Molecular characterization of a dehydroascorbate reductase from Pinus bungeana

Glutathione S-transferase: a versatile protein family

Glutathione-S transferase (GST) is a most ancient protein superfamily of multipurpose roles and evolved principally from gene duplication of an ancestral GSH binding protein. They have implemented in diverse plant functions such as detoxification of xenobiotic, secondary metabolism, growth and development, and majorly against biotic and abiotic stresses. The vital structural features of GSTs like highly divergent functional topographies, conserved integrated architecture with separate binding pockets for substrates and ligand, the stringent structural fidelity with high Tm values (50º-60º), and stress-responsive cis-regulatory elements in the promoter region offer this protein as most flexible plant protein for plant breeding approaches, biotechnological applications, etc. This review article summarizes the recent information of GST evolution, and their distribution and structural features with emphasis on the assorted roles of Ser and Cys GSTs with the signature motifs in their active sites, alongside their recent biotechnological application in the area of agriculture, environment, and nanotechnology have been highlighted.

The pivotal function of dehydroascorbate reductase in glutathione homeostasis in plants

Under natural conditions, plants are exposed to various abiotic and biotic stresses that trigger rapid changes in the production and removal of reactive oxygen species (ROS) such as hydrogen peroxide (H2O2). The ascorbate-glutathione pathway has been recognized to be a key player in H2O2 metabolism, in which reduced glutathione (GSH) regenerates ascorbate by reducing dehydroascorbate (DHA), either chemically or via DHA reductase (DHAR), an enzyme belonging to the glutathione S-transferase (GST) superfamily. Thus, DHAR has been considered to be important in maintaining the ascorbate pool and its redox state. Although some GSTs and peroxiredoxins may contribute to GSH oxidation, analysis of Arabidopsis dhar mutants has identified the key role of DHAR in coupling H2O2 to GSH oxidation. The reaction of DHAR has been proposed to proceed by a ping-pong mechanism, in which binding of DHA to the free reduced form of the enzyme is followed by binding of GSH. Information from crystal structures has shed light on the formation of sulfenic acid at the catalytic cysteine of DHAR that occurs with the reduction of DHA. In this review, we discuss the molecular properties of DHAR and its importance in coupling the ascorbate and glutathione pools with H2O2 metabolism, together with its functions in plant defense, growth, and development.

Enzymatic characterization and crystal structure analysis of Chlamydomonas reinhardtii dehydroascorbate reductase and their implications for oxidative stress

Dehydroascorbate reductase (DHAR) is a key enzyme for glutathione (GSH)-dependent reduction of dehydroascorbate (DHA) to recycled ascorbate (AsA) in plants, and plays a major role against the toxicity of reactive oxygen species (ROS). Previously, we proposed that the increase of AsA regeneration via enhanced DHAR activity modulates the ascorbate-glutathione cycle activity against photooxidative stress in Chlamydomonas reinhardtii. In the present work, we use site-directed mutagenesis and crystal structure analysis to elucidate the molecular basis of how C. reinhardtii DHAR (CrDHAR1) is involved in the detoxification mechanisms. Mutagenesis data show that the D21A, D21N and C22A mutations result in severe loss of the enzyme's function, suggesting crucial roles of Asp-21 and Cys-22 in substrate binding and catalysis. The mutant K11A also exhibits reduced redox activity (∼50%). The crystal structure of apo CrDHAR1 further provides insights into the proposed mechanism centering on the strictly conserved Cys-22, which is suggested to initiate the redox reactions of DHA and GSH. Furthermore, in vitro oxidation of the recombinant CrDHAR1 in the presence of 1 mM H₂O₂ has minor effects on the K_m for the substrates but significantly reduces the k_cat. The enzyme's activity and its mRNA abundance in the C. reinhardtii cells are increased by treatment with 0.2-1 mM H₂O₂ but decreased when H₂O₂ is ≥ 1.5 mM. The latter decrease is accompanied by oxidative damage and lower AsA concentrations. These biochemical and physiological data provide new insights into the catalytic mechanism of CrDHAR1, which protects the C. reinhardtii cells from oxidative stress-induced toxicity.

Dioscorea alata tuber proteome analysis shows over thirty dioscorin isoforms and novel tuber proteins

In Dioscorea, dioscorin (31 kDa) is the major storage protein constituting 85% of the total tuber proteins. An integrated proteomic and biochemical approach was used to understand the physiological role of dioscorin in the two contrasting growth stages (germinating and mature tuber). HPLC analysis showed 3 fold reduction in mannitol and 12.88 and 1.24 fold increase in sucrose and maltose in the germinating tuber. A 1.8 and 3 fold increase in sucrose phosphate synthase and mannitol dehydrogenase activity respectively was observed in the germinating tuber while a 2 fold higher invertase probably lowers the sucrose accumulation in the mature tuber. SDS-PAGE and 2-D maps of the mature and germinating tubers confirmed depletion (more than 50%) of dioscorin on germination. Dioscorin was purified using ion exchange and gel filtration chromatography with 43.32 fold purification and 38.16 yield. Out of a trail of 35 spots at 31 kDa only 12 spots (identified as dioscorin isoforms) were present in the 2D gel of the purified fraction. To search for other tuber proteins besides dioscorin, the unbound fractions of DEAE column were analysed by 2DGE. DREB 1A, caffeic acid 3-O-methyltransferase and Rab-1 small GTP binding protein were identified perhaps for the first time in the Dioscorea proteome. The interactome analysis revealed these to be involved in oxidative stress, carotenoid synthesis and vesicular transport. This is perhaps the first attempt to identify tuber proteome (although limited) and to understand the physiological significance of these proteins.

Insights into ascorbate regeneration in plants: investigating the redox and structural properties of dehydroascorbate reductases from Populus trichocarpa

Dehydroascorbate reductases (DHARs), enzymes belonging to the GST superfamily, catalyse the GSH-dependent reduction of dehydroascorbate into ascorbate in plants. By maintaining a reduced ascorbate pool, they notably participate to H2O2 detoxification catalysed by ascorbate peroxidases (APXs). Despite this central role, the catalytic mechanism used by DHARs is still not well understood and there is no supportive 3D structure. In this context, we have performed a thorough biochemical and structural analysis of the three poplar DHARs and coupled this to the analysis of their transcript expression patterns and subcellular localizations. The transcripts for these genes are mainly detected in reproductive and green organs and the corresponding proteins are expressed in plastids, in the cytosol and in the nucleus, but not in mitochondria and peroxisomes where ascorbate regeneration is obviously necessary. Comparing the kinetic properties and the sensitivity to GSSG-mediated oxidation of DHAR2 and DHAR3A, exhibiting 1 or 3 cysteinyl residues respectively, we observed that the presence of additional cysteines in DHAR3A modifies the regeneration mechanism of the catalytic cysteine by forming different redox states. Finally, from the 3D structure of DHAR3A solved by NMR, we were able to map the residues important for the binding of both substrates (GSH and DHA), showing that DHAR active site is very selective for DHA recognition and providing further insights into the catalytic mechanism and the roles of the additional cysteines found in some DHARs.

Characterization of proteins in soybean roots under flooding and drought stresses

Flooding and drought affect soybean growth because soybean is a stress-sensitive crop. In 2-day-old plants exposed to 2-day flooding or drought, the fresh weight of roots was markedly suppressed, although the root morphology clearly differed between two conditions. To understand the response mechanisms of soybean to flooding and drought stresses, a gel-free proteomic technique was used. A total of 97 and 48 proteins were significantly changed in response to flooding and drought stresses, respectively. Proteins involved in protein synthesis were decreased by flooding stress and increased by drought. Glycolysis-related proteins were increased in roots by both flooding and drought stresses. Fermentation, stress, and cell wall-related proteins were increased in response to flooding stress, whereas cell organization and redox-related proteins were increased under drought stress. Among the identified proteins, three S-adenosylmethionine synthetases were commonly decreased and increased in response to flooding and drought stresses, respectively. The mRNA expression levels of S-adenosylmethionine synthetase genes displayed a similar tendency to the changes in protein abundance. These results suggest that S-adenosylmethionine synthetase is involved in the regulation of stress response because it was changed in response to flooding and drought stresses.

BIOLOGICAL SIGNIFICANCE

This study reported on the response mechanisms of soybean to flooding and drought stresses using the gel-free proteomic technique. Proteins involved in protein synthesis were decreased by flooding stress and increased by drought. Glycolysis-related proteins were increased in roots by both flooding and drought stresses. Fermentation, stress, and cell wall-related proteins were increased in response to flooding stress, whereas cell organization and redox-related proteins were increased under drought stress. Among the identified proteins, three S-adenosylmethionine synthetases were commonly decreased and increased in response to flooding and drought stresses, respectively. The mRNA expression levels of S-adenosylmethionine synthetase genes displayed a similar tendency to the changes in protein abundance. These results suggest that S-adenosylmethionine synthetase is involved in the regulation of stress response because it was changed in response to flooding and drought stresses.

The still mysterious roles of cysteine-containing glutathione transferases in plants

Glutathione transferases (GSTs) represent a widespread multigenic enzyme family able to modify a broad range of molecules. These notably include secondary metabolites and exogenous substrates often referred to as xenobiotics, usually for their detoxification, subsequent transport or export. To achieve this, these enzymes can bind non-substrate ligands (ligandin function) and/or catalyze the conjugation of glutathione onto the targeted molecules, the latter activity being exhibited by GSTs having a serine or a tyrosine as catalytic residues. Besides, other GST members possess a catalytic cysteine residue, a substitution that radically changes enzyme properties. Instead of promoting GSH-conjugation reactions, cysteine-containing GSTs (Cys-GSTs) are able to perform deglutathionylation reactions similarly to glutaredoxins but the targets are usually different since glutaredoxin substrates are mostly oxidized proteins and Cys-GST substrates are metabolites. The Cys-GSTs are found in most organisms and form several classes. While Beta and Omega GSTs and chloride intracellular channel proteins (CLICs) are not found in plants, these organisms possess microsomal ProstaGlandin E-Synthase type 2, glutathionyl hydroquinone reductases, Lambda, Iota and Hemerythrin GSTs and dehydroascorbate reductases (DHARs); the four last classes being restricted to the green lineage. In plants, whereas the role of DHARs is clearly associated to the reduction of dehydroascorbate to ascorbate, the physiological roles of other Cys-GSTs remain largely unknown. In this context, a genomic and phylogenetic analysis of Cys-GSTs in photosynthetic organisms provides an updated classification that is discussed in the light of the recent literature about the functional and structural properties of Cys-GSTs. Considering the antioxidant potencies of phenolic compounds and more generally of secondary metabolites, the connection of GSTs with secondary metabolism may be interesting from a pharmacological perspective.

Molecular and biochemical characterization of dehydroascorbate reductase from a stress adapted C4 plant, pearl millet [Pennisetum glaucum (L.) R. Br]

KEY MESSAGE

PgDHAR was isolated from Pennisetum glaucum. PgDHAR responded to abiotic stress and exhibited enzyme activity at broad ranges of temperature, pH and substrate concentrations suggesting its role in stress tolerance.

ABSTRACT

Dehydroascorbate reductase (EC 1.8.5.1) is a crucial enzyme actively involved in the recycling of ascorbate redox pool in the cellular environment. In this study, the full-length cDNA coding for DHAR polypeptide and its corresponding gene was isolated from Pennisetum glaucum (PgDHAR). PgDHAR encodes a polypeptide of 213 amino acids with a predicted molecular mass of 23.4 kDa and shares 80-75 % sequence homology with DHAR from other plants. The heterologously expressed recombinant PgDHAR protein exhibited activity in a wide range of substrate concentrations. The recombinant PgDHAR is thermostable and retains its activity over a broad pH range. Furthermore, transcript level of PgDHAR is quantitatively up-regulated in response to temperature. On the whole, PgDHAR alone or in combination with other genes of ascorbate-glutathione cycle can be used for the development of stress tolerant as well as nutritionally improved food crop with enhanced ascorbic acid content.

Proteomics analysis of alfalfa response to heat stress

The proteome responses to heat stress have not been well understood. In this study, alfalfa (Medicago sativa L. cv. Huaiyin) seedlings were exposed to 25°C (control) and 40°C (heat stress) in growth chambers, and leaves were collected at 24, 48 and 72 h after treatment, respectively. The morphological, physiological and proteomic processes were negatively affected under heat stress. Proteins were extracted and separated by two-dimensional polyacrylamide gel electrophoresis (2-DE), and differentially expressed protein spots were identified by mass spectrometry (MS). Totally, 81 differentially expressed proteins were identified successfully by MALDI-TOF/TOF. These proteins were categorized into nine classes: including metabolism, energy, protein synthesis, protein destination/storage, transporters, intracellular traffic, cell structure, signal transduction and disease/defence. Five proteins were further analyzed for mRNA levels. The results of the proteomics analyses provide a better understanding of the molecular basis of heat-stress responses in alfalfa.

Proteomic analysis of S-nitrosylated proteins in potato plant

Nitric oxide (NO) has various functions in physiological responses in plants, such as development, hormone signaling and defense. The mechanism of how NO regulates physiological responses has not been well understood. Protein S-nitrosylation, a redox-related modification of cysteine thiol by NO, is known to be one of the important post-translational modifications to regulate activity and interactions of proteins. To elucidate NO function in plants, proteomic analysis of S-nitrosylated proteins in potato (Solanum tuberosum) was performed. Detection and functional analysis of internal S-nitrosylated proteins is technically demanding because of the instability and reversibility of the protein S-nitrosylation. By using a modified biotin switch assay optimized for potato tissues, and nano liquid chromatography combined with mass spectrometry, approximately 80 S-nitrosylated candidate proteins were identified in S-nitrosoglutathione-treated potato leaves and tuber extracts. Identified proteins included redox-related enzymes, defense-related proteins and metabolic enzymes. Some of identified proteins were synthesized in Escherichia coli, and S-nitrosylation of recombinant proteins was confirmed in vitro. Dehydroascorbate reductase 1 (DHAR1, EC 1.8.5.1), one of the identified S-nitrosylated target proteins, showed glutathione-dependent dehydroascorbate-reducing activity. Either point mutation in a target cysteine of S-nitrosylation or treatment with an NO donor, S-nitroso-L-cysteine, significantly reduced the activity of DHAR1, indicating that DHAR1 is negatively regulated by S-nitrosylation of the cysteine residue essential for the enzymatic activity. These results show that the modified method developed in this study can be used to identify proteins regulated by S-nitrosylation in potato tissues.

Cloning, molecular characterization and heterologous expression of a glutathione S-transferase gene in rice

OsGSTL2 is one of three tandem-arranged glutathione S-transferase, lambda class genes in chromosome 3 of rice (Oryza sativa L.). It includes 9 introns and 10 exons, and encodes a protein of 244 amino acid residues with a calculated molecular mass of 27.37 kDa. The predicted three-dimensional structure of OsGSTL2 showed a typical glutathione S-transferase fold. Using semi-quantitative RT-PCR analysis, OsGSTL2 transcript was detected in the roots and leaves of seedling stage and tillering stage, and the roots, leaves and panicles of heading stage from rice plants, and the expression level of OsGSTL2 mRNA in rice roots show significant change under chlorsulfuron stress. The OsGSTL2 gene was cloned into pYTV vector and was transformed into yeast strain PEP4. Western blot analysis showed the exogenous OsGSTL2 was expressed in transformed yeast. GST activity of crude extracts of yeast showed the OsGSTL2 transgenic yeast had higher levels of GST activities than control yeasts. These findings suggested that the OsGSTL2 is a glutathione S-transferase and has potential use in detoxification.