24 h. Although a small amount of Ara8-2AB was present as a contaminant in the acceptor Ara7-2AB, a substantial increase in the amount of Ara8-2AB was observed with a 10-min reaction, whereas no Ara10-2AB was detected until after 2 h (Fig. 1B), suggesting that the newly formed Ara8-2AB and Ara9-2AB themselves became acceptors for ArafT. This indicates that the transfer of Araf residues is nonprocessive because only one arabinosyl residue is added during the reaction. The acceptor oligosaccharide and the reaction products were almost completely digested with arabinofuranosidase (data not shown) that specifically hydrolyzes -Araf residues (Yang et al., 2006), indicating that the transferred arabinosyl residues exist in the furanose form.
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The 2AB-labeled arabino-tetrasaccharide (Ara4-2AB) was one of the most effective acceptor substrates tested (Fig. 2D ). Therefore, we used Ara4-2AB to generate Ara5-2AB in amounts sufficient for its structural characterization. The products from three separate reactions were combined and Ara5-2AB purified by size-exclusion and normal-phase liquid chromatography (LC). The newly formed Ara5-2AB was analyzed by positive ion-mode LC-electrospray ionization (ESI)-mass spectrometry. The ESI mass spectrum contained an intense signal at mass-to-charge ratio (m/z) 799 that corresponds to the [M+H]+ ion of a 2AB-labeled product that contains five pentosyl residues. The product ion mass spectrum of m/z 799 contained ions at m/z 667, 535, 403, and 271 that correspond to the sequential loss of pentosyl residues and confirms the presence of consecutive arabinosyl residues.
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5.065 is assigned to the H-1 resonance of the nonreducing terminal Araf residue (T), which partially overlaps with the H-1 resonance of the internal Araf residue (B). The chemical shift value and broad singlet peak are consistent with an -linkage (Mizutani et al., 1989). The broad singlet peak at 5.032 is the residue adjacent to the terminal nonreducing end (residue E). The resonance of the Araf residue next to the reducing end (residue A) occurs at 5.075. These chemical shift values and the small J1,2 coupling constants are consistent with the presence of -linked L-Araf residues (Mizutani et al., 1989; Ishii et al., 2005b). The ring protons of the enzymatically formed Ara5-2AB were not assigned because the oligosaccharide was available in limited amounts and its ring proton signals overlapped with the proton signals of the residual Triton X-100. Nevertheless, these results strongly suggested that the enzymatically formed product was -(1, 5)-linked arabinan pentasaccharide composed of Araf residue.
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-L-ArafTThe amounts of oligosaccharide products formed were proportional to the amounts of protein (Fig. 2A) and were maximal between pH 6.5 and 7.0 (Fig. 2B). ArafT activity required the presence of divalent cations (Fig. 2C). Mn2+ and Co2+ were more effective than Mg2+, Ca2+, Zn2+, and Cu2+. However, MnCl2 at concentrations >5 mM decreased ArafT activity (data not shown). ArafT activity was increased 2-fold by adding up to 2% Triton X-100 to the reaction mixture (data not shown). This increase may result from a partial solubilization of the enzyme, thereby allowing greater access to the donor and acceptor molecules. The apparent Km and Vmax values of ArafT for UDP-Araf were 243 µM and 243 pmol min–1 mg protein–1, respectively.
The effect of the DP of arabino-oligosaccharides on ArafT activity was determined using arabino-oligosaccharides with DPs between 3 and 8. The tetra- and pentasaccharides were the most effective acceptors tested (Fig. 2D). No new products were formed when the reaction was performed with 2AB-labeled arabinitol.
When arabino-octasaccharide having Arap residue at the nonreducing end (ArapAra7-2AB) was used as an acceptor substrate, no Araf residues transferred onto the ArapAra7-2AB (Fig. 4 ).
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Membrane fractions were isolated from mung bean hypocotyls by discontinuous Suc gradient centrifugation. Each membrane fraction was identified by immunoblotting using an antibody raised against a specific membrane marker protein, calnexin for endoplasmic reticulum (ER), xyloglucan xylosyltransferase (XT1) for Golgi, and proton ATPase for plasma membranes (PMs; Fig. 5A ). Immunoblotting showed that Golgi were enriched in the interface between 25% and 34% Suc, whereas ER and PMs were enriched in the fractions between 18% and 25%, and between supernatant and 50% Suc, respectively (Fig. 5B). ArafT activity was the highest in the fraction between 25% and 34% Suc, indicating that ArafT is mainly localized in the Golgi membrane (Fig. 5B).
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DISCUSSION |
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TOPABSTRACTRESULTSDISCUSSIONMATERIALS AND METHODSLITERATURE CITED |
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-L-arabinan biosynthesis is present in mung bean Golgi membranes. -L-Arabinofuranosidase treatment and 1H-NMR spectroscopic analysis show unequivocally that the enzymatically attached residue is furanose and -(1, 5)-linked. The highest ArafT activity was present in Golgi membranes isolated from actively elongating portions of mung bean hypocotyls (see Fig. 5C). The Suc density gradient procedure used to obtain the membrane fractions is convenient for detecting ArafT activity, but may have some disadvantages. For example, many of the enzymes involved in the interconversion of UDP sugars are cytoplasmic and presumably buffer soluble. Thus, the possibility cannot be discounted that the presence of such enzymes in the membrane fractions reduces the apparent ArafT activity by decreasing the amount of UDP-Araf. However, we found no evidence for the incorporation of Arap residues into the oligosaccharide acceptors, suggesting that little, if any, of the UDP-Araf is converted to UDP-Arap. Such a result also suggests that the biochemical identification of an Arap mutase, if it is indeed present in plants, is likely to be challenging.
Previous studies have shown that a single -linked Arap residue is transferred to O-3 of the nonreducing terminal Araf residue of 2AB-labeled arabino-oligosaccharides when UDP-Arap and arabino-oligosaccharides are incubated with mung bean microsomal membranes (Nunan and Scheller, 2003; Ishii et al., 2005a). In contrast, our results show that UDP-Araf is the donor molecule for the generation of arabino-oligosaccharides composed of Araf residues. We suggest that it is unlikely that plant AraTs convert UDP-Arap to UDP-Araf as suggested earlier (Fry and Northcote, 1983).
Arabinosyl residues are present in numerous cell wall polysaccharides, including arabinan, arabinogalactan, arabinogalactan protein, and xylan. In these polysaccharides, arabinosyl residues exist predominantly in the Araf form. The Araf residues in these polysaccharides are likely to be transferred from UDP-Araf by ArafTs. However, there have been no reports on the formation or presence of UDP-Araf in plant cells (Fry and Northcote, 1983). The possibility that plants contain mutases that interconvert UDP-L-Arap and UDP-L-Araf is suggested by the fact that a small amount of Araf residues are detected in the arabinoxylan formed when UDP-[14C]Arap is incubated with wheat (Triticum aestivum) seedling microsomes (Porchia et al., 2002). A UDP-galactopyranose mutase, which catalyzes the interconversion of UDP-D-Galf and UDP-D-Galp, has been detected in mycobacteria. This enzyme also interconverts UDP-L-Arap and UDP-L-Araf, although the reaction strongly favors the formation of UDP-Arap (Zhang and Liu, 2001). Thus, a possible explanation for the failure to detect UDP-Araf in plant tissues may be that the concentration of UDP-Araf is very low because of the presence of a mutase that favors UDP-Arap formation. No homologs of the microbial UDP-Galp mutases have been identified by BLAST searches of the currently available plant genome databases (Nunan and Scheller, 2003). Thus, biochemical studies are required to determine whether plants do contain mutases that interconvert UDP-Arap and UDP-Araf.
Arap residues are quantitatively minor components of arabinans isolated from several plants (Capek et al., 1983; Stephen, 1983; Kiyohara et al., 1987; Swamy and Salimath, 1991). 3-O--L-Arap-L-Ara has been isolated from partial acid hydrolysates of larchwood arabinogalactan (Odonma
ig et al., 1994). Arap is also present at the nonreducing end of pectic galactan (Huisman et al., 2001). A single internal -L-Arap residue is present in the acetic acid-containing side chain of RG-II (O'Neill et al., 2004).
We have shown that no Araf residues are transferred to ArapAra7-2AB when this acceptor and UDP-Araf are incubated with mung bean Golgi membranes. Thus, -(1, 5)-linked arabinan with nonreducing terminal Arap residues is not likely to be an acceptor substrate for ArafT. This result, when taken together with the fact that virtually all of the Arap in RG-I exist as nonreducing terminal residues, implies that the attachment of an Arap onto an Araf side chain will prevent further elongation of the side chain and may provide a mechanism for controlling side chain length. Side chain length may also be determined by differences in the ability of ArafT to elongate arabino-oligosaccharides. We found that Ara4-2AB and Ara5-2AB were the most effective acceptor substrates tested (Fig. 2D). We have also shown that the arabinan side chains have an average DP of 6 in the RG-I isolated from 3-d-old mung bean hypocotyls (Ishii et al., 2005c). The availability of a facile assay for ArafT activity provides an opportunity to investigate arabinan synthesis in great detail. However, such studies require the development of improved and less expensive methods for the synthesis of UDP-Araf and that the ArafTs are purified to homogeneity or that enzymatically active recombinant protein is obtained by heterologous expression of the appropriate genes.
There are numerous reports suggesting that pectic arabinans have specific biological functions in plants. For example, transgenic potato (Solanum tuberosum) plants with reduced arabinan content (approximately 70% reduction of arabinans compared to wild type) caused by expression of Golgi-localized arabinanase (SkjØt et al., 2002) have been reported to have tubers that are more brittle than their wild-type counterparts (Ulvskov et al., 2005), suggesting that pectic arabinan has a role in cell-cell interactions and tissue cohesion. Reduced cell-cell attachment is also a characteristic of callus tissue of the arabinan-deficient tobacco (Nicotiana plumbaginifolia) mutant nolac-H14 (Iwai et al., 2001). Similarly, cell-cell adhesion is affected in tomato (Lycopersicon esculentum) fruits carrying the Cnr mutation that leads to reduced arabinan deposition in the wall (Orfila et al., 2001). Pectic polysaccharides, including arabinans, have also been implicated in the functioning of stomata because the normal opening and closing of guard cells is prevented by treatment with an -L-arabinanase (Jones et al., 2003, 2005). In contrast, the arabinan-deficient Arabidopsis (Arabidopsis thaliana) mutant arad 1 has no visible phenotype even though the arabinan content of leaf and stem cell walls is reduced by 75% and 46%, respectively (Harholt et al., 2006). Providing a complete description of the roles of arabinan in plant cell walls will be challenging because it will require the identification and functional characterization of all plant genes that encode proteins involved in arabinan synthesis and metabolism.
Identification of ArafT in mung bean Golgi and the availability of a facile assay for identification of this class of glycosyltransferase provide some of the tools required to increase our understanding of pectin synthesis in plants at both the molecular and biochemical levels.
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MATERIALS AND METHODS |
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TOPABSTRACTRESULTSDISCUSSIONMATERIALS AND METHODSLITERATURE CITED |
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Mung bean (Vigna radiata) seeds were purchased from Marutane. Chemically synthesized UDP--L-Araf was obtained from the Peptide Institute. Arabino-oligosaccharides with DP between 2 and 8 were purchased from Megazyme. 2AB-labeled arabino-oligosaccharides were prepared as described (Ishii et al., 2002). Recombinant -L-arabinofuranosidase was obtained by expressing an -L-arabinofuranosidase gene of Aspergillus oryzae in Pichia pastoris (Yang et al., 2006). This exoglycanase hydrolyzes (1, 2)-, (1, 3)-, and (1, 5)-linked Araf residues. HEPES, MES, and Triton X-100 were obtained from Dojindo and Sigma-Aldrich, respectively. All other chemicals and reagents used were purchased from Wako Pure Chemicals.
Preparation of the Membrane
Mung bean seeds were grown for 3 d in the dark at 25°C. All manipulations were done on ice or at 4°C. ER, Golgi, and PMs of mung bean hypocotyls were prepared by a flotation centrifugation method (Buckeridge et al., 1999) with some minor modifications. One-centimeter segments under the hook of etiolated hypocotyls (20 g fresh weight) were chopped with a razor blade and ground with a mortar and pestle in a buffer (1 mL g–1) of 20 mM HEPES-KOH, pH 7.0, containing 84% (w/v) Suc, 20 mM KCl, 5 mM EDTA, 5 mM EGTA, 10 mM dithiothreitol, and EDTA-free complete protease inhibitor cocktail (Roche Diagnostics). The suspension was filtered through miracloth to remove cell debris and then centrifuged at 1,000g for 5 min. The supernatant (approximately 10–13 mL) was loaded onto a 3-mL cushion of 50% Suc solution. A discontinuous gradient was formed by adding 8 mL of 34% Suc, 8 mL of 25% Suc, 7 mL of 18% Suc, and 5 mL of 9.5% Suc solutions on the surface of the supernatant (Fig. 5B). The gradient was centrifuged at 100,000g for 1.5 h and the interfaces at 18%-25%, 25%-34%, and supernatant-50% Suc were collected. Each membrane fraction was identified by immunoblotting using antibodies against calnexin for ER, XT1, Golgi, and proton ATPase for PMs. Immunoblots were visualized using a chemiluminescence detection kit (Pierce). Interfaces at 18%/25%, 25%/34%, and supernatant/50% Suc corresponded to ER, Golgi, and PMs, respectively. For localization analysis of the enzyme activity, the Golgi membrane was prepared from three successive regions (A, B, and C) of mung bean hypocotyls (see Fig. 5C). Regions A, B, and C are young, middle, and mature tissues, respectively (Goldberg et al., 1986). Total protein in the membrane fractions was determined with bovine serum albumin as a standard using a bicinchoninic acid protein assay kit (Pierce) according to the manufacturer's instructions.
Assay for ArafT Activity
ArafT was determined using a standard reaction mixture (10 µL) containing 50 mM MES-KOH, pH 6.5, 5 mM MnCl2, 2 mM UDP-Araf, 10 µM 2AB-labeled arabino-heptasaccharide, and 1% (v/v) Triton X-100. The reaction mixture was incubated for 30 min at 25°C and then the reaction was terminated by heating for 5 min at 100°C. The mixture was centrifuged and the supernatant then analyzed by normal-phase HPLC using fluorescence (
x = 330 mm, em = 420 mm) detection as described (Ishii et al., 2005c). Enzyme activity is expressed as pmol Ara transferred min–1 mg protein–1, based on the concentration curve of 2AB-Ara3 as the calibration standard. The apparent Km and Vmax values for ArafT as the crude enzyme for Ara4-2AB were determined at 25°C for 30 min using UDP-Araf (40–200 µM), and the same concentrations of other components as the standard assay mixture. For determination of the optimal pH, 50 mM MES-KOH (pH 5.5–7.0), 50 mM HEPES-KOH (pH 7.0–8.0), and 50 mM Tris-HCl (pH 8.0–9.0) buffer and the same concentration of other components as the standard assay mixture were used. The enzyme activity toward the oligosaccharides with different DPs was determined at 25°C for 30 min by using 10 µM of 2AB-labeled arabino-oligosaccharides with DP 1 to 7 and the same concentration of the other compounds as the standard assay mixture. The effect of various divalent cations on ArafT activity was examined under the same conditions as the standard assay in the presence of 5 mM of MnCl2, MgCl2, CoCl2, CaCl2, ZnCl2, and CuSO4. The enzyme activity toward ArapAra7-2AB was done at 25°C for 1 h using 10 µM ArapAra7-2AB (Ishii et al., 2005a) and the same concentration of other compounds as the standard assay mixture.
Analysis of Enzymatically Formed Oligosaccharides
Enzyme reactions were performed in a total volume of 100 µL containing 0.1 mM of Ara4-2AB instead of Ara7-2AB. The products from five separate reactions were combined and applied to Bio-Gel P-4 and P-2 columns and the fluorescence-positive fractions were collected. This step was repeated three times to obtain sufficient product for analysis. The fluorescence-positive fractions were freeze dried and the residue then dissolved with water (0.5 mL) and extracted with toluene (1 mL x 10) to remove Triton X-100. The aqueous layer was freeze dried. The residue was dissolved in 75% CH3CN-water (v/v) and separated by normal-phase HPLC (Ishii et al., 2005c). The purified product was analyzed by LC-ESI-mass spectrometry and 1H-NMR spectrometry (Ishii et al., 2005c). The enzymatically formed products from Ara7-2AB were treated with -L-arabinofuranosidase (Yang et al., 2006) in 100 mM sodium acetate buffer (pH 5.0) at 50°C overnight and analyzed by HPLC as described above.
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ACKNOWLEDGMENTS |
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Received March 13, 2006; returned for revision April 26, 2006; accepted April 26, 2006.
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FOOTNOTES |
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The author responsible for the distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Tadashi Ishii (tishii{at}ffpri.affrc.go.jp).
Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.106.080309.
* Corresponding author; e-mail tishii{at}ffpri.affrc.go.jp; fax 81–29–874–3720.
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-L-Arabino-Oligosaccharides
), HEPES-KOH (
), and Tris-HCl (
) buffers. The enzyme activity at pH 6.5 was taken as 100%; bars are the means of three analyses ± SD. C, The effects of divalent cations on ArafT activity. The reactions were performed in the presence of 5 mM of each cation. The relative activities to that of MnCl2 are shown; bars are the means of three analyses ± SD. D, ArafT activity is dependent on the DP of the (1, 5)-linked 
