Yeast thioredoxin genes

Characterization of the Trr/Trx system in the fungal pathogen Candida glabrata

C. glabrata, an opportunistic fungal pathogen, can adapt and resist to different stress conditions. It is highly resistant to oxidant stress compared to other Candida spp and to the phylogenetically related but non-pathogen Saccharomyces cerevisiae. In this work, we describe the Trx/Trr system of C. glabrata composed of Trr1 and Trr2 (thioredoxin reductases) and Trx2 (thioredoxin) that are localized in the cytoplasm and Trx3 present in the mitochondrion. The transcriptional induction of TRR2 and TRX2 by oxidants depends on Yap1 and Skn7 and TRR1 and TRX3 have a low expression level. Both TRR2 and TRX2 play an important role in the oxidative stress response. The absence of TRX2 causes auxotrophy of methionine and cysteine. Trr1 and Trr2 are necessary for survival at high temperatures and for the chronological life span of C. glabrata. Furthermore, the Trx/Trr system is needed for survival in the presence of neutrophils. The role of TRR1 and TRX3 is not clear, but in the presence of neutrophils, they have non-overlapping functions with their TRR2 and TRX2 paralogues.

Endoplasmic reticulum (ER) stress-induced reactive oxygen species (ROS) are detrimental for the fitness of a thioredoxin reductase mutant

The unfolded protein response (UPR) is constitutively active in yeast thioredoxin reductase mutants, suggesting a link between cytoplasmic thiol redox control and endoplasmic reticulum (ER) oxidative protein folding. The unique oxidative environment of the ER lumen requires tight regulatory control, and we show that the active UPR depends on the presence of oxidized thioredoxins rather than arising because of a loss of thioredoxin function. Preventing activation of the UPR by deletion of HAC1, encoding the UPR transcription factor, rescues a number of thioredoxin reductase mutant phenotypes, including slow growth, shortened longevity, and oxidation of the cytoplasmic GSH pool. This is because the constitutive UPR in a thioredoxin reductase mutant results in the generation of hydrogen peroxide. The oxidation of thioredoxins in a thioredoxin reductase mutant requires aerobic metabolism and the presence of the Tsa1 and Tsa2 peroxiredoxins, indicating that a complete cytoplasmic thioredoxin system is crucial for maintaining ER redox homeostasis.

Physiological relevance of plant 2-Cys peroxiredoxin overoxidation level and oligomerization status

Peroxiredoxins are ubiquitous thioredoxin-dependent peroxidases presumed to display, upon environmental constraints, a chaperone function resulting from a redox-dependent conformational switch. In this work, using biochemical and genetic approaches, we aimed to unravel the factors regulating the redox status and the conformation of the plastidial 2-Cys peroxiredoxin (2-Cys PRX) in plants. In Arabidopsis, we show that in optimal growth conditions, the overoxidation level mainly depends on the availability of thioredoxin-related electron donors, but not on sulfiredoxin, the enzyme reducing the 2-Cys PRX overoxidized form. We also observed that upon various physiological temperature, osmotic and light stress conditions, the overoxidation level and oligomerization status of 2-Cys PRX can moderately vary depending on the constraint type. Further, no major change was noticed regarding protein conformation in water-stressed Arabidopsis, barley and potato plants, whereas species-dependent up- and down-variations in overoxidation were observed. In contrast, both 2-Cys PRX overoxidation and oligomerization were strongly induced during a severe oxidative stress generated by methyl viologen. From these data, revealing that the oligomerization status of plant 2-Cys PRX does not exhibit important variation and is not tightly linked to the protein redox status upon physiologically relevant environmental constraints, the possible in planta functions of 2-Cys PRX are discussed.

Beer and wort proteomics

Proteome analysis provides a way to identify proteins related to the quality traits of beer. A number of protein species in beer and wort have been identified by two-dimensional gel electrophoresis combined with enzyme digestion such as trypsin, followed by mass spectrometry analyses and/or liquid chromatography mass/mass spectrometry. In addition, low molecular weight polypeptides in beer have been identified by the combination of non-enzyme digestion and mass analyses. These data sets of various molecular weight polypeptides (i.e., proteomes) provide a platform for analyzing protein functions in beer. Several novel proteins related to beer quality traits such as foam stability and haze formation have been identified by analyzing these proteomes. Some of the proteins have been applied to the development of efficient protein or DNA markers for trait selection in malting barley breeding. In this chapter, recent proteome studies of beer and wort are reviewed, and the methods and protocols of beer and wort proteome analysis are described.

Oxidative stresses and ageing

Oxidative damage to cellular constituents has frequently been associated with aging in a wide range of organisms. The power of yeast genetics and biochemistry has provided the opportunity to analyse in some detail how reactive oxygen and nitrogen species arise in cells, how cells respond to the damage that these reactive species cause, and to begin to dissect how these species may be involved in the ageing process. This chapter reviews the major sources of reactive oxygen species that occur in yeast cells, the damage they cause and how cells sense and respond to this damage.

The response to heat shock and oxidative stress in Saccharomyces cerevisiae

A common need for microbial cells is the ability to respond to potentially toxic environmental insults. Here we review the progress in understanding the response of the yeast Saccharomyces cerevisiae to two important environmental stresses: heat shock and oxidative stress. Both of these stresses are fundamental challenges that microbes of all types will experience. The study of these environmental stress responses in S. cerevisiae has illuminated many of the features now viewed as central to our understanding of eukaryotic cell biology. Transcriptional activation plays an important role in driving the multifaceted reaction to elevated temperature and levels of reactive oxygen species. Advances provided by the development of whole genome analyses have led to an appreciation of the global reorganization of gene expression and its integration between different stress regimens. While the precise nature of the signal eliciting the heat shock response remains elusive, recent progress in the understanding of induction of the oxidative stress response is summarized here. Although these stress conditions represent ancient challenges to S. cerevisiae and other microbes, much remains to be learned about the mechanisms dedicated to dealing with these environmental parameters.

Genome-wide fitness and expression profiling implicate Mga2 in adaptation to hydrogen peroxide

Caloric restriction extends lifespan, an effect once thought to involve attenuation of reactive oxygen species (ROS) generated by aerobic metabolism. However, recent evidence suggests that caloric restriction may in fact raise ROS levels, which in turn provides protection from acute doses of oxidant through a process called adaptation. To shed light on the molecular mechanisms of adaptation, we designed a series of genome-wide deletion fitness and mRNA expression screens to identify genes involved in adaptation to hydrogen peroxide. Combined with known transcriptional interactions, the integrated data implicate Yap1 and Skn7 as central transcription factors of both the adaptive and acute oxidative responses. They also identify the transcription factors Mga2 and Rox1 as active exclusively in the adaptive response and show that Mga2 is essential for adaptation. These findings are striking because Mga2 and Rox1 have been thought to control the response to hypoxic, not oxidative, conditions. Expression profiling of mga2Δ and rox1Δ knockouts shows that these factors most strongly regulate targets in ergosterol, fatty-acid, and zinc metabolic pathways. Direct quantitation of ergosterol reveals that its basal concentration indeed depends on Mga2, but that Mga2 is not required for the decrease in ergosterol observed during adaptation.

Reactive oxygen species (ROS) damage a variety of structures within the cell, resulting in disease and aging. In a seemingly paradoxical effect termed adaptation, it is possible to prevent damage caused by ROS by pre-treating the cell with a small amount of oxidant. We studied this process in order to identify the mechanisms that provide this protection. Our study identified a number of genes and processes with previously unappreciated roles in adaptation. The mechanisms we identified are remarkable because they are distinct from those previously known to protect the cell from ROS. Although this study is conducted in yeast, the wide conservation of adaptation among many organisms suggests that the results from this study may be widely applicable.

Heat-shock dependent oligomeric status alters the function of a plant-specific thioredoxin-like protein, AtTDX

We found that Arabidopsis AtTDX, a heat-stable and plant-specific thioredoxin (Trx)-like protein, exhibits multiple functions, acting as a disulfide reductase, foldase chaperone, and holdase chaperone. The activity of AtTDX, which contains 3 tetratricopeptide repeat (TPR) domains and a Trx motif, depends on its oligomeric status. The disulfide reductase and foldase chaperone functions predominate when AtTDX occurs in the low molecular weight (LMW) form, whereas the holdase chaperone function predominates in the high molecular weight (HMW) complexes. Because deletion of the TPR domains results in a significant enhancement of AtTDX disulfide reductase activity and complete loss of the holdase chaperone function, our data suggest that the TPR domains of AtTDX block the active site of Trx and play a critical role in promoting the holdase chaperone function. The oligomerization status of AtTDX is reversibly regulated by heat shock, which causes a transition from LMW to HMW complexes with concomitant functional switching from a disulfide reductase and foldase chaperone to a holdase chaperone. Overexpression of AtTDX in Arabidopsis conferred enhanced heat shock resistance to plants, primarily via its holdase chaperone activity.

Immunocytochemical localization of Pisum sativum TRXs f and m in non-photosynthetic tissues

Plants are the organisms containing the most complex multigenic family for thioredoxins (TRX). Several types of TRXs are targeted to chloroplasts, which have been classified into four subgroups: m, f, x, and y. Among them, TRXs f and m were the first plastidial TRXs characterized, and their function as redox modulators of enzymes involved in carbon assimilation in the chloroplast has been well-established. Both TRXs, f and m, were named according to their ability to reduce plastidial fructose-1,6-bisphosphatase (FBPase) and malate dehydrogenase (MDH), respectively. Evidence is presented here based on the immunocytochemistry of the localization of f and m-type TRXs from Pisum sativum in non-photosynthetic tissues. Both TRXs showed a different spatial pattern. Whilst PsTRXm was localized to vascular tissues of all the organs analysed (leaves, stems, and roots), PsTRXf was localized to more specific cells next to xylem vessels and vascular cambium. Heterologous complementation analysis of the yeast mutant EMY63, deficient in both yeast TRXs, by the pea plastidial TRXs suggests that PsTRXm, but not PsTRXf, is involved in the mechanism of reactive oxygen species (ROS) detoxification. In agreement with this function, the PsTRXm gene was induced in roots of pea plants in response to hydrogen peroxide.

PaTrx1 and PaTrx3, two cytosolic thioredoxins of the filamentous ascomycete Podospora anserina involved in sexual development and cell degeneration

In various organisms, thioredoxins are known to be involved in the reduction of protein disulfide bonds and in protecting the cell from oxidative stress. Genes encoding thioredoxins were found by searching the complete genome sequence of the filamentous ascomycete Podospora anserina. Among them, PaTrx1, PaTrx2, and PaTrx3 are predicted to be canonical cytosolic proteins without additional domains. Targeted disruption of PaTrx1, PaTrx2, and PaTrx3 shows that PaTrx1 is the major thioredoxin involved in sulfur metabolism. Deletions have no effect on peroxide resistance; however, data show that either PaTrx1 or PaTrx3 is necessary for sexual reproduction and for the development of the crippled growth cell degeneration (CG), processes that also required the PaMpk1 mitogen-activated protein kinase (MAPK) pathway. Since PaTrx1 PaTrx3 mutants show not an enhancement but rather an impairment in CG, it seems unlikely that PaTrx1 and PaTrx3 thioredoxins participate in the inhibition of this MAPK pathway. Altogether, these results underscore a role for thioredoxins in fungal development.

Crystal structure of an unusual thioredoxin protein with a zinc finger domain

Many Gram-negative bacteria have two cytoplasmic thioredoxins, thioredoxin-1 and -2, encoded by the trxA and trxC genes, respectively. Both thioredoxins have the highly conserved WCGPC motif and function as disulfide-bond reductases. However, thioredoxin-2 has unique features: it has an N-terminal motif that binds a zinc ion, and its transcription is under the control of OxyR, which allows it to be up-regulated under oxidative stress. Here, we report the crystal structure of thioredoxin-2 from Rhodobacter capsulatus. The C-terminal region of thioredoxin-2 forms a canonical thioredoxin fold with a central beta-sheet consisting of five strands and four flanking alpha-helices on either side. The N-terminal zinc finger is composed of four short beta-strands (S1-S4) connected by three short loops (L1-L3). The four cysteines are at loops L1 and L3 and form a tetragonal binding site for a zinc ion. The zinc finger is close to the first beta-strand and first alpha-helix of the thioredoxin fold. Nevertheless, the zinc finger may not directly affect the oxidoreductase activity of thioredoxin-2 because the zinc finger is not near the active site of a protomer and because thioredoxin-2 is a monomer in solution. On the basis of structural similarity to the zinc fingers in Npl4 and Vps36, we propose that the N-terminal zinc finger of thioredoxin-2 mediates protein-protein interactions, possibly with its substrates or chaperones.

The txl1+ gene from Schizosaccharomyces pombe encodes a new thioredoxin-like 1 protein that participates in the antioxidant defence against tert-butyl hydroperoxide

Yeasts are equipped with several putative single-domain thioredoxins located in different subcellular compartments. However, additional proteins containing thioredoxin domains are also encoded by the yeast genomes as described for mammals and other eukaryotic organisms. We report here the characterization of the fission yeast orthologue thioredoxin-like 1 (txl1(+)), which has been previously identified in mammals. Similarly to the human protein, the fission yeast Txl1 is a two-domain protein comprising an N-terminal thioredoxin-like domain and a C-terminal domain of unknown function. Many other yeasts and fungi species contain homologues of txl1(+); however, there is no evidence of txl1(+) orthologues in either Saccharomyces cerevisiae or plants. Txl1 is found in both the nucleus and the cytoplasm of Schizosaccharomyces pombe cells and exhibits a strong reducing activity coupled to thioredoxin reductase. In humans, TXL1 expression is induced by glucose deprivation and overexpression of TXL1 confers resistance against this stress. In contrast, a Sz. pombe Deltatxl1 mutant was not affected in the response against glucose starvation but the Deltatxl1 mutant strain showed a clear hypersensitivity to alkyl hydroperoxide. The mRNA levels of txl1(+) in a h20 strain did not change in response to any oxidative insult (hydrogen peroxide or alkyl hydroperoxide) and the overexpression of an integrated copy of the wild-type txl1(+) gene did not confer a significant increased resistance against alkyl hydroperoxide. Overall, these results indicate that the Txl1 role in the cellular detoxification of alkyl hydroperoxide is exerted through a constitutive transcription of txl1(+).

Thioredoxins in chloroplasts

Thioredoxins (TRXs) are small disulfide oxidoreductases of ca. 12 kDa found in all free living organisms. In plants, two chloroplastic TRXs, named TRX f and TRX m, were originally identified as light dependent regulators of several carbon metabolism enzymes including Calvin cycle enzymes. The availability of genome sequences revealed an unsuspected multiplicity of TRXs in photosynthetic eukaryotes, including new chloroplastic TRX types. Moreover, proteomic approaches and focused studies allowed identification of 90 potential chloroplastic TRX targets. Lately, recent studies suggest the existence of a complex interplay between TRXs and other redox regulators such as glutaredoxins (GRXs) or glutathione. The latter is involved in a post-translational modification, named glutathionylation that could be controlled by GRXs. Glutathionylation appears to specifically affect the activity of TRX f and other chloroplastic enzymes and could thereby constitute a previously undescribed regulatory mechanism of photosynthetic metabolism under oxidative stress. After summarizing the initial studies on TRX f and TRX m, this review will focus on the most recent developments with special emphasis on the contributions of genomics and proteomics to the field of TRXs. Finally, new emerging interactions with other redox signaling pathways and perspectives for future studies will also be discussed.

Thioltransferases

A family of small molecular weight proteins with thiol-disulfide exchange activity have been discovered, widely distributed from E. coli to mammalian systems, called thioltransferases or glutaredoxins. There are no substantiated reports of thioltransferases-glutaredoxins in plants; however, partially purified dehydroascorbate reductase from peas had thiol-disulfide exchange catalytic activity using glutathione as reductant and S-sulfocysteine as thiosulfate cosubstrate (unpublished data). Thus, this class of proteins is universally distributed. Based on mutagenesis studies, a sequence of Cys-Pro-Tyr(Phe)-Cys- followed by Arg-Lys- or Lys alone is critical for both the thiol-disulfide exchange reaction and the dehydroascorbate reductase activity. The dithiol-disulfide loop represented by this structure is unique since the cystine closer to the N-terminus has a highly acidic thiol pKa (3.8 as determined for the pig liver enzyme) that contributes to the protein's high S- nucleophilicity. Compared with the microbial enzyme, the mammalian thioltransferases (glutaredoxins) are extended at both N and C termini by 10-12 amino acid residues, including a second pair of cysteines toward the C-terminus with no known special function. Yeast thioltransferase is more like mammalian enzymes in length (106 amino acids) but more like E. coli glutaredoxin in being unblocked at the N-terminus and having only one set of cysteines; that is, at the active center. The three mammalian enzymes, for which sequences are available, are blocked at the N-terminus by an acetyl group linked to alanine with no known special function other than possibly to impart greater cellular turnover stability. A report of carbohydrate (8.6%) content in rat liver thioltransferase has not been verified by more sensitive methods of carbohydrate analysis, nor has carbohydrate been identified in samples of purified glutaredoxin from any source. Thiol transferase and glutaredoxin are two names for the same protein based on similarity of amino acid sequence, immunochemical cross-reactivity, and other enzyme properties. The inability of thioltransferase from some mammalian sources to act as an electron carrier in ribonucleotide reductase systems, whether homologous or heterologous in origin, remains to be explained in future studies.

GPX2, encoding a phospholipid hydroperoxide glutathione peroxidase homologue, codes for an atypical 2-Cys peroxiredoxin in Saccharomyces cerevisiae

We have previously reported that Saccharomyces cerevisiae has three glutathione peroxidase homologues (GPX1, GPX2, and GPX3) (Inoue, Y., Matsuda, T., Sugiyama, K., Izawa, S., and Kimura, A. (1999) J. Biol. Chem. 274, 27002-27009). Of these, the GPX2 gene product (Gpx2) shows the greatest similarity to phospholipid hydroperoxide glutathione peroxidase. Here we show that GPX2 encodes an atypical 2-Cys peroxiredoxin which uses thioredoxin as an electron donor. Gpx2 was essentially in a reduced form even in mutants defective in glutathione reductase or glutaredoxin under oxidative stressed conditions. On the other hand, Gpx2 was partially oxidized in a mutant defective in cytosolic thioredoxin (trx1Deltatrx2Delta) under non-stressed conditions and completely oxidized in tert-butyl hydroperoxide-treated cells of trx1Deltatrx2Delta and thioredoxin reductase-deficient mutant cells. Alanine scanning of cysteine residues of Gpx2 revealed that an intramolecular disulfide bond was formed between Cys37 and Cys83 in vivo. Gpx2 was purified to determine whether it functions as a peroxidase that uses thioredoxin as an electron donor in vitro. Gpx2 reduced H2O2 and tert-butyl hydroperoxide in the presence of thioredoxin, thioredoxin reductase, and NADPH (for H2O2, Km= 20 microm, kcat = 9.57 x 10(2) s(-1); for tert-butyl hydroperoxide, Km= 62.5 microm, kcat = 3.68 x 10(2) s(-1)); however, it showed remarkably less activity toward these peroxides in the presence of glutathione, glutathione reductase, and NADPH. The sensitivity of yeast cells to tert-butyl hydroperoxide was found to be exacerbated by the co-existence of Ca2+, a tendency that was most obvious in gpx2Delta cells. Although the redox state of Gpx2 was not affected by Ca2+, the Gpx2 level was markedly increased in the presence of both tert-butyl hydroperoxide and Ca2+. Gpx2 is likely to play an important role in the protection of cells from oxidative stress in the presence of Ca2+.

Overlapping roles of the cytoplasmic and mitochondrial redox regulatory systems in the yeast Saccharomyces cerevisiae

Thioredoxins are small, highly conserved oxidoreductases which are required to maintain the redox homeostasis of the cell. Saccharomyces cerevisiae contains a cytoplasmic thioredoxin system (TRX1, TRX2, and TRR1) as well as a complete mitochondrial thioredoxin system, comprising a thioredoxin (TRX3) and a thioredoxin reductase (TRR2). In the present study we have analyzed the functional overlap between the two systems. By constructing mutant strains with deletions of both the mitochondrial and cytoplasmic systems (trr1 trr2 and trx1 trx2 trx3), we show that cells can survive in the absence of both systems. Analysis of the redox state of the cytoplasmic thioredoxins reveals that they are maintained independently of the mitochondrial system. Similarly, analysis of the redox state of Trx3 reveals that it is maintained in the reduced form in wild-type cells and in mutants lacking components of the cytoplasmic thioredoxin system (trx1 trx2 or trr1). Surprisingly, the redox state of Trx3 is also unaffected by the loss of the mitochondrial thioredoxin reductase (trr2) and is largely maintained in the reduced form unless cells are exposed to an oxidative stress. Since glutathione reductase (Glr1) has been shown to colocalize to the cytoplasm and mitochondria, we examined whether loss of GLR1 influences the redox state of Trx3. During normal growth conditions, deletion of TRR2 and GLR1 was found to result in partial oxidation of Trx3, indicating that both Trr2 and Glr1 are required to maintain the redox state of Trx3. The oxidation of Trx3 in this double mutant is even more pronounced during oxidative stress or respiratory growth conditions. Taken together, these data indicate that Glr1 and Trr2 have an overlapping function in the mitochondria.

Yeast signaling pathways in the oxidative stress response

Oxidative stress that generates the reactive oxygen species (ROS) is one of the major causes of DNA damage and mutations. The "DNA damage checkpoint" that arrests cell cycle and repairs damaged DNA has been a focus of recent studies, and the genetically amenable model systems provided by yeasts have been playing a leading role in the eukaryotic checkpoint research. However, means to eliminate ROS are likely to be as important as the DNA repair mechanisms in order to suppress mutations in the chromosomal DNA, and yeasts also serve as excellent models to understand how eukaryotes combat oxidative stress. In this article, we present an overview of the signaling pathways that sense oxidative stress and induce expression of various anti-oxidant genes in the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe and the pathogenic yeast Candida albicans. Three conserved signaling modules have been identified in the oxidative stress response of these diverse yeast species: the stress-responsive MAP kinase cascade, the multistep phosphorelay and the AP-1-like transcription factor. The structure and function of these signaling modules are discussed.

Monitoring disulfide bond formation in the eukaryotic cytosol

Glutathione is the most abundant low molecular weight thiol in the eukaryotic cytosol. The compartment-specific ratio and absolute concentrations of reduced and oxidized glutathione (GSH and GSSG, respectively) are, however, not easily determined. Here, we present a glutathione-specific green fluorescent protein–based redox probe termed redox sensitive YFP (rxYFP). Using yeast with genetically manipulated GSSG levels, we find that rxYFP equilibrates with the cytosolic glutathione redox buffer. Furthermore, in vivo and in vitro data show the equilibration to be catalyzed by glutaredoxins and that conditions of high intracellular GSSG confer to these a new role as dithiol oxidases. For the first time a genetically encoded probe is used to determine the redox potential specifically of cytosolic glutathione. We find it to be −289 mV, indicating that the glutathione redox status is highly reducing and corresponds to a cytosolic GSSG level in the low micromolar range. Even under these conditions a significant fraction of rxYFP is oxidized.

The thioredoxin h system of higher plants

In plants, thioredoxins h are encoded by a multigenic family of genes (eight in Arabidopsis thaliana, at least five in Populus sp.). The multiplicity of these isoforms raises the question of their specificity. This review focuses on thioredoxins h in two plant models: Arabidopsis and poplar. Thioredoxins h can be divided into three different subgroups according to the analysis of their primary structure. This paper describes the biochemical properties of each subgroup. Recent data in the field indicate that subgroup members differ by their subcellular localization as well as their reduction pathways suggesting specific functions for each subgroup. The development of proteomic tools has also increased considerably the number of potential thioredoxin targets, showing the importance of thioredoxins h in plants.

Transcriptional control of multidrug resistance in the yeast Saccharomyces

A major problem in chemotherapeutic treatment of many pathological conditions including cancer and fungal infections is the development of a multidrug-resistant state in the target cell. Saccharomyces cerevisiae cells can be isolated that have single genetic alterations that cause the resulting mutant strains to become tolerant of a wide range of compounds that would otherwise be toxic. These mutant cells are referred to as having a pleiotropic drug-resistant (Pdr) phenotype. Studies of these Pdr cells have demonstrated that mutations either within genes encoding transcriptional regulators or in their regulatory inputs lead to overexpression of downstream transporter proteins with associated multidrug resistance. This review is aimed at providing a framework for understanding the networks modulating expression of PDR genes in S. cerevisiae.

Regulation of redox homeostasis in the yeast Saccharomyces cerevisiae

An increasingly important area of research is based on sulphydryl chemistry, since the oxidation of -SH groups is one of the earliest observable events during oxidant-mediated damage and -SH groups play a critical role in the function of many macromolecular structures including enzymes, transcription factors and membrane proteins. Glutaredoxins and thioredoxins are small heat-stable oxidoreductases, conserved throughout evolution, which play key roles in maintaining the cellular redox balance. Much progress has been made in analysing these systems in the yeast Saccharomyces cerevisiae which is a very useful model eukaryote due to its ease of genetic manipulation, its compact genome, the availability of the entire genome sequence, and the current rate of progress in gene function research. Yeast, like all eukaryotes, contains a number of glutaredoxin and thioredoxin isoenzymes located in both the cytoplasm and the mitochondria. This review describes recent findings made in yeast that are leading to a better understanding of the regulation and role of redox homeostasis in eukaryotic cell metabolism.

Characterization of human thioredoxin-like 2. A novel microtubule-binding thioredoxin expressed predominantly in the cilia of lung airway epithelium and spermatid manchette and axoneme

We describe here the cloning and characterization of a novel member of the thioredoxin family, thioredoxin-like protein 2 (Txl-2). The Txl-2 open reading frame codes for a protein of 330 amino acids consisting of two distinct domains: an N-terminal domain typical of thioredoxins and a C-terminal domain belonging to the nucleoside-diphosphate kinase family, separated by a small interface domain. The Txl-2 gene spans approximately 28 kb, is organized into 11 exons, and maps at locus 3q22.3-q23. A splicing variant lacking exon 5 (Delta 5Txl-2) has also been isolated. By quantitative real time PCR we demonstrate that Txl-2 mRNA is ubiquitously expressed, with testis and lung having the highest levels of expression. Unexpectedly, light and electron microscopy analyses show that the protein is associated with microtubular structures such as lung airway epithelium cilia and the manchette and axoneme of spermatids. Using in vitro translated proteins, we demonstrate that full-length Txl-2 weakly associates with microtubules. In contrast, Delta 5Txl-2 specifically binds with very high affinity brain microtubule preparations containing microtubule-binding proteins. Importantly, Delta 5Txl-2 also binds to pure microtubules, proving that it possesses intrinsic microtubule binding capability. Taken together, Delta 5Txl-2 is the first thioredoxin reported to bind microtubules and might therefore be a novel regulator of microtubule physiology.

Non-reciprocal regulation of the redox state of the glutathione-glutaredoxin and thioredoxin systems

Our studies in yeast show that there is an essential requirement for either an active thioredoxin or an active glutathione (GSH)-glutaredoxin system for cell viability. Glutathione reductase (Glr1) and thioredoxin reductase (Trr1) are key regulatory enzymes that determine the redox state of the GSH-glutaredoxin and thioredoxin systems, respectively. Here we show that Trr1 is required during normal cell growth, whereas there is no apparent requirement for Glr1. Analysis of the redox state of thioredoxins and glutaredoxins in glr1 and trr1 mutants reveals that thioredoxins are maintained independently of the glutathione system. In contrast, there is a strong correlation between the redox state of glutaredoxins and the oxidation state of the GSSG/2GSH redox couple. We suggest that independent redox regulation of thioredoxins enables cells to survive in conditions under which the GSH-glutaredoxin system is oxidized.

Thioredoxins are required for protection against a reductive stress in the yeast Saccharomyces cerevisiae

Thioredoxins are small, highly conserved oxidoreductases that are required to maintain the redox homeostasis of the cell. They have been best characterized for their role as antioxidants in protection against reactive oxygen species. We show here that thioredoxins (TRX1, TRX2) and thioredoxin reductase (TRR1) are also required for protection against a reductive stress induced by exposure to dithiothreitol (DTT). This sensitivity to reducing conditions is not a general property of mutants affected in redox control, as mutants lacking components of the glutathione/glutaredoxin system are unaffected. Furthermore, TRX2 gene expression is induced in response to DTT treatment, indicating that thioredoxins form part of the cellular response to a reductive challenge. Our data indicate that the sensitivity of thioredoxin mutants to reducing stress appears to be a consequence of elevated glutathione levels, which is present predominantly in the reduced form (GSH). The elevated GSH levels also result in a constitutively high unfolded protein response (UPR), indicative of an accumulation of unfolded proteins in the endoplasmic reticulum (ER). However, there does not appear to be a general defect in ER function in thioredoxin mutants, as oxidative protein folding of the model protein carboxypeptidase Y occurs with similar kinetics to the wild-type strain, and trx1 trx2 mutants are unaffected in sensitivity to the glycosylation inhibitor tunicamycin. Furthermore, trr1 mutants are resistant to tunicamycin, consistent with their high UPR. The high UPR seen in trr1 mutants can be abrogated by the GSH-specific reagent 1-chloro-2,4-dinitrobenzene. In summary, thioredoxins are required to maintain redox homeostasis in response to both oxidative and reductive stress conditions.

Characterization and regulation of a second gene encoding thioredoxin from the fission yeast

A genomic DNA encoding a second thioredoxin (TRX2) was isolated from the chromosomal DNA of the fission yeast Schizosaccharomyces pombe. The cloned sequence contains 1823 bp and encodes a protein of 121 amino acids. It has extra N-terminal 17 amino acid residues compared to previously identified thioredoxin (TRX1), which are positively charged and hydrophobic amino acids. The additional N-terminal region contains a plausible prepeptidase cleavage site, indicating that the TRX2 protein exists in mitochondria. The cloned TRX2 gene produced functional TRX estimated with insulin reduction assay. The upstream region of the TRX2 gene was fused into the promoterless beta-galactosidase gene of the shuttle vector YEp357R. The 782 bp sequence in the region further upstream of the TRX2 gene was found to be inhibitory in its expression. Synthesis of beta-galactosidase from the fusion plasmid pYFX135-HRL was enhanced by the addition of aluminum chloride and ferrous chloride, indicating that the TRX2 protein is involved in stress response.

Role of thioredoxins in the response of Saccharomyces cerevisiae to oxidative stress induced by hydroperoxides

Glutaredoxins and thioredoxins are highly conserved, small, heat-stable oxidoreductases. The yeast Saccharomyces cerevisiae contains two gene pairs encoding cytoplasmic glutaredoxins (GRX1, GRX2) and thioredoxins (TRX1, TRX2), and we have used multiple mutants to determine their roles in mediating resistance to oxidative stress caused by hydroperoxides. Our data indicate that TRX2 plays the predominant role, as mutants lacking TRX2 are hypersensitive, and mutants containing TRX2 are resistant to these oxidants. However, the requirement for TRX2 is only apparent during stationary phase growth, and we present three lines of evidence that the thioredoxin isoenzymes actually have redundant activities as antioxidants. First, the trx1 and trx2 mutants show wild-type resistance to hydroperoxide during exponential phase growth; secondly, overexpression of either TRX1 or TRX2 leads to increased resistance to hydroperoxides; and, thirdly, both Trx1 and Trx2 are equally able to act as cofactors for the thioredoxin peroxidase, Tsa1. The antioxidant activity of thioredoxins is required for both the survival of yeast cells as well as protection against oxidative stress during stationary phase growth, and correlates with an increase in the expression of both TRX1 and TRX2. We show that the requirement for thioredoxins during this growth phase is dependent on their activity as cofactors for the antioxidant enzyme Tsa1, and for regulation of the redox state and protein-bound levels of the low-molecular-weight antioxidant glutathione.

Role of the glutathione/glutaredoxin and thioredoxin systems in yeast growth and response to stress conditions

Sulphydryl groups (-SH) play a remarkably broad range of roles in the cell, and the redox status of cysteine residues can affect both the structure and the function of numerous enzymes, receptors and transcription factors. The intracellular milieu is usually a reducing environment as a result of high concentrations of the low-molecular-weight thiol glutathione (GSH). However, reactive oxygen species (ROS), which are the products of normal aerobic metabolism, as well as naturally occurring free radical-generating compounds, can alter this redox balance. A number of cellular factors have been implicated in the regulation of redox homeostasis, including the glutathione/glutaredoxin and thioredoxin systems. Glutaredoxins and thioredoxins are ubiquitous small heat-stable oxidoreductases that have proposed functions in many cellular processes, including deoxyribonucleotide synthesis, repair of oxidatively damaged proteins, protein folding and sulphur metabolism. This review describes recent findings in the lower eukaryote Saccharomyces cerevisiae that are leading to a better understanding of their role in redox homeostasis in eukaryotic cell metabolism.

A single glutaredoxin or thioredoxin gene is essential for viability in the yeast Saccharomyces cerevisiae

Glutaredoxins and thioredoxins are small heat-stable oxidoreductases that have been conserved throughout evolution. The yeast Saccharomyces cerevisiae contains two gene pairs encoding cytoplasmic glutaredoxins (GRX1, GRX2) and thioredoxins (TRX1, TRX2). We report here that the quadruple trx1 trx2 grx1 grx2 mutant is inviable and that either a single glutaredoxin or a single thioredoxin (i.e. grx1 grx2 trx1, grx1 grx2 trx2, grx1 trx1 trx2, grx2 trx1 trx2) is essential for viability. Loss of both thioredoxins has been reported previously to lead to methionine auxotrophy consistent with thioredoxins being the sole reductants for 3'-phosphoadenosine 5'-phosphosulphate reductase (PAPS) in yeast. However, we present evidence for the existence of a novel yeast hydrogen donor for PAPS reductase, as strains lacking both thioredoxins assimilated sulphate under conditions that minimized the generation of reactive oxygen species (low aeration and absence of functional mitochondria). In addition, the assimilation of [35S]-sulphate was approximately 60-fold higher in the trx1 trx2 grx1 and trx1 trx2 grx2 mutants compared with the trx1 trx2 mutant. Furthermore, in contrast to the trx1 trx2 mutant, the trx1 trx2 grx2 mutant grew on minimal agar plates, and the trx1 trx2 grx1 mutant grew on minimal agar plates under anaerobic conditions. We propose a model in which the novel reductase activity normally functions in the repair of oxidant-mediated protein damage but, under conditions that minimize the generation of reactive oxygen species, it can serve as a hydrogen donor for PAPS reductase.

Isolation and characterization of Nrf1p, a novel negative regulator of the Cdc42p GTPase in Schizosaccharomyces pombe

The Cdc42p GTPase and its regulators, such as the Saccharomyces cerevisiae Cdc24p guanine-nucleotide exchange factor, control signal-transduction pathways in eukaryotic cells leading to actin rearrangements. A cross-species genetic screen was initiated based on the ability of negative regulators of Cdc42p to reverse the Schizosaccharomyces pombe Cdc42p suppression of a S. cerevisiae cdc24(ts) mutant. A total of 32 S. pombe nrf (negative regulator of Cdc forty two) cDNAs were isolated that reversed the suppression. One cDNA, nrf1(+), encoded an approximately 15 kD protein with three potential transmembrane domains and 78% amino-acid identity to a S. cerevisiae gene, designated NRF1. A S. pombe Deltanrf1 mutant was viable but overexpression of nrf1(+) in S. pombe resulted in dose-dependent lethality, with cells exhibiting an ellipsoidal morphology indicative of loss of polarized cell growth along with partially delocalized cortical actin and large vacuoles. nrf1(+) also displayed synthetic overdose phenotypes with cdc42 and pak1 alleles. Green fluorescent protein (GFP)-Cdc42p and GFP-Nrf1p colocalized to intracellular membranes, including vacuolar membranes, and to sites of septum formation during cytokinesis. GFP-Nrf1p vacuolar localization depended on the S. pombe Cdc24p homolog Scd1p. Taken together, these data are consistent with Nrf1p functioning as a negative regulator of Cdc42p within the cell polarity pathway.

The mitochondrial thioredoxin system

Eukaryotic organisms from yeast to human possess a mitochondrial thioredoxin system composed of thioredoxin and thioredoxin reductase, similar to the cytosolic thioredoxin system that exists in the same cells. Yeast and mammalian mitochondrial thioredoxins are monomers of approximately 12 kDa and contain the typical conserved active site WCGPC. However, there are important differences between yeast and mammalian mitochondrial thioredoxin reductases that resemble the differences between their cytosolic counterparts. Mammalian mitochondrial thioredoxin reductase is a selenoprotein that forms a homodimer of 55 kDa/subunit; while yeast mitochondrial thioredoxin reductase is a homodimer of 37 kDa/subunit and does not contain selenocysteine. A function of the mitochondrial thioredoxin system is as electron donor for a mitochondrial peroxiredoxin, an enzyme that detoxifies the hydrogen peroxide generated by the mitochondrial metabolism. Experiments with yeast mutants lacking both the mitochondrial thioredoxin system as well as the mitochondrial peroxiredoxin system suggest an important role for mitochondrial thioredoxin, thioredoxin reductase, and peroxiredoxin in the protection against oxidative stress.

Thioredoxin deficiency causes the constitutive activation of Yap1, an AP-1-like transcription factor in Saccharomyces cerevisiae

Yap1 is a transcription factor that responds to oxidative stress in Saccharomyces cerevisiae. The activity of Yap1 is regulated at the level of its intracellular localization, and a cysteine-rich domain at the C terminus of Yap1 is involved in this regulation. We investigated the effects of redox-regulatory proteins, thioredoxin and glutaredoxin, on the regulation of Yap1, using the deficient mutants of these thiol-disulfide oxidoreductases. In the thioredoxin-deficient mutant (trx1Delta/trx2Delta), Yap1 was constitutively concentrated in the nucleus and the level of expression of the Yap1 target genes was high under normal conditions, while this was not the case for the glutaredoxin-deficient mutant (grx1Delta/grx2Delta). No distinct difference was observed in the levels of Yap1 protein between the wild type and trx1Delta/trx2Delta. The constitutive activation of Yap1 in trxDelta/trx2Delta was observed under aerobic conditions but not under anaerobic conditions. These findings suggest that thioredoxin has negative effects on this regulation via the redox states. We also show the synthetic lethality between yap1Delta and trx1Delta/trx2Delta mutation, but the yap1Delta/grx1Delta/grx2Delta triple mutant was viable, suggesting a difference of the functions between thioredoxin and glutaredoxin and a genetic interaction between Yap1 and thioredoxin in vivo.

Activation of the Saccharomyces cerevisiae filamentation/invasion pathway by osmotic stress in high-osmolarity glycogen pathway mutants

Mitogen-activated protein kinase (MAPK) cascades are frequently used signal transduction mechanisms in eukaryotes. Of the five MAPK cascades in Saccharomyces cerevisiae, the high-osmolarity glycerol response (HOG) pathway functions to sense and respond to hypertonic stress. We utilized a partial loss-of-function mutant in the HOG pathway, pbs2-3, in a high-copy suppressor screen to identify proteins that modulate growth on high-osmolarity media. Three high-copy suppressors of pbs2-3 osmosensitivity were identified: MSG5, CAK1, and TRX1. Msg5p is a dual-specificity phosphatase that was previously demonstrated to dephosphorylate MAPKs in yeast. Deletions of the putative MAPK targets of Msg5p revealed that kss1delta could suppress the osmosensitivity of pbs2-3. Kss1p is phosphorylated in response to hyperosmotic shock in a pbs2-3 strain, but not in a wild-type strain nor in a pbs2-3 strain overexpressing MSG5. Both TEC1 and FRE::lacZ expressions are activated in strains lacking a functional HOG pathway during osmotic stress in a filamentation/invasion-pathway-dependent manner. Additionally, the cellular projections formed by a pbs2-3 mutant on high osmolarity are absent in strains lacking KSS1 or STE7. These data suggest that the loss of filamentation/invasion pathway repression contributes to the HOG mutant phenotype.

Identification and functional characterization of a novel mitochondrial thioredoxin system in Saccharomyces cerevisiae

The so-called thioredoxin system, thioredoxin (Trx), thioredoxin reductase (Trr), and NADPH, acts as a disulfide reductase system and can protect cells against oxidative stress. In Saccharomyces cerevisiae, two thioredoxins (Trx1 and Trx2) and one thioredoxin reductase (Trr1) have been characterized, all of them located in the cytoplasm. We have identified and characterized a novel thioredoxin system in S. cerevisiae. The TRX3 gene codes for a 14-kDa protein containing the characteristic thioredoxin active site (WCGPC). The TRR2 gene codes for a protein of 37 kDa with the active-site motif (CAVC) present in prokaryotic thioredoxin reductases and binding sites for NADPH and FAD. We cloned and expressed both proteins in Escherichia coli, and the recombinant Trx3 and Trr2 proteins were active in the insulin reduction assay. Trx3 and Trr2 proteins have N-terminal domain extensions with characteristics of signals for import into mitochondria. By immunoblotting analysis of Saccharomyces subcellular fractions, we provide evidence that these proteins are located in mitochondria. We have also constructed S. cerevisiae strains null in Trx3 and Trr2 proteins and tested them for sensitivity to hydrogen peroxide. The Deltatrr2 mutant was more sensitive to H2O2, whereas the Deltatrx3 mutant was as sensitive as the wild type. These results suggest an important role of the mitochondrial thioredoxin reductase in protection against oxidative stress in S. cerevisiae.

Saccharomyces cerevisiae ACR2 gene encodes an arsenate reductase

The ACR2 gene of Saccharomyces cerevisiae was disrupted by insertion of a HIS3 gene. Cells with the disruption were sensitive to arsenate. This phenotype could be complemented by ACR2 on a plasmid. The ACR2 gene was cloned and expressed in Escherichia coli as a malE gene fusion with a C-terminal histidine tag. The combination of chimeric MBP-Acr2-6H protein and yeast cytosol from an ACR2-disrupted strain exhibited arsenate reductase activity.

The yeast Saccharomyces cerevisiae contains two glutaredoxin genes that are required for protection against reactive oxygen species

Glutaredoxins are small heat-stable proteins that act as glutathione-dependent disulfide oxidoreductases. Two genes, designated GRX1 and GRX2, which share 40-52% identity and 61-76% similarity with glutaredoxins from bacterial and mammalian species, were identified in the yeast Saccharomyces cerevisiae. Strains deleted for both GRX1 and GRX2 were viable but lacked heat-stable oxidoreductase activity using beta-hydroxyethylene disulfide as a substrate. Surprisingly, despite the high degree of homology between Grx1 and Grx2 (64% identity), the grx1 mutant was unaffected in oxidoreductase activity, whereas the grx2 mutant displayed only 20% of the wild-type activity, indicating that Grx2 accounted for the majority of this activity in vivo. Expression analysis indicated that this difference in activity did not arise as a result of differential expression of GRX1 and GRX2. In addition, a grx1 mutant was sensitive to oxidative stress induced by the superoxide anion, whereas a strain that lacked GRX2 was sensitive to hydrogen peroxide. Sensitivity to oxidative stress was not attributable to altered glutathione metabolism or cellular redox state, which did not vary between these strains. The expression of both genes was similarly elevated under various stress conditions, including oxidative, osmotic, heat, and stationary phase growth. Thus, Grx1 and Grx2 function differently in the cell, and we suggest that glutaredoxins may act as one of the primary defenses against mixed disulfides formed following oxidative damage to proteins.

In vivo functional discrimination between plant thioredoxins by heterologous expression in the yeast Saccharomyces cerevisiae

Whereas vertebrates possess only two thioredoxin genes, higher plants present a much greater diversity of thioredoxins. For example, Arabidopsis thaliana has five cytoplasmic thioredoxins (type h) and at least as many chloroplastic thioredoxins. The abundance of plant thioredoxins leads to the question whether the various plant thioredoxins play a similar role or have specific functions. Because most of these proteins display very similar activities on artificial or biological substrates in vitro, we developed an in vivo approach to answer this question. The disruption of both of the two Saccharomyces cerevisiae thioredoxin genes leads to pleiotropic effects including methionine auxotrophy, H2O2 hypersensitivity, altered cell cycle characteristics, and a limited ability to use methionine sulfoxide as source of methionine. We expressed eight plant thioredoxins (six cytoplasmic and two chloroplastic) in yeast trx1, trx2 double mutant cells and analyzed the different phenotypes. Arabidopsis type h thioredoxin 2 efficiently restored sulfate assimilation whereas Arabidopsis type h thioredoxin 3 conferred H2O2 tolerance. All thioredoxins tested could complement for reduction of methionine sulfoxide, whereas only type h thioredoxins were able to complement the cell cycle defect. These findings clearly indicate that specific interactions between plant thioredoxins and their targets occur in vivo.