An atypical catalytic mechanism involving three cysteines of thioredoxin

Genome-wide analysis and transcriptional regulation of the typical and atypical thioredoxins in Arabidopsis thaliana

Thioredoxins (TRXs), a large subclass of ubiquitous oxidoreductases, are involved in thiol redox regulation. Here, we performed a comprehensive analysis of TRXs in the Arabidopsis thaliana genome, revealing 41 genes encoding 18 typical and 23 atypical TRXs, and 6 genes encoding thioredoxin reductases (TRs). The high number of atypical TRXs indicates special functions in plants that mostly await elucidation. We identified an atypical class of thioredoxins called TRX-c in the genomes of photosynthetic eukaryotes. Localized to the chloroplast, TRX-c displays atypical CPLC, CHLC and CNLC motifs in the active sites. In silico analysis of the transcriptional regulations of TRXs revealed high expression of TRX-c in leaves and strong regulation under cold, osmotic, salinity and metal ion stresses.

Genome-wide analysis and expression profiling of H-type Trx family in Phaseolus vulgaris revealed distinctive isoforms associated with symbiotic N2-fixing performance and abiotic stress response

Thioredoxins (Trxs) are implicated in plant development and stress tolerance through redox regulation of target proteins. Trxs of Type h (Trxhs) constitute the largest and the most complicated cluster in the Trx family because of their unknown individual functions. Here, we identified and characterized the Phaseolus vulgaris Trxh family during development, mutualistic interactions and in response to abiotic stress. P. vulgaris (common bean) Trxh gene family (PvTrxh) encompasses 12 isoforms (PvTrxh1-h12), subdivided into 3 groups according to their amino acid sequence features. In silico RNA-seq -based expression analysis showed a differential expression of PvTrxh genes during development. RT-qPCR analysis of PvTrxh genes during nodule organogenesis revealed their highest expression in the nodule primordium (NP). Interestingly, in response to symbiosis, specific PvTrxh isoforms (PvTrxh3 and h5) were found to be highly upregulated compared to mock-inoculated plants. In addition, their expression patterns in the NP positively correlated with the symbiotic N₂-fixing efficiency of the Rhizobium strain, as revealed by a number of symbiotic efficiency parameters (ARA, leghemoglobin content, biomass, and total soluble proteins), concomitantly with increased amounts of hydrogen peroxide (H₂O₂). On the other hand, distinctive PvTrxh isoforms were found to be upregulated in plant leaves, where H₂O₂ amounts were elevated, in response to both salt and drought constraints. When exogenously applied, H₂O₂ upregulated specific PvTrxh isoforms in plant leaves and roots. These findings point to a specific, rather than redundant, function for Trxh proteins in common bean beside the association of distinctive Trxh isoforms with symbiosis and abiotic stress response.

Single-molecule enzymatic reaction dynamics and mechanisms of GPX3 and TRXh9 from Arabidopsis thaliana

Glutathione peroxidases (GPXs) regulate the levels of reactive oxygen species in cells and tissues. During the redox cycling, the plant GPX is regenerated by thioredoxins (TRXs) as reductant rather than glutathione as the electron donor. However, the direct experimental observation on the interaction dynamics between GPXs and TRXs has not been reported, and the redox mechanism is unclear. In this work, the protein interactions between oxidized AtGPX3 and reduced AtTRXh9 have been studied using single-molecule fluorescence resonance energy transfer (smFRET). The obtained results indicate there are four processes in these two protein interaction, including biological recognition, binding, intermediate and unbinding state. Two enzymatic reaction intermediate states have been identified in the dissociation of AtGPX3-AtTRXh9 complex from binding to unbinding state, suggesting two types of interaction pathways and intermediate complexes. In particular, the dynamical study reveals that the redox reaction between oxidized AtGPX3 and reduced AtTRXh9 is realized through the forming and breaking of disulfide bonds via the active sites of Cys4 and Cys57 in AtTRXh9. These findings are of significant for deep understanding the redox reaction and mechanism between GPXs and TRXs enzymes, and studying other protein dynamics at single-molecule level.

Redox Homeostasis in Photosynthetic Organisms: Novel and Established Thiol-Based Molecular Mechanisms

Significance: Redox homeostasis consists of an intricate network of reactions in which reactive molecular species, redox modifications, and redox proteins act in concert to allow both physiological responses and adaptation to stress conditions. Recent Advances: This review highlights established and novel thiol-based regulatory pathways underlying the functional facets and significance of redox biology in photosynthetic organisms. In the last decades, the field of redox regulation has largely expanded and this work is aimed at giving the right credit to the importance of thiol-based regulatory and signaling mechanisms in plants. Critical Issues: This cannot be all-encompassing, but is intended to provide a comprehensive overview on the structural/molecular mechanisms governing the most relevant thiol switching modifications with emphasis on the large genetic and functional diversity of redox controllers (i.e., redoxins). We also summarize the different proteomic-based approaches aimed at investigating the dynamics of redox modifications and the recent evidence that extends the possibility to monitor the cellular redox state in vivo. The physiological relevance of redox transitions is discussed based on reverse genetic studies confirming the importance of redox homeostasis in plant growth, development, and stress responses. Future Directions: In conclusion, we can firmly assume that redox biology has acquired an established significance that virtually infiltrates all aspects of plant physiology.

Structural and Biochemical Insights into the Reactivity of Thioredoxin h1 from Chlamydomonas reinhardtii

Thioredoxins (TRXs) are major protein disulfide reductases of the cell. Their redox activity relies on a conserved Trp-Cys-(Gly/Pro)-Pro-Cys active site bearing two cysteine (Cys) residues that can be found either as free thiols (reduced TRXs) or linked together by a disulfide bond (oxidized TRXs) during the catalytic cycle. Their reactivity is crucial for TRX activity, and depends on the active site microenvironment. Here, we solved and compared the 3D structure of reduced and oxidized TRX h1 from Chlamydomonas reinhardtii (CrTRXh1). The three-dimensional structure was also determined for mutants of each active site Cys. Structural alignments of CrTRXh1 with other structurally solved plant TRXs showed a common spatial fold, despite the low sequence identity. Structural analyses of CrTRXh1 revealed that the protein adopts an identical conformation independently from its redox state. Treatment with iodoacetamide (IAM), a Cys alkylating agent, resulted in a rapid and pH-dependent inactivation of CrTRXh1. Starting from fully reduced CrTRXh1, we determined the acid dissociation constant (pK_a) of each active site Cys by Matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry analyses coupled to differential IAM-based alkylation. Based on the diversity of catalytic Cys deprotonation states, the mechanisms and structural features underlying disulfide redox activity are discussed.

The thioredoxin-mediated recycling of Arabidopsis thaliana GRXS16 relies on a conserved C-terminal cysteine

BACKGROUND

Glutaredoxins (GRXs) are oxidoreductases involved in diverse cellular processes through their capacity to reduce glutathionylated proteins and/or to coordinate iron‑sulfur (Fe-S) clusters. Among class II GRXs, the plant-specific GRXS16 is a bimodular protein formed by an N-terminal endonuclease domain fused to a GRX domain containing a ¹⁵⁸CGFS signature.

METHODS

The biochemical properties (redox activity, sensitivity to oxidation, pK_a of cysteine residues, midpoint redox potential) of Arabidopsis thaliana GRXS16 were investigated by coupling oxidative treatments to alkylation shift assays, activity measurements and mass spectrometry analyses.

RESULTS

Activity measurements using redox-sensitive GFP2 (roGFP2) as target protein did not reveal any significant glutathione-dependent reductase activity of A. thaliana GRXS16 whereas it was able to catalyze the oxidation of roGFP2 in the presence of glutathione disulfide. Accordingly, Arabidopsis GRXS16 reacted efficiently with oxidized forms of glutathione, leading to the formation of an intramolecular disulfide between Cys¹⁵⁸ and the semi-conserved Cys²¹⁵, which has a midpoint redox potential of - 298 mV at pH 7.0 and is reduced by plastidial thioredoxins (TRXs) but not GSH. By promoting the formation of this disulfide, Cys²¹⁵ modulates GRXS16 oxidoreductase activity.

CONCLUSION

The reduction of AtGRXS16, which is mandatory for its oxidoreductase activity and the binding of Fe-S clusters, depends on light through the plastidial FTR/TRX system. Hence, disulfide formation may constitute a redox switch mechanism controlling GRXS16 function in response to day/night transition or oxidizing conditions.

GENERAL SIGNIFICANCE

From the in vitro data obtained with roGFP2, one can postulate that GRXS16 would mediate protein glutathionylation/oxidation in plastids but not their deglutathionylation.

Structural snapshots along the reaction mechanism of the atypical poplar thioredoxin-like2.1

Plastidial thioredoxin (TRX)-like2.1 proteins are atypical thioredoxins possessing a WCRKC active site signature and using glutathione for recycling. To obtain structural information supporting the peculiar catalytic mechanisms and target proteins of these TRXs, we solved the crystal structures of poplar TRX-like2.1 in oxidized and reduced states and of mutated variants. These structures share similar folding with TRXs exhibiting the canonical WCGPC signature. Moreover, the overall conformation is not altered by reduction of the catalytic disulfide bond or in a C45S/C67S variant that formed a disulfide-bridged dimer possibly mimicking reaction intermediates with target proteins. Modeling of the interaction of TRX-like2.1 with both NADPH- and ferredoxin-thioredoxin reductases (FTR) indicates that the presence of Arg43 and Lys44 residues likely precludes reduction by the plastidial FTR.

Fats and function: protein lipid modifications in plant cell signalling

The post-translational lipid modifications N-myristoylation, prenylation and S-acylation are traditionally associated with increasing protein membrane affinity and localisation. However this is an over-simplification, with evidence now implicating these modifications in a variety of roles such as membrane microdomain partitioning, protein trafficking, protein complex assembly and polarity maintenance. Evidence for a regulatory role is also emerging, with changes or manipulation of lipid modifications offering a means of directly controlling various aspects of protein function. Proteomics advances have revealed an enrichment of signalling proteins in the lipid-modified proteome, potentially indicating an important role for these modifications in responding to stimuli. This review highlights some of the key themes and possible functions of lipid modification during signalling processes in plants.

Dithiol disulphide exchange in redox regulation of chloroplast enzymes in response to evolutionary and structural constraints

Redox regulation of chloroplast enzymes via disulphide reduction is believed to control the rates of CO₂ fixation. The study of the thioredoxin reduction pathways and of various target enzymes lead to the following guidelines.

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.

Seed thioredoxin h

Thioredoxins are nearly ubiquitous disulfide reductases involved in a wide range of biochemical pathways in various biological systems, and also implicated in numerous biotechnological applications. Plants uniquely synthesize an array of thioredoxins targeted to different cell compartments, for example chloroplastic f- and m-type thioredoxins involved in regulation of the Calvin-Benson cycle. The cytosolic h-type thioredoxins act as key regulators of seed germination and are recycled by NADPH-dependent thioredoxin reductase. The present review on thioredoxin h systems in plant seeds focuses on occurrence, reaction mechanisms, specificity, target protein identification, three-dimensional structure and various applications. The aim is to provide a general background as well as an update covering the most recent findings. This article is part of a Special Issue entitled: Plant Proteomics--a bridge between fundamental processes and crop production, edited by Dr. Hans-Peter Mock.

Involvement of thiol-based mechanisms in plant development

BACKGROUND

Increasing knowledge has been recently gained regarding the redox regulation of plant developmental stages.

SCOPE OF VIEW

The current state of knowledge concerning the involvement of glutathione, glutaredoxins and thioredoxins in plant development is reviewed.

MAJOR CONCLUSIONS

The control of the thiol redox status is mainly ensured by glutathione (GSH), a cysteine-containing tripeptide and by reductases sharing redox-active cysteines, glutaredoxins (GRXs) and thioredoxins (TRXs). Indeed, thiol groups present in many regulatory proteins and metabolic enzymes are prone to oxidation, ultimately leading to post-translational modifications such as disulfide bond formation or glutathionylation. This review focuses on the involvement of GSH, GRXs and TRXs in plant development. Recent studies showed that the proper functioning of root and shoot apical meristems depends on glutathione content and redox status, which regulate, among others, cell cycle and hormone-related processes. A critical role of GRXs in the formation of floral organs has been uncovered, likely through the redox regulation of TGA transcription factor activity. TRXs fulfill many functions in plant development via the regulation of embryo formation, the control of cell-to-cell communication, the mobilization of seed reserves, the biogenesis of chloroplastic structures, the metabolism of carbon and the maintenance of cell redox homeostasis. This review also highlights the tight relationships between thiols, hormones and carbon metabolism, allowing a proper development of plants in relation with the varying environment and the energy availability.

GENERAL SIGNIFICANCE

GSH, GRXs and TRXs play key roles during the whole plant developmental cycle via their antioxidant functions and the redox-regulation of signaling pathways. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

Thioredoxin h isoforms from rice are differentially reduced by NADPH/thioredoxin or GSH/glutaredoxin systems

Rice (Oryza sativa L.) has multiple potential genes encoding thioredoxin (Trx) h and NADP-thioredoxin reductase (NTR). These NTR and Trx h isoforms, known as cytoplasmic NTR/Trx system along with multiple members of glutaredoxin (Grx) family constitute a complex redox control system in rice. In the present study, we investigated the kinetic parameters of two rice NTRs, OsNTRA and OsNTRB, toward three endogenous Trx h isoforms, OsTrx1, OsTrx20, and OsTrx23. The results showed that in contrast with OsTrx1 and OsTrx23, the isoform OsTrx20 was not reduced by OsNTR isoforms. The kcat/Km values of OsNTRB and OsNTRA toward OsTrx1 was six- and 13-fold higher than those values toward OsTrx23, respectively, suggesting that OsNTR isoforms do not reduce different OsTrx h isoforms, equivalently. Furthermore, the possible reduction of OsTrx isoforms by the glutathione (GSH)/Grx system was investigated through the heterologous expression of a gene encoding OsGrx9, a bicysteinic CPYC Grx found in rice. Whereas OsTrx23 was not reduced by GSH, OsTrx20 and with less efficiently OsTrx1 were reduced by GSH or GSH/Grx. Therefore, it seems that OsTrx1 can be reduced either by OsNTR or GSH/Grx. These data for the first time provides an evidence for cross-talking between NTR/Trx and GSH/Grx systems in rice.

NTR/NRX define a new thioredoxin system in the nucleus of Arabidopsis thaliana cells

Thioredoxins (TRX) are key components of cellular redox balance, regulating many target proteins through thiol/disulfide exchange reactions. In higher plants, TRX constitute a complex multigenic family whose members have been found in almost all cellular compartments. Although chloroplastic and cytosolic TRX systems have been largely studied, the presence of a nuclear TRX system has been elusive for a long time. Nucleoredoxins (NRX) are potential nuclear TRX found in most eukaryotic organisms. In contrast to mammals, which harbor a unique NRX, angiosperms generally possess multiple NRX organized in three subfamilies. Here, we show that Arabidopsis thaliana has two NRX genes (AtNRX1 and AtNRX2), respectively, belonging to subgroups I and III. While NRX1 harbors typical TRX active sites (WCG/PPC), NRX2 has atypical active sites (WCRPC and WCPPF). Nevertheless, both NRX1 and NRX2 have disulfide reduction capacities, although NRX1 alone can be reduced by the thioredoxin reductase NTRA. We also show that both NRX1 and NRX2 have a dual nuclear/cytosolic localization. Interestingly, we found that NTRA, previously identified as a cytosolic protein, is also partially localized in the nucleus, suggesting that a complete TRX system is functional in the nucleus. We show that NRX1 is mainly found as a dimer in vivo. nrx1 and nrx2 knockout mutant plants exhibit no phenotypic perturbations under standard growth conditions. However, the nrx1 mutant shows a reduced pollen fertility phenotype, suggesting a specific role of NRX1 at the haploid phase.

Thioredoxin 1 is inactivated due to oxidation induced by peroxiredoxin under oxidative stress and reactivated by the glutaredoxin system

The mammalian cytosolic thioredoxin system, comprising thioredoxin (Trx), Trx reductase, and NADPH, is the major protein-disulfide reductase of the cell and has numerous functions. Besides the active site thiols, human Trx1 contains three non-active site cysteine residues at positions 62, 69, and 73. A two-disulfide form of Trx1, containing an active site disulfide between Cys-32 and Cys-35 and a non-active site disulfide between Cys-62 and Cys-69, is inactive either as a disulfide reductase or as a substrate for Trx reductase. This could possibly provide a structural switch affecting Trx1 function during oxidative stress and redox signaling. We found that two-disulfide Trx1 was generated in A549 cells under oxidative stress. In vitro data showed that two-disulfide Trx1 was generated from oxidation of Trx1 catalyzed by peroxiredoxin 1 in the presence of H2O2. The redox Western blot data indicated that the glutaredoxin system protected Trx1 in HeLa cells from oxidation caused by ebselen, a superfast oxidant for Trx1. Our results also showed that physiological concentrations of glutathione, NADPH, and glutathione reductase reduced the non-active site disulfide in vitro. This reaction was stimulated by glutaredoxin 1 via the so-called monothiol mechanism. In conclusion, reversible oxidation of the non-active site disulfide of Trx1 is suggested to play an important role in redox regulation and cell signaling via temporal inhibition of its protein-disulfide reductase activity for the transmission of oxidative signals under oxidative stress.

Thioredoxin and glutaredoxin systems in plants: molecular mechanisms, crosstalks, and functional significance

Thioredoxins (Trx) and glutaredoxins (Grx) constitute families of thiol oxidoreductases. Our knowledge of Trx and Grx in plants has dramatically increased during the last decade. The release of the Arabidopsis genome sequence revealed an unexpectedly high number of Trx and Grx genes. The availability of several genomes of vascular and nonvascular plants allowed the establishment of a clear classification of the genes and the chronology of their appearance during plant evolution. Proteomic approaches have been developed that identified the putative Trx and Grx target proteins which are implicated in all aspects of plant growth, including basal metabolism, iron/sulfur cluster formation, development, adaptation to the environment, and stress responses. Analyses of the biochemical characteristics of specific Trx and Grx point to a strong specificity toward some target enzymes, particularly within plastidial Trx and Grx. In apparent contradiction with this specificity, genetic approaches show an absence of phenotype for most available Trx and Grx mutants, suggesting that redundancies also exist between Trx and Grx members. Despite this, the isolation of mutants inactivated in multiple genes and several genetic screens allowed the demonstration of the involvement of Trx and Grx in pathogen response, phytohormone pathways, and at several control points of plant development. Cytosolic Trxs are reduced by NADPH-thioredoxin reductase (NTR), while the reduction of Grx depends on reduced glutathione (GSH). Interestingly, recent development integrating biochemical analysis, proteomic data, and genetics have revealed an extensive crosstalk between the cytosolic NTR/Trx and GSH/Grx systems. This crosstalk, which occurs at multiple levels, reveals the high plasticity of the redox systems in plants.

New substrates and activity of Phanerochaete chrysosporium Omega glutathione transferases

Omega glutathione transferases (GSTO) constitute a family of proteins with variable distribution throughout living organisms. It is notably expanded in several fungi and particularly in the wood-degrading fungus Phanerochaete chrysosporium, raising questions concerning the function(s) and potential redundancy of these enzymes. Within the fungal families, GSTOs have been poorly studied and their functions remain rather sketchy. In this study, we have used fluorescent compounds as activity reporters to identify putative ligands. Experiments using 5-chloromethylfluorescein diacetate as a tool combined with mass analyses showed that GSTOs are able to cleave ester bonds. Using this property, we developed a specific activity-based profiling method for identifying ligands of PcGSTO3 and PcGSTO4. The results suggest that GSTOs could be involved in the catabolism of toxic compounds like tetralone derivatives. Biochemical investigations demonstrated that these enzymes are able to catalyze deglutathionylation reactions thanks to the presence of a catalytic cysteine residue. To access the physiological function of these enzymes and notably during the wood interaction, recombinant proteins have been immobilized on CNBr Sepharose and challenged with beech wood extracts. Coupled with GC-MS experiments this ligand fishing method allowed to identify terpenes as potential substrates of Omega GST suggesting a physiological role during the wood-fungus interactions.

Characterization of a Phanerochaete chrysosporium glutathione transferase reveals a novel structural and functional class with ligandin properties

Glutathione S-transferases (GSTs) form a superfamily of multifunctional proteins with essential roles in cellular detoxification processes. A new fungal specific class of GST has been highlighted by genomic approaches. The biochemical and structural characterization of one isoform of this class in Phanerochaete chrysosporium revealed original properties. The three-dimensional structure showed a new dimerization mode and specific features by comparison with the canonical GST structure. An additional β-hairpin motif in the N-terminal domain prevents the formation of the regular GST dimer and acts as a lid, which closes upon glutathione binding. Moreover, this isoform is the first described GST that contains all secondary structural elements, including helix α4' in the C-terminal domain, of the presumed common ancestor of cytosolic GSTs (i.e. glutaredoxin 2). A sulfate binding site has been identified close to the glutathione binding site and allows the binding of 8-anilino-1-naphtalene sulfonic acid. Competition experiments between 8-anilino-1-naphtalene sulfonic acid, which has fluorescent properties, and various molecules showed that this GST binds glutathionylated and sulfated compounds but also wood extractive molecules, such as vanillin, chloronitrobenzoic acid, hydroxyacetophenone, catechins, and aldehydes, in the glutathione pocket. This enzyme could thus function as a classical GST through the addition of glutathione mainly to phenethyl isothiocyanate, but alternatively and in a competitive way, it could also act as a ligandin of wood extractive compounds. These new structural and functional properties lead us to propose that this GST belongs to a new class that we name GSTFuA, for fungal specific GST class A.

Atypical thioredoxins in poplar: the glutathione-dependent thioredoxin-like 2.1 supports the activity of target enzymes possessing a single redox active cysteine

Plant thioredoxins (Trxs) constitute a complex family of thiol oxidoreductases generally sharing a WCGPC active site sequence. Some recently identified plant Trxs (Clot, Trx-like1 and -2, Trx-lilium1, -2, and -3) display atypical active site sequences with altered residues between the two conserved cysteines. The transcript expression patterns, subcellular localizations, and biochemical properties of some representative poplar (Populus spp.) isoforms were investigated. Measurements of transcript levels for the 10 members in poplar organs indicate that most genes are constitutively expressed. Using transient expression of green fluorescent protein fusions, Clot and Trx-like1 were found to be mainly cytosolic, whereas Trx-like2.1 was located in plastids. All soluble recombinant proteins, except Clot, exhibited insulin reductase activity, although with variable efficiencies. Whereas Trx-like2.1 and Trx-lilium2.2 were efficiently regenerated both by NADPH-Trx reductase and glutathione, none of the proteins were reduced by the ferredoxin-Trx reductase. Only Trx-like2.1 supports the activity of plastidial thiol peroxidases and methionine sulfoxide reductases employing a single cysteine residue for catalysis and using a glutathione recycling system. The second active site cysteine of Trx-like2.1 is dispensable for this reaction, indicating that the protein possesses a glutaredoxin-like activity. Interestingly, the Trx-like2.1 active site replacement, from WCRKC to WCGPC, suppresses its capacity to use glutathione as a reductant but is sufficient to allow the regeneration of target proteins employing two cysteines for catalysis, indicating that the nature of the residues composing the active site sequence is crucial for substrate selectivity/recognition. This study provides another example of the cross talk existing between the glutathione/glutaredoxin and Trx-dependent pathways.

Glutathione transferases of Phanerochaete chrysosporium: S-glutathionyl-p-hydroquinone reductase belongs to a new structural class

The white rot fungus Phanerochaete chrysosporium, a saprophytic basidiomycete, possesses a large number of cytosolic glutathione transferases, eight of them showing similarity to the Omega class. PcGSTO1 (subclass I, the bacterial homologs of which were recently proposed, based on their enzymatic function, to constitute a new class of glutathione transferase named S-glutathionyl-(chloro)hydroquinone reductases) and PcGSTO3 (subclass II related to mammalian homologs) have been investigated in this study. Biochemical investigations demonstrate that both enzymes are able to catalyze deglutathionylation reactions thanks to the presence of a catalytic cysteinyl residue. This reaction leads to the formation of a disulfide bridge between the conserved cysteine and the removed glutathione from their substrate. The substrate specificity of each isoform differs. In particular PcGSTO1, in contrast to PcGSTO3, was found to catalyze deglutathionylation of S-glutathionyl-p-hydroquinone substrates. The three-dimensional structure of PcGSTO1 presented here confirms the hypothesis that it belongs not only to a new biological class but also to a new structural class that we propose to name GST xi. Indeed, it shows specific features, the most striking ones being a new dimerization mode and a catalytic site that is buried due to the presence of long loops and that contains the catalytic cysteine.

Identification and characterization of thioredoxin h isoforms differentially expressed in germinating seeds of the model legume Medicago truncatula

Thioredoxins (Trxs) h, small disulfide reductases, and NADP-thioredoxin reductases (NTRs) have been shown to accumulate in seeds of different plant species and play important roles in seed physiology. However, little is known about the identity, properties, and subcellular location of Trx h isoforms that are abundant in legume seeds. To fill this gap, in this work, we characterized the Trx h family of Medicago truncatula, a model legume, and then explored the activity and localization of Trx h isoforms accumulating in seeds. Twelve Trx h isoforms were identified in M. truncatula. They belong to the groups previously described: h1 to h3 (group I), h4 to h7 (group II), and h8 to h12 (group III). Isoforms of groups I and II were found to be reduced by M. truncatula NTRA, but with different efficiencies, Trxs of group II being more efficiently reduced than Trxs of group I. In contrast, their insulin disulfide-reducing activity varies greatly and independently of the group to which they belong. Furthermore, Trxs h1, h2, and h6 were found to be present in dry and germinating seeds. Trxs h1 and, to a lesser extent, h2 are abundant in both embryonic axes and cotyledons, while Trx h6 is mainly present in cotyledons. Thus, M. truncatula seeds contain distinct isoforms of Trx h that differ in spatial distribution and kinetic properties, suggesting that they play different roles. Because we show that Trx h6 is targeted to the tonoplast, the possible role of this isoform during germination is finally discussed.

Redox regulation of auxin signaling and plant development in Arabidopsis

Thioredoxin (NTR/TRX) and glutathione (GSH/GRX) are the two major systems which play a key role in the maintenance of cellular redox homeostasis. They are essential for plant development, cell division or the response to environmental stresses. In a recent article, we studied the interplay between the NADP-linked thioredoxin and glutathione systems in auxin signaling genetically, by associating TRX reductase (ntra ntrb) and glutathione biosynthesis (cad2) mutations. We show that these two thiol reduction pathways interfere with developmental processes. This occurs through modulation of auxin activity as shown by genetic analyses of loss of function mutations in a triple ntra ntrb cad2 mutant. The triple mutant develops almost normally at the rosette stage but fails to generate lateral organs from the inflorescence meristem, producing almost naked stems that are reminiscent of mutants affected in PAT (polar auxin transport) or biosynthesis. The triple mutant exhibits other defects in processes regulated by auxin, including a loss of apical dominance, vasculature defects and reduced secondary root production. Furthermore, it has lower auxin (IAA) levels and decreased capacity for PAT, suggesting that the NTR and glutathione pathways influence inflorescence meristem development through regulation of auxin transport and metabolism.

Structure, function, and mechanism of thioredoxin proteins

Thioredoxins are ubiquitous antioxidant enzymes that play important roles in many health-related cellular processes. As such, the fundamental knowledge of how these enzymes work is of prime importance for understanding cellular redox mechanisms and for laying the ground for the development of future therapeutic approaches. Over the past 40 years, a really impressive amount of data has been published on thioredoxins. Here, we review the most significant results that have contributed to our knowledge regarding the structure, the function, and the mechanism of these crucial enzymes.

A membrane-associated thioredoxin required for plant growth moves from cell to cell, suggestive of a role in intercellular communication

Thioredoxins (Trxs) are small ubiquitous regulatory disulfide proteins. Plants have an unusually complex complement of Trxs composed of six well-defined types (Trxs f, m, x, y, h, and o) that reside in different cell compartments and function in an array of processes. The extraplastidic h type consists of multiple members that in general have resisted isolation of a specific phenotype. In analyzing mutant lines in Arabidopsis thaliana, we identified a phenotype of dwarf plants with short roots and small yellowish leaves for AtTrx h9 (henceforth, Trx h9), a member of the Arabidopsis Trx h family. Trx h9 was found to be associated with the plasma membrane and to move from cell to cell. Controls conducted in conjunction with the localization of Trx h9 uncovered another h-type Trx in mitochondria (Trx h2) and a Trx in plastids earlier described as a cytosolic form in tomato. Analysis of Trx h9 revealed a 17-amino acid N-terminal extension in which the second Gly (Gly(2)) and fourth cysteine (Cys(4)) were highly conserved. Mutagenesis experiments demonstrated that Gly(2) was required for membrane binding, possibly via myristoylation. Both Gly(2) and Cys(4) were needed for movement, the latter seemingly for protein structure and palmitoylation. A three-dimensional model was consistent with these predictions as well as with earlier evidence showing that a poplar ortholog is reduced by a glutaredoxin rather than NADP-thioredoxin reductase. In demonstrating the membrane location and intercellular mobility of Trx h9, the present results extend the known boundaries of Trx and suggest a role in cell-to-cell communication.

Interplay between the NADP-linked thioredoxin and glutathione systems in Arabidopsis auxin signaling

Intracellular redox status is a critical parameter determining plant development in response to biotic and abiotic stress. Thioredoxin (TRX) and glutathione are key regulators of redox homeostasis, and the TRX and glutathione pathways are essential for postembryonic meristematic activities. Here, we show by associating TRX reductases (ntra ntrb) and glutathione biosynthesis (cad2) mutations that these two thiol reduction pathways interfere with developmental processes through modulation of auxin signaling. The triple ntra ntrb cad2 mutant develops normally at the rosette stage, undergoes the floral transition, but produces almost naked stems, reminiscent of the phenotype of several mutants affected in auxin transport or biosynthesis. In addition, the ntra ntrb cad2 mutant shows a loss of apical dominance, vasculature defects, and reduced secondary root production, several phenotypes tightly regulated by auxin. We further show that auxin transport capacities and auxin levels are perturbed in the mutant, suggesting that the NTR-glutathione pathways alter both auxin transport and metabolism. Analysis of ntr and glutathione biosynthesis mutants suggests that glutathione homeostasis plays a major role in auxin transport as both NTR and glutathione pathways are involved in auxin homeostasis.

Thioredoxins and glutaredoxins: unifying elements in redox biology

Since their discovery as a substrate for ribonucleotide reductase (RNR), the role of thioredoxin (Trx) and glutaredoxin (Grx) has been largely extended through their regulatory function. Both proteins act by changing the structure and activity of a broad spectrum of target proteins, typically by modifying redox status. Trx and Grx are members of families with multiple and partially redundant genes. The number of genes clearly increased with the appearance of multicellular organisms, in part because of new types of Trx and Grx with orthologs throughout the animal and plant kingdoms. The function of Trx and Grx also broadened as cells achieved increased complexity, especially in the regulation arena. In view of these progressive changes, the ubiquitous distribution of Trx and the wide occurrence of Grx enable these proteins to serve as indicators of the evolutionary history of redox regulation. In so doing, they add a unifying element that links the diverse forms of life to one another in an uninterrupted continuum. It is anticipated that future research will embellish this continuum and further elucidate the properties of these proteins and their impact on biology. The new information will be important not only to our understanding of the role of Trx and Grx in fundamental cell processes but also to future societal benefits as the proteins find new applications in a range of fields.

Regulation by glutathionylation of isocitrate lyase from Chlamydomonas reinhardtii

Post-translational modification of protein cysteine residues is emerging as an important regulatory and signaling mechanism. We have identified numerous putative targets of redox regulation in the unicellular green alga Chlamydomonas reinhardtii. One enzyme, isocitrate lyase (ICL), was identified both as a putative thioredoxin target and as an S-thiolated protein in vivo. ICL is a key enzyme of the glyoxylate cycle that allows growth on acetate as a sole source of carbon. The aim of the present study was to clarify the molecular mechanism of the redox regulation of Chlamydomonas ICL using a combination of biochemical and biophysical methods. The results clearly show that purified C. reinhardtii ICL can be inactivated by glutathionylation and reactivated by glutaredoxin, whereas thioredoxin does not appear to regulate ICL activity, and no inter- or intramolecular disulfide bond could be formed under any of the conditions tested. Glutathionylation of the protein was investigated by mass spectrometry analysis, Western blotting, and site-directed mutagenesis. The enzyme was found to be protected from irreversible oxidative inactivation by glutathionylation of its catalytic Cys(178), whereas a second residue, Cys(247), becomes artifactually glutathionylated after prolonged incubation with GSSG. The possible functional significance of this post-translational modification of ICL in Chlamydomonas and other organisms is discussed.

Structural and evolutionary aspects of thioredoxin reductases in photosynthetic organisms

Thioredoxins (Trxs) are small oxidoreductases that are involved in redox homeostasis and are found in large numbers in the subcellular compartments of eukaryotic plant cells, including the chloroplasts. Also present in chloroplasts are two forms of thioredoxin reductase (TR), which use either NADPH or ferredoxin as an electron donor. In other compartments, two additional TR forms also use NADPH: one is distributed in all photosynthetic organisms and is similar to prokaryotic enzymes, whereas the other is restricted to algae and is similar to mammalian selenoproteins. Here, we review current knowledge of the different forms of TRs across organisms and discuss the possible evolutionary fate of this class of enzymes, which provide an example of convergent functional evolution.

Comparative genomic study of the thioredoxin family in photosynthetic organisms with emphasis on Populus trichocarpa

The recent genome sequencing of Populus trichocarpa and Vitis vinifera, two models of woody plants, of Sorghum bicolor, a model of monocot using C4 metabolism, and of the moss Physcomitrella patens, together with the availability of photosynthetic organism genomes allows performance of a comparative genomic study with organisms having different ways of life, reproduction modes, biological traits, and physiologies. Thioredoxins (Trxs) are small ubiquitous proteins involved in the reduction of disulfide bridges in a variety of target enzymes present in all sub-cellular compartments and involved in many biochemical reactions. The genes coding for these enzymes have been identified in these newly sequenced genomes and annotated. The gene content, organization and distribution were compared to other photosynthetic organisms, leading to a refined classification. This analysis revealed that higher plants and bryophytes have a more complex family compared to algae and cyanobacteria and to non-photosynthetic organisms, since poplar exhibits 49 genes coding for typical and atypical thioredoxins and thioredoxin reductases, namely one-third more than monocots such as Oryza sativa and S. bicolor. The higher number of Trxs in poplar is partially explained by gene duplication in the Trx m, h, and nucleoredoxin classes. Particular attention was paid to poplar genes with emphasis on Trx-like classes called Clot, thioredoxin-like, thioredoxins of the lilium type and nucleoredoxins, which were not described in depth in previous genomic studies.