Glutathionylation in the photosynthetic model organism Chlamydomonas reinhardtii: a proteomic survey

The glyceraldehyde-3-phosphate dehydrogenase GapDH of Corynebacterium diphtheriae is redox-controlled by protein S-mycothiolation under oxidative stress

Mycothiol (MSH) is the major low molecular weight (LMW) thiol in Actinomycetes and functions in post-translational thiol-modification by protein S-mycothiolation as emerging thiol-protection and redox-regulatory mechanism. Here, we have used shotgun-proteomics to identify 26 S-mycothiolated proteins in the pathogen Corynebacterium diphtheriae DSM43989 under hypochlorite stress that are involved in energy metabolism, amino acid and nucleotide biosynthesis, antioxidant functions and translation. The glyceraldehyde-3-phosphate dehydrogenase (GapDH) represents the most abundant S-mycothiolated protein that was modified at its active site Cys153 in vivo. Exposure of purified GapDH to H₂O₂ and NaOCl resulted in irreversible inactivation due to overoxidation of the active site in vitro. Treatment of GapDH with H₂O₂ or NaOCl in the presence of MSH resulted in S-mycothiolation and reversible GapDH inactivation in vitro which was faster compared to the overoxidation pathway. Reactivation of S-mycothiolated GapDH could be catalyzed by both, the Trx and the Mrx1 pathways in vitro, but demycothiolation by Mrx1 was faster compared to Trx. In summary, we show here that S-mycothiolation can function in redox-regulation and protection of the GapDH active site against overoxidation in C. diphtheriae which can be reversed by both, the Mrx1 and Trx pathways.

Modulation of the specific glutathionylation of mitochondrial proteins in the yeast Saccharomyces cerevisiae under basal and stress conditions

The potential biological consequences of oxidative stress and changes in glutathione levels include the oxidation of susceptible protein thiols and reversible covalent binding of glutathione to the -SH groups of proteins by S-glutathionylation. Mitochondria are central to the response to oxidative stress and redox signaling. It is therefore crucial to explore the adaptive response to changes in thiol-dependent redox status in these organelles. We optimized the purification protocol of glutathionylated proteins in the yeast Saccharomyces cerevisiae and present a detailed proteomic analysis of the targets of protein glutathionylation in cells undergoing constitutive metabolism and after exposure to various stress conditions. This work establishes the physiological importance of the glutathionylation process in S. cerevisiae under basal conditions and provides evidence for an atypical and unexpected cellular distribution of the process between the cytosol and mitochondria. In addition, our data indicate that each oxidative condition (diamide, GSSG, H₂O₂, or the presence of iron) elicits an adaptive metabolic response affecting specific mitochondrial metabolic pathways, mainly involved in the energetic maintenance of the cells. The correlation of protein modifications with intracellular glutathione levels suggests that protein deglutathionylation may play a role in protecting mitochondria from oxidative stress. This work provides further insights into the diversity of proteins undergoing glutathionylation and the role of this post-translational modification as a regulatory process in the adaptive response of the cell.

Proteomic Characterization of Reversible Thiol Oxidations in Proteomes and Proteins

SIGNIFICANCE

Reactive oxygen species are produced during normal metabolism in cells, and their excesses have been implicated in protein damage and toxicity, as well as in the activation of signaling events. In particular, hydrogen peroxide participates in the regulation of different physiological processes as well as in the induction of antioxidant cascades, and often the redox molecular events triggering these pathways are based on reversible cysteine (Cys) oxidation. Recent Advances: Increases in peroxides can cause the accumulation of reversible Cys oxidations in proteomes, which may be either protecting thiols from irreversible oxidations or may just be reporters of future toxicity. It is also becoming clear, however, that only a few proteins, such as the bacterial OxyR or peroxidases, can suffer direct oxidation of their Cys residues by hydrogen peroxide and, therefore, may be the only true sensors initiating signaling events.

CRITICAL ISSUES

We will in this study describe some of the methodologies used to characterize at the proteome level reversible thiol oxidations, specifically those combining gel-free approaches with mass spectrometry. In the second part of this review, we will summarize some of the electrophoretic and proteomic techniques used to monitor Cys oxidation at the protein level, needed to confirm that a protein contains redox Cys involved in signaling relays, using as examples some of the best characterized redox sensors such as bacterial OxyR or yeast Tpx1/Pap1.

FUTURE DIRECTIONS

While Cys oxidations are often detected in proteomes and in specific proteins, major efforts have to be made to establish that they are physiologically relevant. Antioxid. Redox Signal. 26, 329-344.

Post-translational Modifications in Regulation of Chloroplast Function: Recent Advances

Post-translational modifications (PTMs) of proteins enable fast modulation of protein function in response to metabolic and environmental changes. Phosphorylation is known to play a major role in regulating distribution of light energy between the Photosystems (PS) I and II (state transitions) and in PSII repair cycle. In addition, thioredoxin-mediated redox regulation of Calvin cycle enzymes has been shown to determine the efficiency of carbon assimilation. Besides these well characterized modifications, recent methodological progress has enabled identification of numerous other types of PTMs in various plant compartments, including chloroplasts. To date, at least N-terminal and Lys acetylation, Lys methylation, Tyr nitration and S-nitrosylation, glutathionylation, sumoylation and glycosylation of chloroplast proteins have been described. These modifications impact DNA replication, control transcriptional efficiency, regulate translational machinery and affect metabolic activities within the chloroplast. Moreover, light reactions of photosynthesis as well as carbon assimilation are regulated at multiple levels by a number of PTMs. It is likely that future studies will reveal new metabolic pathways to be regulated by PTMs as well as detailed molecular mechanisms of PTM-mediated regulation.

Glutathionylation of Pea Chloroplast 2-Cys Prx and Mitochondrial Prx IIF Affects Their Structure and Peroxidase Activity and Sulfiredoxin Deglutathionylates Only the 2-Cys Prx

Together with thioredoxins (Trxs), plant peroxiredoxins (Prxs), and sulfiredoxins (Srxs) are involved in antioxidant defense and redox signaling, while their regulation by post-translational modifications (PTMs) is increasingly regarded as a key component for the transduction of the bioactivity of reactive oxygen and nitrogen species. Among these PTMs, S-glutathionylation is considered a protective mechanism against overoxidation, it also modulates protein activity and allows signaling. This study explores the glutathionylation of recombinant chloroplastic 2-Cys Prx and mitochondrial Prx IIF from Pisum sativum. Glutathionylation of the decameric form of 2-Cys Prx produced a change in the elution volume after FPLC chromatography and converted it to its dimeric glutathionylated form, while Prx IIF in its reduced dimeric form was glutathionylated without changing its oligomeric state. Mass spectrometry demonstrated that oxidized glutathione (GSSG) can glutathionylate resolving cysteine (Cys¹⁷⁴), but not the peroxidatic equivalent (Cys⁵²), in 2-Cys Prx. In contrast, GSSG was able to glutathionylate both peroxidatic (Cys⁵⁹) and resolving (Cys⁸⁴) cysteine in Prx IIF. Glutathionylation was seen to be dependent on the GSH/GSSG ratio, although the exact effect on the 2-Cys Prx and Prx IIF proteins differed. However, the glutathionylation provoked a similar decrease in the peroxidase activity of both peroxiredoxins. Despite growing evidence of the importance of post-translational modifications, little is known about the enzymatic systems that specifically regulate the reversal of this modification. In the present work, sulfiredoxin from P. sativum was seen to be able to deglutathionylate pea 2-Cys Prx but not pea Prx IIF. Redox changes during plant development and the response to stress influence glutathionylation/deglutathionylation processes, which may represent an important event through the modulation of peroxiredoxin and sulfiredoxin proteins.

Evaluating the Oxidative Stress in Renal Diseases: What Is the Role for S-Glutathionylation?

SIGNIFICANCE

Reactive oxygen species (ROS) have long been considered as toxic derivatives of aerobic metabolism displaying a harmful effect to living cells. Deregulation of redox homeostasis and production of excessive free radicals may contribute to the pathogenesis of kidney diseases. In line, oxidative stress increases in patients with renal dysfunctions due to a general increase of ROS paralleled by impaired antioxidant ability.

RECENT ADVANCES

Emerging evidence revealed that physiologically, ROS can act as signaling molecules interplaying with several transduction pathways such as proliferation, differentiation, and apoptosis. ROS can exert signaling functions by modulating, at different layers, protein oxidation since proteins have "cysteine switches" that can be reversibly reduced or oxidized, supporting the dynamic signaling regulation function. In this scenario, S-glutathionylation is a posttranslational modification involved in oxidative cellular response.

CRITICAL ISSUES

Although it is widely accepted that renal dysfunctions are often associated with altered redox signaling, the relative role of S-glutathionylation on the pathogenesis of specific renal diseases remains unclear and needs further investigations. In this review, we discuss the impact of ROS in renal health and diseases and the role of selective S-glutathionylation proteins potentially relevant to renal physiology.

FUTURE DIRECTIONS

The paucity of studies linking the reversible protein glutathionylation with specific renal disorders remains unmet. The growing number of S-glutathionylated proteins indicates that this is a fascinating area of research. In this respect, further studies on the association of reversible glutathionylation with renal diseases, characterized by oxidative stress, may be useful to develop new pharmacological molecules targeting protein S-glutathionylation. Antioxid. Redox Signal. 25, 147-164.

Redox Regulation of Cytosolic Translation in Plants

Control of protein homeostasis is crucial for environmental acclimation of plants. In this context, translational control is receiving increasing attention, particularly since post-translational modifications of the translational apparatus allow very fast and highly effective control of protein synthesis. Reduction and oxidation (redox) reactions decisively control translation by modifying initiation, elongation, and termination of translation. This opinion article compiles information on the redox sensitivity of cytosolic translation factors and the significance of redox regulation as a key modulator of translation for efficient acclimation to changing environmental conditions.

Tuning Cysteine Reactivity and Sulfenic Acid Stability by Protein Microenvironment in Glyceraldehyde-3-Phosphate Dehydrogenases of Arabidopsis thaliana

AIMS

Cysteines and H2O2 are fundamental players in redox signaling. Cysteine thiol deprotonation favors the reaction with H2O2 that generates sulfenic acids with dual electrophilic/nucleophilic nature. The protein microenvironment surrounding the target cysteine is believed to control whether sulfenic acid can be reversibly regulated by disulfide formation or irreversibly oxidized to sulfinates/sulfonates. In this study, we present experimental oxidation kinetics and a quantum mechanical/molecular mechanical (QM/MM) investigation to elucidate the reaction of H2O2 with glycolytic and photosynthetic glyceraldehyde-3-phosphate dehydrogenase from Arabidopsis thaliana (cytoplasmic AtGAPC1 and chloroplastic AtGAPA, respectively).

RESULTS

Although AtGAPC1 and AtGAPA have almost identical 3D structure and similar acidity of their catalytic Cys149, AtGAPC1 is more sensitive to H2O2 and prone to irreversible oxidation than AtGAPA. As a result, sulfenic acid is more stable in AtGAPA.

INNOVATION

Based on crystallographic structures of AtGAPC1 and AtGAPA, the reaction potential energy surface for Cys149 oxidation by H2O2 was calculated by QM. In both enzymes, sulfenic acid formation was characterized by a lower energy barrier than sulfinate formation, and sulfonate formation was prevented by very high energy barriers. Activation energies for both oxidation steps were lower in AtGAPC1 than AtGAPA, supporting the higher propensity of AtGAPC1 toward irreversible oxidation.

CONCLUSIONS

QM/MM calculations coupled to fingerprinting analyses revealed that two Arg of AtGAPA (substituted by Gly and Val in AtGAPC1), located at 8-15 Å distance from Cys149, are the major factors responsible for sulfenic acid stability, underpinning the importance of long-distance polar interactions in tuning sulfenic acid stability in native protein microenvironments.

Protein S-nitrosylation in photosynthetic organisms: A comprehensive overview with future perspectives

BACKGROUND

The free radical nitric oxide (NO) and derivative reactive nitrogen species (RNS) play essential roles in cellular redox regulation mainly through protein S-nitrosylation, a redox post-translational modification in which specific cysteines are converted to nitrosothiols.

SCOPE OF VIEW

This review aims to discuss the current state of knowledge, as well as future perspectives, regarding protein S-nitrosylation in photosynthetic organisms.

MAJOR CONCLUSIONS

NO, synthesized by plants from different sources (nitrite, arginine), provides directly or indirectly the nitroso moiety of nitrosothiols. Biosynthesis, reactivity and scavenging systems of NO/RNS, determine the NO-based signaling including the rate of protein nitrosylation. Denitrosylation reactions compete with nitrosylation in setting the levels of nitrosylated proteins in vivo.

GENERAL SIGNIFICANCE

Based on a combination of proteomic, biochemical and genetic approaches, protein nitrosylation is emerging as a pervasive player in cell signaling networks. Specificity of protein nitrosylation and integration among different post-translational modifications are among the major challenges for future experimental studies in the redox biology field. 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.

Cytosolic Triosephosphate Isomerase from Arabidopsis thaliana Is Reversibly Modified by Glutathione on Cysteines 127 and 218

In plant cells, an increase in cellular oxidants can have multiple effects, including the promotion of mixed disulfide bonds between glutathione and some proteins (S-glutathionylation). The present study focuses on the cytosolic isoform of the glycolytic enzyme triosephosphate isomerase (cTPI) from Arabidopsis thaliana and its reversible modification by glutathione. We used purified recombinant cTPI to demonstrate the enzyme sensitivity to inhibition by N-ethylmaleimide, hydrogen peroxide and diamide. Treatment of cTPI with diamide in the presence of reduced glutathione (GSH) led to a virtually complete inhibition of its enzymatic activity by S-glutathionylation. Recombinant cTPI was also sensitive to the oxidized form of glutathione (GSSG) in the micromolar range. Activity of cTPI was restored after reversion of S-glutathionylation by two purified recombinant A. thaliana cytosolic glutaredoxins (GRXs). GRXs-mediated deglutathionylation of cTPI was dependent on a GSH-regenerating system. Analysis of cTPI by mass spectrometry after S-glutathionylation by GSSG revealed that two Cys residues (Cys127 and Cys218) were modified by glutathione. The role of these two residues was assessed using site-directed mutagenesis. Mutation of Cys127 and Cys218 to Ser separately or together caused different levels of decrease in enzyme activity, loss of stability, as well as alteration of intrinsic fluorescence, underlining the importance of these Cys residues in protein conformation. Comparison of wild-type and mutant proteins modified with biotinyl glutathione ethyl ester (BioGEE) showed partial binding with single mutants and total loss of binding with the double mutant, demonstrating that both Cys residues were significantly S-glutathionylated. cTPI modification with BioGEE was reversed using DTT. Our study provides the first identification of the amino acid residues involved in cTPI S-glutathionylation and supports the hypothesis that this reversible modification could be part of an oxidative stress response pathway.

Glutathione--linking cell proliferation to oxidative stress

SIGNIFICANCE

The multifaceted functions of reduced glutathione (gamma-glutamyl-cysteinyl-glycine; GSH) continue to fascinate plants and animal scientists, not least because of the dynamic relationships between GSH and reactive oxygen species (ROS) that underpin reduction/oxidation (redox) regulation and signalling. Here we consider the respective roles of ROS and GSH in the regulation of plant growth, with a particular focus on regulation of the plant cell cycle. Glutathione is discussed not only as a crucial low molecular weight redox buffer that shields nuclear processes against oxidative challenge but also a flexible regulator of genetic and epigenetic functions.

RECENT ADVANCES

The intracellular compartmentalization of GSH during the cell cycle is remarkably consistent in plants and animals. Moreover, measurements of in vivo glutathione redox potentials reveal that the cellular environment is much more reducing than predicted from GSH/GSSG ratios measured in tissue extracts. The redox potential of the cytosol and nuclei of non-dividing plant cells is about -300 mV. This relatively low redox potential maintained even in cells experiencing oxidative stress by a number of mechanisms including vacuolar sequestration of GSSG. We propose that regulated ROS production linked to glutathione-mediated signalling events are the hallmark of viable cells within a changing and challenging environment.

CRITICAL ISSUES

The concept that the cell cycle in animals is subject to redox controls is well established but little is known about how ROS and GSH regulate this process in plants. However, it is increasingly likely that redox controls exist in plants, although possibly through different pathways. Moreover, redox-regulated proteins that function in cell cycle checkpoints remain to be identified in plants. While GSH-responsive genes have now been identified, the mechanisms that mediate and regulate protein glutathionylation in plants remain poorly defined.

FUTURE DIRECTIONS

The nuclear GSH pool provides an appropriate redox environment for essential nuclear functions. Future work will focus on how this essential thiol interacts with the nuclear thioredoxin system and nitric oxide to regulate genetic and epigenetic mechanisms. The characterization of redox-regulated cell cycle proteins in plants, and the elucidation of mechanisms that facilitate GSH accumulation in the nucleus are keep steps to unravelling the complexities of nuclear redox controls.

Posttranslational Protein Modifications in Plant Metabolism

Posttranslational modifications (PTMs) of proteins greatly expand proteome diversity, increase functionality, and allow for rapid responses, all at relatively low costs for the cell. PTMs play key roles in plants through their impact on signaling, gene expression, protein stability and interactions, and enzyme kinetics. Following a brief discussion of the experimental and bioinformatics challenges of PTM identification, localization, and quantification (occupancy), a concise overview is provided of the major PTMs and their (potential) functional consequences in plants, with emphasis on plant metabolism. Classic examples that illustrate the regulation of plant metabolic enzymes and pathways by PTMs and their cross talk are summarized. Recent large-scale proteomics studies mapped many PTMs to a wide range of metabolic functions. Unraveling of the PTM code, i.e. a predictive understanding of the (combinatorial) consequences of PTMs, is needed to convert this growing wealth of data into an understanding of plant metabolic regulation.

The Expanding Landscape of the Thiol Redox Proteome

Cysteine occupies a unique place in protein chemistry. The nucleophilic thiol group allows cysteine to undergo a broad range of redox modifications beyond classical thiol-disulfide redox equilibria, including S-sulfenylation (-SOH), S-sulfinylation (-SO(2)H), S-sulfonylation (-SO(3)H), S-nitrosylation (-SNO), S-sulfhydration (-SSH), S-glutathionylation (-SSG), and others. Emerging evidence suggests that these post-translational modifications (PTM) are important in cellular redox regulation and protection against oxidative damage. Identification of protein targets of thiol redox modifications is crucial to understanding their roles in biology and disease. However, analysis of these highly labile and dynamic modifications poses challenges. Recent advances in the design of probes for thiol redox forms, together with innovative mass spectrometry based chemoproteomics methods make it possible to perform global, site-specific, and quantitative analyses of thiol redox modifications in complex proteomes. Here, we review chemical proteomic strategies used to expand the landscape of thiol redox modifications.

Development of a Clickable Probe for Profiling of Protein Glutathionylation in the Central Cellular Metabolism of E. coli and Drosophila

Protein glutathionylation is an important post-translational modification that regulates many cellular processes, including energy metabolism, signal transduction, and protein homeostasis. Global profiling of glutathionylated proteins (denoted as glutathionylome) is crucial for understanding redox-regulated signal transduction. Here, we developed a novel method based on click reaction and proteomics to enrich and identify the glutathionylated peptides in Escherichia coli and Drosophila lysates, in which 937 and 1,930 potential glutathionylated peptides were identified, respectively. Bioinformatics analysis showed that the cysteine residue next to negatively charged amino acid residues has a higher frequency of glutathionylation. Importantly, we found that most proteins associated with metabolic pathways were glutathionylated and that the glutathionylation sites of metabolic enzymes were highly conserved among different species. Our results indicate that the glutathione analog is a useful tool to characterize protein glutathionylation, and glutathionylation of metabolic enzymes, which play important roles in regulating cellular metabolism, is conserved.

New insights into redox control of starch degradation

Starch is one of the major sinks of fixed carbon in photosynthetic tissues of higher plants. Carbon fixation and the synthesis of primary starch occur during the day in the chloroplast stroma, whereas starch degradation typically occurs during the following night to fuel the whole plant with energy and carbon in the absence of photosynthesis. Redox-based regulatory systems play a central role in the modulation of several chloroplastic pathways. Reversible oxidations of cysteine residues are post-translational modifications that orchestrate the precise functioning of chloroplast pathways together with changes in pH, Mg(2+) and concentrations of metabolic intermediates. Leaf starch metabolism has been intensively studied. The enzymes involved in starch synthesis and degradation have been identified and characterized. However, the redox control of the enzymes responsible for starch degradation at night remains elusive, and their response to redox transitions conflicts with the timing of the physiological events. Most of the enzymes of starch degradation are activated by reducing conditions, characteristic of daytime. Thus, redox control may have only a minor role during starch degradation at night, but could become relevant for daily stomatal opening in guard cells or in the re-allocation of fixed carbon in mesophyll cells in response to stress conditions.

Identification of a transporter Slr0982 involved in ethanol tolerance in cyanobacterium Synechocystis sp. PCC 6803

Cyanobacteria have been engineered to produce ethanol through recent synthetic biology efforts. However, one major challenge to the cyanobacterial systems for high-efficiency ethanol production is their low tolerance to the ethanol toxicity. With a major goal to identify novel transporters involved in ethanol tolerance, we constructed gene knockout mutants for 58 transporter-encoding genes of Synechocystis sp. PCC 6803 and screened their tolerance change under ethanol stress. The efforts allowed discovery of a mutant of slr0982 gene encoding an ATP-binding cassette transporter which grew poorly in BG11 medium supplemented with 1.5% (v/v) ethanol when compared with the wild type, and the growth loss could be recovered by complementing slr0982 in the Δslr0982 mutant, suggesting that slr0982 is involved in ethanol tolerance in Synechocystis. To decipher the tolerance mechanism involved, a comparative metabolomic and network-based analysis of the wild type and the ethanol-sensitive Δslr0982 mutant was performed. The analysis allowed the identification of four metabolic modules related to slr0982 deletion in the Δslr0982 mutant, among which metabolites like sucrose and L-pyroglutamic acid which might be involved in ethanol tolerance, were found important for slr0982 deletion in the Δslr0982 mutant. This study reports on the first transporter related to ethanol tolerance in Synechocystis, which could be a useful target for further tolerance engineering. In addition, metabolomic and network analysis provides important findings for better understanding of the tolerance mechanism to ethanol stress in Synechocystis.

The CO2 concentrating mechanism and photosynthetic carbon assimilation in limiting CO2 : how Chlamydomonas works against the gradient

The CO2 concentrating mechanism (CCM) represents an effective strategy for carbon acquisition that enables microalgae to survive and proliferate when the CO2 concentration limits photosynthesis. The CCM improves photosynthetic performance by raising the CO2 concentration at the site of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), simultaneously enhancing carbon fixation and suppressing photorespiration. Active inorganic carbon (Ci) uptake, Rubisco sequestration and interconversion between different Ci species catalyzed by carbonic anhydrases (CAs) are key components in the CCM, and an array of molecular regulatory elements is present to facilitate the sensing of CO2 availability, to regulate the expression of the CCM and to coordinate interplay between photosynthetic carbon metabolism and other metabolic processes in response to limiting CO2 conditions. This review intends to integrate our current understanding of the eukaryotic algal CCM and its interaction with carbon assimilation, based largely on Chlamydomonas as a model, and to illustrate how Chlamydomonas acclimates to limiting CO2 conditions and how its CCM is regulated.

Cysteines under ROS attack in plants: a proteomics view

Plants generate reactive oxygen species (ROS) as part of their metabolism and in response to various external stress factors, potentially causing significant damage to biomolecules and cell structures. During the course of evolution, plants have adapted to ROS toxicity, and use ROS as signalling messengers that activate defence responses. Cysteine (Cys) residues in proteins are one of the most sensitive targets for ROS-mediated post-translational modifications, and they have become key residues for ROS signalling studies. The reactivity of Cys residues towards ROS, and their ability to react to different oxidation states, allow them to appear at the crossroads of highly dynamic oxidative events. As such, a redox-active cysteine can be present as S-glutathionylated (-SSG), disulfide bonded (S-S), sulfenylated (-SOH), sulfinylated (-SO2H), and sulfonylated (-SO3H). The sulfenic acid (-SOH) form has been considered as part of ROS-sensing pathways, as it leads to further modifications which affect protein structure and function. Redox proteomic studies are required to understand how and why cysteines undergo oxidative post-translational modifications and to identify the ROS-sensor proteins. Here, we update current knowledge of cysteine reactivity with ROS. Further, we give an overview of proteomic techniques that have been applied to identify different redox-modified cysteines in plants. There is a particular focus on the identification of sulfenylated proteins, which have the potential to be involved in plant signal transduction.

Redox regulation by reversible protein S-thiolation in bacteria

Low molecular weight (LMW) thiols function as thiol-redox buffers to maintain the reduced state of the cytoplasm. The best studied LMW thiol is the tripeptide glutathione (GSH) present in all eukaryotes and Gram-negative bacteria. Firmicutes bacteria, including Bacillus and Staphylococcus species utilize the redox buffer bacillithiol (BSH) while Actinomycetes produce the related redox buffer mycothiol (MSH). In eukaryotes, proteins are post-translationally modified to S-glutathionylated proteins under conditions of oxidative stress. S-glutathionylation has emerged as major redox-regulatory mechanism in eukaryotes and protects active site cysteine residues against overoxidation to sulfonic acids. First studies identified S-glutathionylated proteins also in Gram-negative bacteria. Advances in mass spectrometry have further facilitated the identification of protein S-bacillithiolations and S-mycothiolation as BSH- and MSH-mixed protein disulfides formed under oxidative stress in Firmicutes and Actinomycetes, respectively. In Bacillus subtilis, protein S-bacillithiolation controls the activities of the redox-sensing OhrR repressor and the methionine synthase MetE in vivo. In Corynebacterium glutamicum, protein S-mycothiolation was more widespread and affected the functions of the maltodextrin phosphorylase MalP and thiol peroxidase (Tpx). In addition, novel bacilliredoxins (Brx) and mycoredoxins (Mrx1) were shown to function similar to glutaredoxins in the reduction of BSH- and MSH-mixed protein disulfides. Here we review the current knowledge about the functions of the bacterial thiol-redox buffers glutathione, bacillithiol, and mycothiol and the role of protein S-thiolation in redox regulation and thiol protection in model and pathogenic bacteria.

Phosphoribulokinase from Chlamydomonas reinhardtii: a Benson–Calvin cycle enzyme enslaved to its cysteine residues

In this study, focused on C. reinhardtii phosphoribulokinase, we showed that CP12 catalyses a disulfide bridge between Cys243 and Cys249 on PRK. This disulfide bridge is essential for the GAPDH–CP12–PRK complex formation.

Metabolomic basis of laboratory evolution of butanol tolerance in photosynthetic Synechocystis sp. PCC 6803

BACKGROUND

Recent efforts demonstrated the potential application of cyanobacteria as a "microbial cell factory" to produce butanol directly from CO2. However, cyanobacteria have very low tolerance to the toxic butanol, which limits the economic viability of this renewable system.

RESULTS

Through a long-term experimental evolution process, we achieved a 150% increase of the butanol tolerance in a model cyanobacterium Synechocystis sp. PCC 6803 after a continuous 94 passages for 395 days in BG11 media amended with gradually increased butanol concentration from 0.2% to 0.5% (v/v). To decipher the molecular mechanism responsible for the tolerance increase, we employed an integrated GC-MS and LC-MS approach to determine metabolomic profiles of the butanol-tolerant Synechocystis strains isolated from several stages of the evolution, and then applied PCA and WGCNA network analyses to identify the key metabolites and metabolic modules related to the increased tolerance. The results showed that unstable metabolites of 3-phosphoglyceric acid (3PG), D-fructose 6-phosphate (F6P), D-glucose 6-phosphate (G6P), NADPH, phosphoenolpyruvic acid (PEP), D-ribose 5-phosphate (R5P), and stable metabolites of glycerol, L-serine and stearic acid were differentially regulated during the evolution process, which could be related to tolerance increase to butanol in Synechocystis.

CONCLUSIONS

The study provided the first time-series description of the metabolomic changes related to the gradual increase of butanol tolerance, and revealed a metabolomic basis important for rational tolerance engineering in Synechocystis.

Evolutionary development of redox regulation in chloroplasts

SIGNIFICANCE

The post-translational modification of thiol groups stands out as a key strategy that cells employ for metabolic regulation and adaptation to changing environmental conditions. Nowhere is this more evident than in chloroplasts-the O2-evolving photosynthetic organelles of plant cells that are fitted with multiple redox systems, including the thioredoxin (Trx) family of oxidoreductases functional in the reversible modification of regulatory thiols of proteins in all types of cells. The best understood member of this family in chloroplasts is the ferredoxin-linked thioredoxin system (FTS) by which proteins are modified via light-dependent disulfide/dithiol (S-S/2SH) transitions.

RECENT ADVANCES

Discovered in the reductive activation of enzymes of the Calvin-Benson cycle in illuminated chloroplast preparations, recent studies have extended the role of the FTS far beyond its original boundaries to include a spectrum of cellular processes. Together with the NADP-linked thioredoxin reductase C-type (NTRC) and glutathione/glutaredoxin systems, the FTS also plays a central role in the response of chloroplasts to different types of stress.

CRITICAL ISSUES

The comparisons of redox regulatory networks functional in chloroplasts of land plants with those of cyanobacteria-prokaryotes considered to be the ancestors of chloroplasts-and different types of algae summarized in this review have provided new insight into the evolutionary development of redox regulation, starting with the simplest O2-evolving organisms.

FUTURE DIRECTIONS

The evolutionary appearance, mode of action, and specificity of the redox regulatory systems functional in chloroplasts, as well as the types of redox modification operating under diverse environmental conditions stand out as areas for future study.

The redox-sensitive chloroplast trehalose-6-phosphate phosphatase AtTPPD regulates salt stress tolerance

AIMS

High salinity stress impairs plant growth and development. Trehalose metabolism has been implicated in sugar signaling, and enhanced trehalose metabolism can positively regulate abiotic stress tolerance. However, the molecular mechanism(s) of the stress-related trehalose pathway and the role of individual trehalose biosynthetic enzymes for stress tolerance remain unclear.

RESULTS

Trehalose-6-phosphate phosphatase (TPP) catalyzes the final step of trehalose metabolism. Investigating the subcellular localization of the Arabidopsis thaliana TPP family members, we identified AtTPPD as a chloroplast-localized enzyme. Plants deficient in AtTPPD were hypersensitive, whereas plants overexpressing AtTPPD were more tolerant to high salinity stress. Elevated stress tolerance of AtTPPD overexpressors correlated with high starch levels and increased accumulation of soluble sugars, suggesting a role for AtTPPD in regulating sugar metabolism under salinity conditions. Biochemical analyses indicate that AtTPPD is a target of post-translational redox regulation and can be reversibly inactivated by oxidizing conditions. Two cysteine residues were identified as the redox-sensitive sites. Structural and mutation analyses suggest that the formation of an intramolecular disulfide bridge regulates AtTPPD activity.

INNOVATION

The activity of different AtTPP isoforms, located in the cytosol, nucleus, and chloroplasts, can be redox regulated, suggesting that the trehalose metabolism might relay the redox status of different cellular compartments to regulate diverse biological processes such as stress responses.

CONCLUSION

The evolutionary conservation of the two redox regulatory cysteine residues of TPPs in spermatophytes indicates that redox regulation of TPPs might be a common mechanism enabling plants to rapidly adjust trehalose metabolism to the prevailing environmental and developmental conditions.

Insight into protein S-nitrosylation in Chlamydomonas reinhardtii

AIMS

Protein S-nitrosylation, a post-translational modification (PTM) consisting of the covalent binding of nitric oxide (NO) to a cysteine thiol moiety, plays a major role in cell signaling and is recognized to be involved in numerous physiological processes and diseases in mammals. The importance of nitrosylation in photosynthetic eukaryotes has been less studied. The aim of this study was to expand our knowledge on protein nitrosylation by performing a large-scale proteomic analysis of proteins undergoing nitrosylation in vivo in Chlamydomonas reinhardtii cells under nitrosative stress.

RESULTS

Using two complementary proteomic approaches, 492 nitrosylated proteins were identified. They participate in a wide range of biological processes and pathways, including photosynthesis, carbohydrate metabolism, amino acid metabolism, translation, protein folding or degradation, cell motility, and stress. Several proteins were confirmed in vitro by western blot, site-directed mutagenesis and activity measurements. Moreover, 392 sites of nitrosylation were also identified. These results strongly suggest that S-nitrosylation could constitute a major mechanism of regulation in C. reinhardtii under nitrosative stress conditions.

INNOVATION

This study constitutes the largest proteomic analysis of protein nitrosylation reported to date.

CONCLUSION

The identification of 381 previously unrecognized targets of nitrosylation further extends our knowledge on the importance of this PTM in photosynthetic eukaryotes. The data have been deposited to the ProteomeXchange repository with identifier PXD000569.

First proteomic study of S-glutathionylation in cyanobacteria

Glutathionylation, the reversible post-translational formation of a mixed disulfide between a cysteine residue and glutathione (GSH), is a crucial mechanism for signal transduction and regulation of protein function. Until now this reversible redox modification was studied mainly in eukaryotic cells. Here we report a large-scale proteomic analysis of glutathionylation in a photosynthetic prokaryote, the model cyanobacterium Synechocystis sp. PCC6803. Treatment of acellular extracts with N,N-biotinyl glutathione disulfide (BioGSSG) induced glutathionylation of numerous proteins, which were subsequently isolated by affinity chromatography on streptavidin columns and identified by nano LC-MS/MS analysis. Potential sites of glutathionylation were also determined for 125 proteins following tryptic cleavage, streptavidin-affinity purification, and mass spectrometry analysis. Taken together the two approaches allowed the identification of 383 glutathionylatable proteins that participate in a wide range of cellular processes and metabolic pathways such as carbon and nitrogen metabolisms, cell division, stress responses, and H2 production. In addition, the glutathionylation of two putative targets, namely, peroxiredoxin (Sll1621) involved in oxidative stress tolerance and 3-phosphoglycerate dehydrogenase (Sll1908) acting on amino acids metabolism, was confirmed by biochemical studies on the purified recombinant proteins. These results suggest that glutathionylation constitutes a major mechanism of global regulation of the cyanobacterial metabolism under oxidative stress conditions.

Thioredoxin-dependent redox regulation of chloroplastic phosphoglycerate kinase from Chlamydomonas reinhardtii

In photosynthetic organisms, thioredoxin-dependent redox regulation is a well established mechanism involved in the control of a large number of cellular processes, including the Calvin-Benson cycle. Indeed, 4 of 11 enzymes of this cycle are activated in the light through dithiol/disulfide interchanges controlled by chloroplastic thioredoxin. Recently, several proteomics-based approaches suggested that not only four but all enzymes of the Calvin-Benson cycle may withstand redox regulation. Here, we characterized the redox features of the Calvin-Benson enzyme phosphoglycerate kinase (PGK1) from the eukaryotic green alga Chlamydomonas reinhardtii, and we show that C. reinhardtii PGK1 (CrPGK1) activity is inhibited by the formation of a single regulatory disulfide bond with a low midpoint redox potential (-335 mV at pH 7.9). CrPGK1 oxidation was found to affect the turnover number without altering the affinity for substrates, whereas the enzyme activation appeared to be specifically controlled by f-type thioredoxin. Using a combination of site-directed mutagenesis, thiol titration, mass spectrometry analyses, and three-dimensional modeling, the regulatory disulfide bond was shown to involve the not strictly conserved Cys(227) and Cys(361). Based on molecular mechanics calculation, the formation of the disulfide is proposed to impose structural constraints in the C-terminal domain of the enzyme that may lower its catalytic efficiency. It is therefore concluded that CrPGK1 might constitute an additional light-modulated Calvin-Benson cycle enzyme with a low activity in the dark and a TRX-dependent activation in the light. These results are also discussed from an evolutionary point of view.

Sub-proteome S-nitrosylation analysis in Brassica juncea hints at the regulation of Brassicaceae specific as well as other vital metabolic pathway(s) by nitric oxide and suggests post-translational modifications cross-talk

Abiotic stress affects the normal physiology of the plants and results in crop loss. Brassica juncea is an oil yielding crop affected by abiotic stress. In future, over 30% yield loss by abiotic stress is predicted in India. Understanding the mechanism of plant response to stress would help in developing stress tolerant crops. Nitric oxide (NO) is now viewed as a remarkably important signaling molecule, involved in regulating stress responses. S-Nitrosylation is a NO based post-translational modification (PTM), linked with the regulation of many physiologically relevant targets. In the last decade, over 700 functionally varied S-nitrosylated proteins were identified, which suggested broad-spectrum regulation. To understand the physiological significance of S-nitrosylation, it was analyzed in cold stress. Functional categorization and validation of some of the B. juncea S-nitrosylated targets, suggested that NO produced during stress regulates cellular detoxification by modulating enzymes of ascorbate glutathione cycle, superoxide dismutase, glutathione S-transferase and glyoxalase I by S-nitrosylation in crude, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) depleted and apoplastic fractions. Interestingly, S-nitrosylation of enzymes associated with glucosinolate hydrolysis pathway, suggests a novel regulation of this Brassicaceae specific pathway by NO. Moreover, identification of enzymes of Glycolysis and Calvin cycle in crude and RuBisCO depleted fractions showed the regulation of metabolic as well as photosynthetic pathways by S-nitrosylation. S-Nitrosylation of cell wall modifying and proteolytic enzymes in the apoplast suggested differential and spatial regulation by S-nitrosylation. To have an overview of physiological role(s) of NO, collective information on NO based signaling (mainly by S-nitrosylation) is presented in this review.

Glutathionylation of the aquaporin-2 water channel: a novel post-translational modification modulated by the oxidative stress

Aquaporin-2 (AQP2) is the vasopressin-regulated water channel that controls renal water reabsorption and urine concentration. AQP2 undergoes different regulated post-translational modifications, including phosphorylation and ubiquitylation, which are fundamental for controlling AQP2 cellular localization, stability, and function. The relationship between AQP2 and S-glutathionylation is of potential interest because reactive oxygen species (ROS), produced under renal failure or nephrotoxic drugs, may influence renal function as well as the expression and the activity of different transporters and channels, including aquaporins. Here, we show for the first time that AQP2 is subjected to S-glutathionylation in kidney and in HEK-293 cells stably expressing AQP2. S-Glutathionylation is a redox-dependent post-translational modification controlling several signal transduction pathways and displaying an acute effect on free cytosolic calcium concentration. Interestingly, we found that in fresh kidney slices, the increased AQP2 S-glutathionylation correlated with tert-butyl hydroperoxide-induced ROS generation. Moreover, we also found that cells expressing wild-type human calcium-sensing receptor (hCaSR-wt) and its gain of function (hCaSR-R990G; hCaSR-N124K) had a significant decrease in AQP2 S-glutathionylation secondary to reduced ROS levels and reduced basal intracellular calcium concentration compared with mock cells. Together, these new findings provide fundamental insight into cell biological aspects of AQP2 function and may be relevant to better understand and explain pathological states characterized by an oxidative stress and AQP2-dependent water reabsorption disturbs.

Thioredoxin targets fundamental processes in a methane-producing archaeon, Methanocaldococcus jannaschii

Thioredoxin (Trx), a small redox protein, controls multiple processes in eukaryotes and bacteria by changing the thiol redox status of selected proteins. The function of Trx in archaea is, however, unexplored. To help fill this gap, we have investigated this aspect in methanarchaea--strict anaerobes that produce methane, a fuel and greenhouse gas. Bioinformatic analyses suggested that Trx is nearly universal in methanogens. Ancient methanogens that produce methane almost exclusively from H2 plus CO2 carried approximately two Trx homologs, whereas nutritionally versatile members possessed four to eight. Due to its simplicity, we studied the Trx system of Methanocaldococcus jannaschii--a deeply rooted hyperthermophilic methanogen growing only on H2 plus CO2. The organism carried two Trx homologs, canonical Trx1 that reduced insulin and accepted electrons from Escherichia coli thioredoxin reductase and atypical Trx2. Proteomic analyses with air-oxidized extracts treated with reduced Trx1 revealed 152 potential targets representing a range of processes--including methanogenesis, biosynthesis, transcription, translation, and oxidative response. In enzyme assays, Trx1 activated two selected targets following partial deactivation by O2, validating proteomics observations: methylenetetrahydromethanopterin dehydrogenase, a methanogenesis enzyme, and sulfite reductase, a detoxification enzyme. The results suggest that Trx assists methanogens in combating oxidative stress and synchronizing metabolic activities with availability of reductant, making it a critical factor in the global carbon cycle and methane emission. Because methanogenesis developed before the oxygenation of Earth, it seems possible that Trx functioned originally in metabolic regulation independently of O2, thus raising the question whether a complex biological system of this type evolved at least 2.5 billion years ago.

Proteomic identification and quantification of S-glutathionylation in mouse macrophages using resin-assisted enrichment and isobaric labeling

S-Glutathionylation (SSG) is an important regulatory posttranslational modification on protein cysteine (Cys) thiols, yet the role of specific cysteine residues as targets of modification is poorly understood. We report a novel quantitative mass spectrometry (MS)-based proteomic method for site-specific identification and quantification of S-glutathionylation across different conditions. Briefly, this approach consists of initial blocking of free thiols by alkylation, selective reduction of glutathionylated thiols, and covalent capture of reduced thiols using thiol affinity resins, followed by on-resin tryptic digestion and isobaric labeling with iTRAQ (isobaric tags for relative and absolute quantitation) for MS-based identification and quantification. The overall approach was initially validated by application to RAW 264.7 mouse macrophages treated with different doses of diamide to induce glutathionylation. A total of 1071 Cys sites from 690 proteins were identified in response to diamide treatment, with ~90% of the sites displaying >2-fold increases in SSG modification compared to controls. This approach was extended to identify potential SSG-modified Cys sites in response to H2O2, an endogenous oxidant produced by activated macrophages and many pathophysiological stimuli. The results revealed 364 Cys sites from 265 proteins that were sensitive to S-glutathionylation in response to H2O2 treatment, thus providing a database of proteins and Cys sites susceptible to this modification under oxidative stress. Functional analysis revealed that the most significantly enriched molecular function categories for proteins sensitive to SSG modifications were free radical scavenging and cell death/survival. Overall the results demonstrate that our approach is effective for site-specific identification and quantification of SSG-modified proteins. The analytical strategy also provides a unique approach to determining the major pathways and cellular processes most susceptible to S-glutathionylation under stress conditions.

Lack of isocitrate lyase in Chlamydomonas leads to changes in carbon metabolism and in the response to oxidative stress under mixotrophic growth

Isocitrate lyase is a key enzyme of the glyoxylate cycle. This cycle plays an essential role in cell growth on acetate, and is important for gluconeogenesis as it bypasses the two oxidative steps of the tricarboxylic acid (TCA) cycle in which CO₂ is evolved. In this paper, a null icl mutant of the green microalga Chlamydomonas reinhardtii is described. Our data show that isocitrate lyase is required for growth in darkness on acetate (heterotrophic conditions), as well as for efficient growth in the light when acetate is supplied (mixotrophic conditions). Under these latter conditions, reduced acetate assimilation and concomitant reduced respiration occur, and biomass composition analysis reveals an increase in total fatty acid content, including neutral lipids and free fatty acids. Quantitative proteomic analysis by ¹⁴N/¹⁵N labelling was performed, and more than 1600 proteins were identified. These analyses reveal a strong decrease in the amounts of enzymes of the glyoxylate cycle and gluconeogenesis in parallel with a shift of the TCA cycle towards amino acid synthesis, accompanied by an increase in free amino acids. The decrease of the glyoxylate cycle and gluconeogenesis, as well as the decrease in enzymes involved in β-oxidation of fatty acids in the icl mutant are probably major factors that contribute to remodelling of lipids in the icl mutant. These modifications are probably responsible for the elevation of the response to oxidative stress, with significantly augmented levels and activities of superoxide dismutase and ascorbate peroxidase, and increased resistance to paraquat.

Redox regulation of glycogen biosynthesis in the cyanobacterium Synechocystis sp. PCC 6803: analysis of the AGP and glycogen synthases

Glycogen constitutes the major carbon storage source in cyanobacteria, as starch in algae and higher plants. Glycogen and starch synthesis is linked to active photosynthesis and both of them are degraded to glucose in the dark to maintain cell metabolism. Control of glycogen biosynthesis in cyanobacteria could be mediated by the regulation of the enzymes involved in this process, ADP-glucose pyrophosphorylase (AGP) and glycogen synthase, which were identified as putative thioredoxin targets. We have analyzed whether both enzymes were subjected to redox modification using purified recombinant enzymes or cell extracts in the model cyanobacterium Synechocystis sp. PCC 6803. Our results indicate that both AGP and glycogen synthases are sensitive to copper oxidation. However, only AGP exhibits a decrease in its enzymatic activity, which is recovered after reduction by DTT or reduced thioredoxin (TrxA), suggesting a redox control of AGP. In order to elucidate the role in redox control of the cysteine residues present on the AGP sequence (C45, C185, C320, and C337), they were replaced with serine. All AGP mutant proteins remained active when expressed in Synechocystis, although they showed different electrophoretic mobility profiles after copper oxidation, reflecting a complex pattern of cysteines interaction.

Heavy-metal-induced reactive oxygen species: phytotoxicity and physicochemical changes in plants

As a result of the industrial revolution, anthropogenic activities have enhanced there distribution of many toxic heavy metals from the earth's crust to different environmental compartments. Environmental pollution by toxic heavy metals is increasing worldwide, and poses a rising threat to both the environment and to human health.Plants are exposed to heavy metals from various sources: mining and refining of ores, fertilizer and pesticide applications, battery chemicals, disposal of solid wastes(including sewage sludge), irrigation with wastewater, vehicular exhaust emissions and adjacent industrial activity.Heavy metals induce various morphological, physiological, and biochemical dysfunctions in plants, either directly or indirectly, and cause various damaging effects. The most frequently documented and earliest consequence of heavy metal toxicity in plants cells is the overproduction of ROS. Unlike redox-active metals such as iron and copper, heavy metals (e.g, Pb, Cd, Ni, AI, Mn and Zn) cannot generate ROS directly by participating in biological redox reactions such as Haber Weiss/Fenton reactions. However, these metals induce ROS generation via different indirect mechanisms, such as stimulating the activity of NADPH oxidases, displacing essential cations from specific binding sites of enzymes and inhibiting enzymatic activities from their affinity for -SH groups on the enzyme.Under normal conditions, ROS play several essential roles in regulating the expression of different genes. Reactive oxygen species control numerous processes like the cell cycle, plant growth, abiotic stress responses, systemic signalling, programmed cell death, pathogen defence and development. Enhanced generation of these species from heavy metal toxicity deteriorates the intrinsic antioxidant defense system of cells, and causes oxidative stress. Cells with oxidative stress display various chemical,biological and physiological toxic symptoms as a result of the interaction between ROS and biomolecules. Heavy-metal-induced ROS cause lipid peroxidation, membrane dismantling and damage to DNA, protein and carbohydrates. Plants have very well-organized defense systems, consisting of enzymatic and non-enzymatic antioxidation processes. The primary defense mechanism for heavy metal detoxification is the reduced absorption of these metals into plants or their sequestration in root cells.Secondary heavy metal tolerance mechanisms include activation of antioxidant enzymes and the binding of heavy metals by phytochelatins, glutathione and amino acids. These defense systems work in combination to manage the cascades of oxidative stress and to defend plant cells from the toxic effects of ROS.In this review, we summarized the biochemiCal processes involved in the over production of ROS as an aftermath to heavy metal exposure. We also described the ROS scavenging process that is associated with the antioxidant defense machinery.Despite considerable progress in understanding the biochemistry of ROS overproduction and scavenging, we still lack in-depth studies on the parameters associated with heavy metal exclusion and tolerance capacity of plants. For example, data about the role of glutathione-glutaredoxin-thioredoxin system in ROS detoxification in plant cells are scarce. Moreover, how ROS mediate glutathionylation (redox signalling)is still not completely understood. Similarly, induction of glutathione and phytochelatins under oxidative stress is very well reported, but it is still unexplained that some studied compounds are not involved in the detoxification mechanisms. Moreover,although the role of metal transporters and gene expression is well established for a few metals and plants, much more research is needed. Eventually, when results for more metals and plants are available, the mechanism of the biochemical and genetic basis of heavy metal detoxification in plants will be better understood. Moreover, by using recently developed genetic and biotechnological tools it may be possible to produce plants that have traits desirable for imparting heavy metal tolerance.

High-resolution crystal structure and redox properties of chloroplastic triosephosphate isomerase from Chlamydomonas reinhardtii

Triosephosphate isomerase (TPI) catalyzes the interconversion of glyceraldehyde-3-phosphate to dihydroxyacetone phosphate. Photosynthetic organisms generally contain two isoforms of TPI located in both cytoplasm and chloroplasts. While the cytoplasmic TPI is involved in the glycolysis, the chloroplastic isoform participates in the Calvin-Benson cycle, a key photosynthetic process responsible for carbon fixation. Compared with its cytoplasmic counterpart, the functional features of chloroplastic TPI have been poorly investigated and its three-dimensional structure has not been solved. Recently, several studies proposed TPI as a potential target of different redox modifications including dithiol/disulfide interchanges, glutathionylation, and nitrosylation. However, neither the effects on protein activity nor the molecular mechanisms underlying these redox modifications have been investigated. Here, we have produced recombinantly and purified TPI from the unicellular green alga Chlamydomonas reinhardtii (Cr). The biochemical properties of the enzyme were delineated and its crystallographic structure was determined at a resolution of 1.1 Å. CrTPI is a homodimer with subunits containing the typical (β/α)8-barrel fold. Although no evidence for TRX regulation was obtained, CrTPI was found to undergo glutathionylation by oxidized glutathione and trans-nitrosylation by nitrosoglutathione, confirming its sensitivity to multiple redox modifications.

Dissecting the integrative antioxidant and redox systems in plant mitochondria. Effect of stress and S-nitrosylation

Mitochondrial respiration provides the energy needed to drive metabolic and transport processes in cells. Mitochondria are a significant site of reactive oxygen species (ROS) production in plant cells, and redox-system components obey fine regulation mechanisms that are essential in protecting the mitochondrial integrity. In addition to ROS, there are compelling indications that nitric oxide can be generated in this organelle by both reductive and oxidative pathways. ROS and reactive nitrogen species play a key role in signaling but they can also be deleterious via oxidation of macromolecules. The high production of ROS obligates mitochondria to be provided with a set of ROS scavenging mechanisms. The first line of mitochondrial antioxidants is composed of superoxide dismutase and the enzymes of the ascorbate-glutathione cycle, which are not only able to scavenge ROS but also to repair cell damage and possibly serve as redox sensors. The dithiol-disulfide exchanges form independent signaling nodes and act as antioxidant defense mechanisms as well as sensor proteins modulating redox signaling during development and stress adaptation. The presence of thioredoxin (Trx), peroxiredoxin (Prx) and sulfiredoxin (Srx) in the mitochondria has been recently reported. Cumulative results obtained from studies in salt stress models have demonstrated that these redox proteins play a significant role in the establishment of salt tolerance. The Trx/Prx/Srx system may be subjected to a fine regulated mechanism involving post-translational modifications, among which S-glutathionylation and S-nitrosylation seem to exhibit a critical role that is just beginning to be understood. This review summarizes our current knowledge in antioxidative systems in plant mitochondria, their interrelationships, mechanisms of compensation and some unresolved questions, with special focus on their response to abiotic stress.

Redox regulation of the Calvin-Benson cycle: something old, something new

Reversible redox post-translational modifications such as oxido-reduction of disulfide bonds, S-nitrosylation, and S-glutathionylation, play a prominent role in the regulation of cell metabolism and signaling in all organisms. These modifications are mainly controlled by members of the thioredoxin and glutaredoxin families. Early studies in photosynthetic organisms have identified the Calvin-Benson cycle, the photosynthetic pathway responsible for carbon assimilation, as a redox regulated process. Indeed, 4 out of 11 enzymes of the cycle were shown to have a low activity in the dark and to be activated in the light through thioredoxin-dependent reduction of regulatory disulfide bonds. The underlying molecular mechanisms were extensively studied at the biochemical and structural level. Unexpectedly, recent biochemical and proteomic studies have suggested that all enzymes of the cycle and several associated regulatory proteins may undergo redox regulation through multiple redox post-translational modifications including glutathionylation and nitrosylation. The aim of this review is to detail the well-established mechanisms of redox regulation of Calvin-Benson cycle enzymes as well as the most recent reports indicating that this pathway is tightly controlled by multiple interconnected redox post-translational modifications. This redox control is likely allowing fine tuning of the Calvin-Benson cycle required for adaptation to varying environmental conditions, especially during responses to biotic and abiotic stresses.

Plastid-localized glutathione reductase2-regulated glutathione redox status is essential for Arabidopsis root apical meristem maintenance

Glutathione is involved in thiol redox signaling and acts as a major redox buffer against reactive oxygen species, helping to maintain a reducing environment in vivo. Glutathione reductase (GR) catalyzes the reduction of glutathione disulfide (GSSG) into reduced glutathione (GSH). The Arabidopsis thaliana genome encodes two GRs: GR1 and GR2. Whereas the cytosolic/peroxisomal GR1 is not crucial for plant development, we show here that the plastid-localized GR2 is essential for root growth and root apical meristem (RAM) maintenance. We identify a GR2 mutant, miao, that displays strong inhibition of root growth and severe defects in the RAM, with GR activity being reduced to ∼50%. miao accumulates high levels of GSSG and exhibits increased glutathione oxidation. The exogenous application of GSH or the thiol-reducing agent DTT can rescue the root phenotype of miao, demonstrating that the RAM defects in miao are triggered by glutathione oxidation. Our in silico analysis of public microarray data shows that auxin and glutathione redox signaling generally act independently at the transcriptional level. We propose that glutathione redox status is essential for RAM maintenance through both auxin/PLETHORA (PLT)-dependent and auxin/PLT-independent redox signaling pathways.

Thiol-based redox control of enzymes involved in the tetrapyrrole biosynthesis pathway in plants

The last decades of research brought substantial insights into tetrapyrrole biosynthetic pathway in photosynthetic organisms. Almost all genes have been identified and roles of seemingly all essential proteins, leading to the synthesis of heme, siroheme, phytochromobilin, and chlorophyll (Chl), have been characterized. Detailed studies revealed the existence of a complex network of transcriptional and post-translational control mechanisms for maintaining a well-adjusted tetrapyrrole biosynthesis during plant development and adequate responses to environmental changes. Among others one of the known post-translational modifications is regulation of enzyme activities by redox modulators. Thioredoxins and NADPH-dependent thioredoxin reductase C (NTRC) adjust the activity of tetrapyrrole synthesis to the redox status of plastids. Excessive excitation energy of Chls in both photosystems and accumulation of light-absorbing unbound tetrapyrrole intermediates generate reactive oxygen species, which interfere with the plastid redox poise. Recent reports highlight ferredoxin-thioredoxin and NTRC-dependent control of key steps in tetrapyrrole biosynthesis in plants. In this review we introduce the regulatory impact of these reductants on the stability and activity of enzymes involved in 5-aminolevulinic acid synthesis as well as in the Mg-branch of the tetrapyrrole biosynthetic pathway and we propose molecular mechanisms behind this redox control.

Protein disulfide isomerase 2 of Chlamydomonas reinhardtii is involved in circadian rhythm regulation

Protein disulfide isomerases (PDIs) are known to play important roles in the folding of nascent proteins and in the formation of disulfide bonds. Recently, we identified a PDI from Chlamydomonas reinhardtii (CrPDI2) by a mass spectrometry approach that is specifically enriched by heparin affinity chromatography in samples taken during the night phase. Here, we show that the recombinant CrPDI2 is a redox-active protein. It is reduced by thioredoxin reductase and catalyzes itself the reduction of insulin chains and the oxidative refolding of scrambled RNase A. By immunoblots, we confirm a high-amplitude change in abundance of the heparin-bound CrPDI2 during subjective night. Interestingly, we find that CrPDI2 is present in protein complexes of different sizes at both day and night. Among three identified interaction partners, one (a 2-cys peroxiredoxin) is present only during the night phase. To study a potential function of CrPDI2 within the circadian system, we have overexpressed its gene. Two transgenic lines were used to measure the rhythm of phototaxis. In the transgenic strains, a change in the acrophase was observed. This indicates that CrPDI2 is involved in the circadian signaling pathway and, together with the night phase-specific interaction of CrPDI2 and a peroxiredoxin, these findings suggest a close coupling of redox processes and the circadian clock in C. reinhardtii.

Characterization of the LOV1-mediated, victorin-induced, cell-death response with virus-induced gene silencing

Victoria blight, caused by Cochliobolus victoriae, is a disease originally described on oat and recapitulated on Arabidopsis. C. victoriae pathogenesis depends upon production of the toxin victorin. In oat, victorin sensitivity is conferred by the Vb gene, which is genetically inseparable from the Pc2 resistance gene. Concurrently, in Arabidopsis, sensitivity is conferred by the LOCUS ORCHESTRATING VICTORIN EFFECTS1 (LOV1) gene. LOV1 encodes a nucleotide-binding site leucine-rich repeat protein, a type of protein commonly associated with disease resistance, and LOV1 "guards" the defense thioredoxin, TRX-h5. Expression of LOV1 and TRX-h5 in Nicotiana benthamiana is sufficient to confer victorin sensitivity. Virus-induced gene silencing was used to characterize victorin-induced cell death in N. benthamiana. We determined that SGT1 is required for sensitivity and involved in LOV1 protein accumulation. We screened a normalized cDNA library and identified six genes that, when silenced, suppressed LOV1-mediated, victorin-induced cell death and cell death induced by expression of the closely related RPP8 resistance gene: a mitochondrial phosphate transporter, glycolate oxidase, glutamine synthetase, glyceraldehyde 3-phosphate dehydrogenase, and the P- and T-protein of the glycine decarboxylase complex. Silencing the latter four also inhibited cell death and disease resistance mediated by the PTO resistance gene. Together, these results provide evidence that the victorin response mediated by LOV1 is a defense response.

Causes and consequences of cysteine S-glutathionylation

Post-translational S-glutathionylation occurs through the reversible addition of a proximal donor of glutathione to thiolate anions of cysteines in target proteins, where the modification alters molecular mass, charge, and structure/function and/or prevents degradation from sulfhydryl overoxidation or proteolysis. Catalysis of both the forward (glutathione S-transferase P) and reverse (glutaredoxin) reactions creates a functional cycle that can also regulate certain protein functional clusters, including those involved in redox-dependent cell signaling events. For translational application, S-glutathionylated serum proteins may be useful as biomarkers in individuals (who may also have polymorphic expression of glutathione S-transferase P) exposed to agents that cause oxidative or nitrosative stress.

Mitochondrial energy and redox signaling in plants

SIGNIFICANCE

For a plant to grow and develop, energy and appropriate building blocks are a fundamental requirement. Mitochondrial respiration is a vital source for both. The delicate redox processes that make up respiration are affected by the plant's changing environment. Therefore, mitochondrial regulation is critically important to maintain cellular homeostasis. This involves sensing signals from changes in mitochondrial physiology, transducing this information, and mounting tailored responses, by either adjusting mitochondrial and cellular functions directly or reprogramming gene expression.

RECENT ADVANCES

Retrograde (RTG) signaling, by which mitochondrial signals control nuclear gene expression, has been a field of very active research in recent years. Nevertheless, no mitochondrial RTG-signaling pathway is yet understood in plants. This review summarizes recent advances toward elucidating redox processes and other bioenergetic factors as a part of RTG signaling of plant mitochondria.

CRITICAL ISSUES

Novel insights into mitochondrial physiology and redox-regulation provide a framework of upstream signaling. On the other end, downstream responses to modified mitochondrial function have become available, including transcriptomic data and mitochondrial phenotypes, revealing processes in the plant that are under mitochondrial control.

FUTURE DIRECTIONS

Drawing parallels to chloroplast signaling and mitochondrial signaling in animal systems allows to bridge gaps in the current understanding and to deduce promising directions for future research. It is proposed that targeted usage of new technical approaches, such as quantitative in vivo imaging, will provide novel leverage to the dissection of plant mitochondrial signaling.

Nitric oxide-cold stress signalling cross-talk, evolution of a novel regulatory mechanism

Plants enhance their cold stress tolerance by cold acclimation, a process which results in vast reprogramming of transcriptome, proteome and metabolome. Evidence suggests nitric oxide (NO) production during cold stress which regulates genes (especially the C-repeat binding factor (CBF) cold stress signalling pathway), diverse proteins including transcription factors (TFs) and phosphosphingolipids. About 59% (redox), 50% (defence/stress) and 30% (signalling) cold responsive proteins are modulated by NO-based post translational modifications (PTMs) namely S-nitrosylation, tyrosine nitration and S-glutathionylation, suggesting a cross-talk between NO and cold. Analysis of cold stress responsive deep proteome in apoplast, mitochondria, chloroplast and nucleus suggested continuation of this cross-talk in sub-cellular systems. Modulation of cold responsive proteins by these PTMs right from cytoskeletal elements in plasma membrane to TFs in nucleus suggests a novel regulation of cold stress signalling. NO-mediated altered protein transport in nucleus seems an important stress regulatory mechanism. This review addresses the NO and cold stress signalling cross-talk to present the overview of this novel regulatory mechanism.

Glutaredoxins in thiol/disulfide exchange

SIGNIFICANCE

Glutaredoxins (Grxs) are small oxidoreductases of the thioredoxin family of proteins regulating the thiol redox state of several proteins. Thereby, Grxs are key elements in redox signaling.

RECENT ADVANCES

Redox signaling via protein thiols depends on reversible oxidative modifications induced mainly by reactive oxygen/nitrogen species and glutathione (GSH) in form of its oxidized disulfide or S-nitroso-glutathione. Grxs contribute to redox signaling by the catalysis of glutathionylation, de-glutathionylation, as well as reduction of disulfide bridges via two distinct enzymatic mechanisms. The dithiol mechanism utilizes both active site cysteines to reduce disulfides, whereas the monothiol mechanism utilizes only the N-terminal active site cysteine for the reduction of GSH mixed disulfides. The sphere of action of Grxs continues to grow with the recent identification of novel targets.

CRITICAL ISSUES

Because of limited methodological tools, the identification of new substrates for oxidoreductases in general is one of the biggest challenges in this research area.

FUTURE DIRECTIONS

With this review, we provide a condensed summary of the current knowledge of thiol/disulfide exchange reactions catalyzed by Grxs regarding the mechanistic, structural, and functional aspects. The latter will be of high importance for future research directions, gaining novel insights into redox signaling in general, and the role of Grxs in particular.

CP12 residues involved in the formation and regulation of the glyceraldehyde-3-phosphate dehydrogenase-CP12-phosphoribulokinase complex in Chlamydomonas reinhardtii

CP12, a member of the intrinsically disordered protein family, forms a stable complex with glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and phosphoribulokinase (PRK). To understand the function of conserved residues of CP12 in the formation of the GAPDH-CP12-PRK complex and in the regulation of the enzymes within this complex, we have produced mutants of CP12 by site-directed mutagenesis. The GAPDH, CP12 and PRK recombinant proteins are able to reconstitute spontaneously the ternary complex that has been described in Chlamydomonas reinhardtii. Our analysis reveals that the central part ((35)WXXVEE(47)) of CP12 is required to form the GAPDH-CP12-PRK complex. Using the same series of single amino acid replacements, we have identified individual residues, which seem to represent also contact points for GAPDH. Most notably, substitution of glutamate 74 prevents the binding of GAPDH to CP12. This is similar to the mutant C66S, with which the GAPDH-CP12-PRK complex is not formed. In contrast, replacement of the three last residues ((78)YED(80)) of CP12 has no effect on the formation of the ternary supra-molecular complex. However, our findings strongly suggest that Y78 and D80 are involved in the regulation of the GAPDH activity within the supra-molecular complex, since the mutants, D80K and Y78S, do not down-regulate the activity of GAPDH. The replacement of the amino acid E79 weakens the interaction between GAPDH and CP12 as no GAPDH-CP12 sub-complex is formed. In this case, nevertheless, the supra-molecular complex is formed when PRK is present indicating that PRK strengthens the interaction between GAPDH and CP12 within the supra-molecular complex.