Saccharomyces cerevisiae cells have three Omega class glutathione S-transferases acting as 1-Cys thiol transferases

S-glutathionyl-(chloro)hydroquinone reductases: a new class of glutathione transferases functioning as oxidoreductases

Glutathione transferases (GSTs) are best known for transferring glutathione (GSH) to hydrophobic organic compounds, making the conjugates more soluble. However, the omega-class GSTs of animals and the lambda-class GSTs and dehydroascorbate reductases (DHARs) of plants have little or no activity for GSH transfer. Instead, they catalyze GSH-dependent oxidoreductions. The lambda-class GSTs reduce disulfide bonds, the DHARs reduce the disulfide bonds and dehydroascorbate, and the omega-class GSTs can reduce more substrates, including disulfide bonds, dehydroascorbate, and dimethylarsinate. Glutathionyl-(chloro)hydroquinone reductases (GS-HQRs) are the newest class of GSTs that mainly catalyze oxidoreductions. Besides the activities of the other three classes, GS-HQRs also reduce GS-hydroquinones, including GS-trichloro-p-hydroquinone, GS-dichloro-p-hydroquinone, GS-2-hydroxy-p-hydroquinone, and GS-p-hydroquinone. They are conserved and widely distributed in bacteria, fungi, protozoa, and plants, but not in animals. The four classes are phylogenetically more related to each other than to other GSTs, and they share a Cys-Pro motif at the GSH-binding site. Hydroquinones are metabolic intermediates of certain aromatic compounds. They can be auto-oxidized by O(2) to benzoquinones, which spontaneously react with GSH to form GS-hydroquinones via Michael's addition. GS-HQRs are expected to channel GS-hydroquinones, formed spontaneously or enzymatically, back to hydroquinones. When the released hydroquinones are intermediates of metabolic pathways, GS-HQRs play a maintenance role for the pathways. Further, the common presence of GS-HQRs in plants, green algae, cyanobacteria, and halobacteria suggest a beneficial role in the light-using organisms.

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

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

Selenite-induced cell death in Saccharomyces cerevisiae: protective role of glutaredoxins

Unlike in higher organisms, selenium is not essential for growth in Saccharomyces cerevisiae. In this species, it causes toxic effects at high concentrations. In the present study, we show that when supplied as selenite to yeast cultures growing under fermentative metabolism, its effects can be dissected into two death phases. From the time of initial treatment, it causes loss of membrane integrity and genotoxicity. Both effects occur at higher levels in mutants lacking Grx1p and Grx2p than in wild-type cells, and are reversed by expression of a cytosolic version of the membrane-associated Grx7p glutaredoxin. Grx7p can also rescue the high levels of protein carbonylation damage that occur in selenite-treated cultures of the grx1 grx2 mutant. After longer incubation times, selenite causes abnormal nuclear morphology and the appearance of TUNEL-positive cells, which are considered apoptotic markers in yeast cells. This effect is independent of Grx1p and Grx2p. Therefore, the protective role of the two glutaredoxins is restricted to the initial stages of selenite treatment. Lack of Yca1p metacaspase or of a functional mitochondrial electron transport chain only moderately diminishes apoptotic-like death by selenite. In contrast, selenite-induced apoptosis is dependent on the apoptosis-inducing factor Aif1p. In the absence of the latter, intracellular protein carbonylation is reduced after prolonged selenite treatment, supporting the supposition that part of the oxidative damage is contributed by apoptotic cells.

S-Glutathionyl-(chloro)hydroquinone reductases: a novel class of glutathione transferases

Sphingobium chlorophenolicum completely mineralizes PCP (pentachlorophenol). Two GSTs (glutathione transferases), PcpC and PcpF, are involved in the degradation. PcpC uses GSH to reduce TeCH (tetrachloro-p-hydroquinone) to TriCH (trichloro-p-hydroquinone) and then to DiCH (dichloro-p-hydroquinone) during PCP degradation. However, oxidatively damaged PcpC produces GS-TriCH (S-glutathionyl-TriCH) and GS-DiCH (S-glutathionyl-TriCH) conjugates. PcpF converts the conjugates into TriCH and DiCH, re-entering the degradation pathway. PcpF was further characterized in the present study. It catalysed GSH-dependent reduction of GS-TriCH via a Ping Pong mechanism. First, PcpF reacted with GS-TriCH to release TriCH and formed disulfide bond between its Cys53 residue and the GS moiety. Then, a GSH came in to regenerate PcpF and release GS-SG. A TBLASTN search revealed that PcpF homologues were widely distributed in bacteria, halobacteria (archaea), fungi and plants, and they belonged to ECM4 (extracellular mutant 4) group COG0435 in the conserved domain database. Phylogenetic analysis grouped PcpF and homologues into a distinct group, separated from Omega class GSTs. The two groups shared conserved amino acid residues, for GSH binding, but had different residues for the binding of the second substrate. Several recombinant PcpF homologues and two human Omega class GSTs were produced in Escherichia coli and purified. They had zero or low activities for transferring GSH to standard substrates, but all had reasonable activities for GSH-dependent reduction of disulfide bond (thiol transfer), dehydroascorbate and dimethylarsinate. All the tested PcpF homologues reduced GS-TriCH, but the two Omega class GSTs did not. Thus PcpF homologues were tentatively named S-glutathionyl-(chloro)hydroquinone reductases for catalysing the GSH-dependent reduction of GS-TriCH.

The enigma of the CLIC proteins: Ion channels, redox proteins, enzymes, scaffolding proteins?

Chloride intracellular channel proteins (CLICs) are distinct from most ion channels in that they have both soluble and integral membrane forms. CLICs are highly conserved in chordates, with six vertebrate paralogues. CLIC-like proteins are found in other metazoans. CLICs form channels in artificial bilayers in a process favoured by oxidising conditions and low pH. They are structurally plastic, with CLIC1 adopting two distinct soluble conformations. Phylogenetic and structural data indicate that CLICs are likely to have enzymatic function. The physiological role of CLICs appears to be maintenance of intracellular membranes, which is associated with tubulogenesis but may involve other substructures.

Dissection of glutathione conjugate turnover in yeast

Xenobiotics are widely used as pesticides. The detoxification of xenobiotics frequently involves conjugation to glutathione prior to compartmentalization and catabolism. In plants, degradation of glutathione-S-conjugates is initiated either by aminoterminal or carboxyterminal amino acid cleavage catalyzed by a gamma-glutamyl transpeptidase and phytochelatin synthase, respectively. In order to establish yeast as a model system for the analysis of the plant pathway, we used monochlorobimane as a model xenobiotic in Saccharomyces cerevisiae and mutants thereof. The catabolism of monochlorobimane is initiated by conjugation to form glutathione-S-bimane, which is then turned over into a gamma-GluCys-bimane conjugate by the vacuolar serine carboxypeptidases CPC and CPY. Alternatively, the glutathione-S-bimane conjugate is catabolized by the action of the gamma-glutamyl transpeptidase Cis2p to a CysGly-conjugate. The turnover of glutathione-S-bimane was impaired in yeast cells deficient in Cis2p and completely abolished by the additional inactivation of CPC and CPY in the corresponding triple knockout. Inducible expression of the Arabidopsis phytochelatin synthase AtPCS1 in the triple knockout resulted in the turnover of glutathione-S-bimane to the gamma-GluCys-bimane conjugate as observed in plants. Challenge of AtPCS1-expressing yeast cells with zinc, cadmium, and copper ions, which are known to activate AtPCS1, enhanced gamma-GluCys-bimane accumulation. Thus, initial catabolism of glutathione-S-conjugates is similar in plants and yeast, and yeast is a suitable system for a study of enzymes of the plant pathway.

The fungal glutathione S-transferase system. Evidence of new classes in the wood-degrading basidiomycete Phanerochaete chrysosporium

The recent release of several basidiomycete genome sequences allows an improvement of the classification of fungal glutathione S-transferases (GSTs). GSTs are well-known detoxification enzymes which can catalyze the conjugation of glutathione to non-polar compounds that contain an electrophilic carbon, nitrogen, or sulfur atom. Following this mechanism, they are able to metabolize drugs, pesticides, and many other xenobiotics and peroxides. A genomic and phylogenetic analysis of GST classes in various sequenced fungi--zygomycetes, ascomycetes, and basidiomycetes--revealed some particularities in GST distribution, in comparison with previous analyses with ascomycetes only. By focusing essentially on the wood-degrading basidiomycete Phanerochaete chrysosporium, this analysis highlighted a new fungal GST class named GTE, which is related to bacterial etherases, and two new subclasses of the omega class GSTs. Moreover, our phylogenetic analysis suggests a relationship between the saprophytic behavior of some fungi and the number and distribution of some GST isoforms within specific classes.

Novel omega glutathione S-transferases in disk abalone: Characterization and protective roles against environmental stress

Omega glutathione S-transferases (GSTs) are a newly identified class of GSTs with unique properties compared to other members in GST superfamily. This present study reports the cloning, characterization and stress-induced expression analysis of two omega GST genes in disk abalone, Haliotis discus discus. Two disk abalone omega GST genes, HdGSTO1 and HdGSTO2, encode two polypeptides with calculated molecular mass of 27.4 and 26.9 kDa, respectively. Their deduced amino acid sequences showed highest similarity with another molluscan omega GST from Crassostrea gigas. Three-dimensional structures of two omega GSTs were generated by homology modeling and exhibited typical omega GST structural characteristics. The recombinant proteins of HdGSTO1 and HdGSTO2 showed glutathione-dependent thioltransferase and dehydroascorbate reductase activities; however, no activity towards other common GST substrates was detected. Of the two genes, protein encoded by HdGSTO1 showed much higher catalytic ability than the other one. HdGSTO1 mRNA was expressed ubiquitously with high levels in all examined tissues, while HdGSTO2 showed specific expression in gonad and digestive tract. The transcriptional levels of HdGSTO1 in gill were dramatically elevated when abalones were subjected to heat shock, heavy metals and endocrine-disrupting chemical (EDC) exposure, indicating that HdGSTO1 might play important protective roles against environmental stress. HdGSTO2 expression was also significantly induced by heavy metals and EDCs although with much lower fold change than HdGSTO1. But under thermal stress, HdGSTO2 expression was repressed in a time-dependent pattern, implying its different physiological roles under stress. These results indicate that omega GSTs of the disk abalone, especially HdGSTO1, have great potentials as highly sensitive biomarkers of environmental stress.

An atlas of the thioredoxin fold class reveals the complexity of function-enabling adaptations

The group of proteins that contain a thioredoxin (Trx) fold is huge and diverse. Assessment of the variation in catalytic machinery of Trx fold proteins is essential in providing a foundation for understanding their functional diversity and predicting the function of the many uncharacterized members of the class. The proteins of the Trx fold class retain common features-including variations on a dithiol CxxC active site motif-that lead to delivery of function. We use protein similarity networks to guide an analysis of how structural and sequence motifs track with catalytic function and taxonomic categories for 4,082 representative sequences spanning the known superfamilies of the Trx fold. Domain structure in the fold class is varied and modular, with 2.8% of sequences containing more than one Trx fold domain. Most member proteins are bacterial. The fold class exhibits many modifications to the CxxC active site motif-only 56.8% of proteins have both cysteines, and no functional groupings have absolute conservation of the expected catalytic motif. Only a small fraction of Trx fold sequences have been functionally characterized. This work provides a global view of the complex distribution of domains and catalytic machinery throughout the fold class, showing that each superfamily contains remnants of the CxxC active site. The unifying context provided by this work can guide the comparison of members of different Trx fold superfamilies to gain insight about their structure-function relationships, illustrated here with the thioredoxins and peroxiredoxins.

Structures of yeast glutathione-S-transferase Gtt2 reveal a new catalytic type of GST family

Glutathione-S-transferases (GSTs) are ubiquitous detoxification enzymes that catalyse the conjugation of electrophilic substrates to glutathione. Here, we present the crystal structures of Gtt2, a GST of Saccharomyces cerevisiae, in apo and two ligand-bound forms, at 2.23 A, 2.20 A and 2.10 A, respectively. Although Gtt2 has the overall structure of a GST, the absence of the classic catalytic essential residues--tyrosine, serine and cysteine--distinguishes it from all other cytosolic GSTs of known structure. Site-directed mutagenesis in combination with activity assays showed that instead of the classic catalytic residues, a water molecule stabilized by Ser129 and His123 acts as the deprotonator of the glutathione sulphur atom. Furthermore, only glycine and alanine are allowed at the amino-terminus of helix-alpha1 because of stereo-hindrance. Taken together, these results show that yeast Gtt2 is a novel atypical type of cytosolic GST.

Sugar metabolism, redox balance and oxidative stress response in the respiratory yeast Kluyveromyces lactis

A lot of studies have been carried out on Saccharomyces cerevisiae, an yeast with a predominant fermentative metabolism under aerobic conditions, which allows exploring the complex response induced by oxidative stress. S. cerevisiae is considered a eukaryote model for these studies. We propose Kluyveromyces lactis as a good alternative model to analyse variants in the oxidative stress response, since the respiratory metabolism in this yeast is predominant under aerobic conditions and it shows other important differences with S. cerevisiae in catabolic repression and carbohydrate utilization. The knowledge of oxidative stress response in K. lactis is still a developing field. In this article, we summarize the state of the art derived from experimental approaches and we provide a global vision on the characteristics of the putative K. lactis components of the oxidative stress response pathway, inferred from their sequence homology with the S. cerevisiae counterparts. Since K. lactis is also a well-established alternative host for industrial production of native enzymes and heterologous proteins, relevant differences in the oxidative stress response pathway and their potential in biotechnological uses of this yeast are also reviewed.

Novel glutaredoxin activity of the yeast prion protein Ure2 reveals a native-like dimer within fibrils

Ure2 is the protein determinant of the Saccharomyces cerevisiae prion [URE3]. Ure2 has structural similarity to glutathione transferases, protects cells against heavy metal and oxidant toxicity in vivo, and shows glutathione-dependent peroxidase activity in vitro. Here we report that Ure2 (which has no cysteine residues) also shows thiol-disulfide oxidoreductase activity similar to that of glutaredoxin enzymes. This demonstrates that disulfide reductase activity can be independent of the classical glutaredoxin CXXC/CXXS motif or indeed an intrinsic catalytic cysteine residue. The kinetics of the glutaredoxin activity of Ure2 showed positive cooperativity for the substrate glutathione in both the soluble native state and in amyloid-like fibrils, indicating native-like dimeric structure within Ure2 fibrils. Characterization of the glutaredoxin activity of Ure2 sheds light on its ability to protect yeast from heavy metal ion and oxidant toxicity and suggests a role in reversible protein glutathionylation signal transduction. Observation of allosteric enzyme behavior within amyloid-like Ure2 fibrils not only provides insight into the molecular structure of the fibrils but also has implications for the mechanism of [URE3] prion formation.

The glutathione system of Aspergillus nidulans involves a fungus-specific glutathione S-transferase

The tripeptide glutathione is involved in cellular defense mechanisms for xenobiotics and reactive oxygen species. This study investigated glutathione-dependent mechanisms in the model organism Aspergillus nidulans. A recombinant dimeric protein of A. nidulans glutathione reductase (GR) contained FAD and reduced oxidized glutathione (GSSG) using NADPH as an electron donor. A deletion strain of the GR gene (glrA) accumulated less intracellular reduced glutathione (GSH), indicating that the fungal GR contributes to GSSG reduction in vivo. Growth of the deletion strain of glrA was temperature-sensitive, and this phenotype was suppressed by adding GSH to the medium. The strain subsequently accumulated more intracellular superoxide, and cell-free respiration activity was partly defective. Growth of the strain decreased in the presence of oxidants, which induced glrA expression 1.5-6-fold. These results indicated that the fungal glutathione system functions as an antioxidant mechanism in A. nidulans. Our findings further revealed an initial proteomic differential display on GR-depleted and wild type strains. Up-regulation of thioredoxin reductase, peroxiredoxins, catalases, and cytochrome c peroxidase in the glrA-deletion strain revealed interplay between the glutathione system and both the thioredoxin system and hydrogen peroxide defense mechanisms. We also identified a hypothetical, up-regulated protein in the GR-depleted strains as glutathione S-transferase, which is unique among Ascomycetes fungi.

Involvement of glutathione transferases, Gtt1and Gtt2, with oxidative stress response generated by H2O2 during growth of Saccharomyces cerevisiae

Glutathione transferases are detoxifying enzymes responsible for eliminating toxic compounds generated under a variety of stress conditions. Saccharomyces cerevisiae control cells and glutathione transferase mutant strains (gtt1 and gtt2) were used to analyze tolerance, lipid and protein oxidation as oxidative stress markers during growth in the presence of H2O2. Glucose 6-phosphate dehydrogenase (G6PD) and glutathione reductase were assayed to monitor the capacity of cells to recycle glutathione. Although a reduction in growth was observed, deletion of GTT1 showed less inhibition by H2O2 than the control strain. Cells showed a significant reduction in cellular viability during the first hours of growth, the gtt1 mutant being hypersensitive even after 24 h of H2O2 exposure. As a consequence of oxidative stress caused by exposure to H2O2, an increase in lipid peroxidation was observed, mainly in the glutathione transferase mutant strains. While protein carbonylation increased by 17% and 23%, respectively, after 2 h in the presence of H2O2 in the control and gtt2 mutant, a 40% increase was observed in the gtt1 strain after 24-h exposure. The antioxidant G6PD and glutathione reductase activities were affected in the gtt1 mutant during H2O2 exposure, which could be critical for recycling glutathione. The same was observed for the gtt2 mutant after 2-h treatment, indicating that glutathione recycling might be associated with the detoxification process. Thus, glutathione transferases, Gtt1 and Gtt2, seem to be crucial in the response to H2O2 stress.

Nutrient-regulated antisense and intragenic RNAs modulate a signal transduction pathway in yeast

The budding yeast Saccharomyces cerevisiae alters its gene expression profile in response to a change in nutrient availability. The PHO system is a well-studied case in the transcriptional regulation responding to nutritional changes in which a set of genes (PHO genes) is expressed to activate inorganic phosphate (Pi) metabolism for adaptation to Pi starvation. Pi starvation triggers an inhibition of Pho85 kinase, leading to migration of unphosphorylated Pho4 transcriptional activator into the nucleus and enabling expression of PHO genes. When Pi is sufficient, the Pho85 kinase phosphorylates Pho4, thereby excluding it from the nucleus and resulting in repression (i.e., lack of transcription) of PHO genes. The Pho85 kinase has a role in various cellular functions other than regulation of the PHO system in that Pho85 monitors whether environmental conditions are adequate for cell growth and represses inadequate (untimely) responses in these cellular processes. In contrast, Pho4 appears to activate some genes involved in stress response and is required for G1 arrest caused by DNA damage. These facts suggest the antagonistic function of these two players on a more general scale when yeast cells must cope with stress conditions. To explore general involvement of Pho4 in stress response, we tried to identify Pho4-dependent genes by a genome-wide mapping of Pho4 and Rpo21 binding (Rpo21 being the largest subunit of RNA polymerase II) using a yeast tiling array. In the course of this study, we found Pi- and Pho4-regulated intragenic and antisense RNAs that could modulate the Pi signal transduction pathway. Low-Pi signal is transmitted via certain inositol polyphosphate (IP) species (IP7) that are synthesized by Vip1 IP6 kinase. We have shown that Pho4 activates the transcription of antisense and intragenic RNAs in the KCS1 locus to down-regulate the Kcs1 activity, another IP6 kinase, by producing truncated Kcs1 protein via hybrid formation with the KCS1 mRNA and translation of the intragenic RNA, thereby enabling Vip1 to utilize more IP6 to synthesize IP7 functioning in low-Pi signaling. Because Kcs1 also can phosphorylate these IP7 species to synthesize IP8, reduction in Kcs1 activity can ensure accumulation of the IP7 species, leading to further stimulation of low-Pi signaling (i.e., forming a positive feedback loop). We also report that genes apparently not involved in the PHO system are regulated by Pho4 either dependent upon or independent of the Pi conditions, and many of the latter genes are involved in stress response. In S. cerevisiae, a large-scale cDNA analysis and mapping of RNA polymerase II binding using a high-resolution tiling array have identified a large number of antisense RNA species whose functions are yet to be clarified. Here we have shown that nutrient-regulated antisense and intragenic RNAs as well as direct regulation of structural gene transcription function in the response to nutrient availability. Our findings also imply that Pho4 is present in the nucleus even under high-Pi conditions to activate or repress transcription, which challenges our current understanding of Pho4 regulation.

Cytotoxicity mechanism of two naphthoquinones (menadione and plumbagin) in Saccharomyces cerevisiae

Background

Quinones are compounds extensively used in studies of oxidative stress due to their role in plants as chemicals for defense. These compounds are of great interest for pharmacologists and scientists, in general, because several cancer chemotherapeutic agents contain the quinone nucleus. However, due to differences in structures and diverse pharmacological effects, the exact toxicity mechanisms exerted by quinones are far from elucidatation.

Methodology/Principal Findings

Using Saccharomyces cerevisiae, we evaluated the main mechanisms of toxicity of two naphthoquinones, menadione and plumbagin, by determining tolerance and oxidative stress biomarkers such as GSH and GSSG, lipid peroxidation levels, as well as aconitase activity. The importance of glutathione transferases (GST) in quinone detoxification was also addressed. The GSSG/GSH ratio showed that menadione seemed to exert its toxicity mainly through the generation of ROS while plumbagin acted as an electrophile reacting with GSH. However, the results showed that, even by different pathways, both drugs were capable of generating oxidative stress through their toxic effects. Our results showed that the control strain, BY4741, and the glutathione transferase deficient strains (gtt1Δ and gtt2Δ) were sensitive to both compounds. With respect to the role of GST isoforms in cellular protection against quinone toxicity, we observed that the Gtt2 deficient strain was unable to overcome lipid peroxidation, even after a plumbagin pre-treatment, indicating that this treatment did not improve tolerance when compared with the wild type strain. Cross-tolerance experiments confirmed distinct cytotoxicity mechanisms for these naphthoquinones since only a pre-treatment with menadione was able to induce acquisition of tolerance against stress with plumbagin.

Conclusions/Significance

These results suggest different responses to menadione and plumbagin which could be due to the fact that these compounds use different mechanisms to exert their toxicity. In addition, the Gtt2 isoform seemed to act as a general protective factor involved in quinone detoxification.

Saccharomyces cerevisiae Grx6 and Grx7 are monothiol glutaredoxins associated with the early secretory pathway

Saccharomyces cerevisiae Grx6 and Grx7 are two monothiol glutaredoxins whose active-site sequences (CSYS and CPYS, respectively) are reminiscent of the CPYC active-site sequence of classical dithiol glutaredoxins. Both proteins contain an N-terminal transmembrane domain which is responsible for their association to membranes of the early secretory pathway vesicles, facing the luminal side. Thus, Grx6 localizes at the endoplasmic reticulum and Golgi compartments, while Grx7 is mostly at the Golgi. Expression of GRX6 is modestly upregulated by several stresses (calcium, sodium, and peroxides) in a manner dependent on the Crz1-calcineurin pathway. Some of these stresses also upregulate GRX7 expression under the control of the Msn2/4 transcription factor. The N glycosylation inhibitor tunicamycin induces the expression of both genes along with protein accumulation. Mutants lacking both glutaredoxins display reduced sensitivity to tunicamycin, although the drug is still able to manifest its inhibitory effect on a reporter glycoprotein. Grx6 and Grx7 have measurable oxidoreductase activity in vivo, which is increased in the presence of tunicamycin. Both glutaredoxins could be responsible for the regulation of the sulfhydryl oxidative state at the oxidant conditions of the early secretory pathway vesicles. However, the differences in location and expression responses against stresses suggest that their functions are not totally overlapping.

The multiple functions of the thiol-based electron flow pathways of Escherichia coli: Eternal concepts revisited

Electron flow via thiols is a theme with many variations in all kingdoms of life. The favourable physichochemical properties of the redox active couple of two cysteines placed in the optimised environment of the thioredoxin fold allow for two electron transfers in between top biological reductants and ultimate oxidants. The reduction of ribonucleotide reductases by thioredoxin and thioredoxin reductase of Escherichia coli (E. coli) was one of the first pathways to be elucidated. Diverse functions such as protein folding in the periplasm, maturation of respiratory enzymes, detoxification of hydrogen peroxide and prevention of oxidative damage may be based on two electron transfers via thiols. A growing field is the relation of thiol reducing pathways and the interaction of E. coli with different organisms. This concept combined with the sequencing of the genomes of different bacteria may allow for the identification of fine differences in the systems employing thiols for electron flow between pathogens and their corresponding mammalian hosts. The emerging possibility is the development of novel antibiotics.

Oxidative stress in Caenorhabditis elegans: protective effects of the Omega class glutathione transferase (GSTO-1)

To elucidate the function of Omega class glutathione transferases (GSTs) (EC 2.5.1.18) in multicellular organisms, the GSTO-1 from Caenorhabditis elegans (GSTO-1; C29E4.7) was investigated. Disc diffusion assays using Escherichia coli overexpressing GSTO-1 provided a test of resistance to long-term exposure under oxidative stress. After affinity purification, the recombinant GSTO-1 had minimal catalytic activity toward classic GST substrates but displayed significant thiol oxidoreductase and dehydroascorbate reductase activity. Microinjection of the GSTO-1-promoter green fluorescent protein construct and immunolocalization by electron microscopy localized the protein exclusively in the intestine of all postembryonic stages of C. elegans. Deletion analysis identified an approximately 300-nucleotide sequence upstream of the ATG start site necessary for GSTO-1 expression. Site-specific mutagenesis of a GATA transcription factor binding motif in the minimal promoter led to the loss of reporter expression. Similarly, RNA interference (RNAi) of Elt-2 indicated the involvement of this gut-specific transcription factor in GSTO-1 expression. Transcriptional up-regulation under stress conditions of GSTO-1 was confirmed by analyzing promoter-reporter constructs in transgenic C. elegans strains. To investigate the function of GSTO-1 in vivo, transgenic animals overexpressing GSTO-1 were generated exhibiting an increased resistance to juglone-, paraquat-, and cumene hydroperoxide-induced oxidative stress. Specific silencing of the GSTO-1 by RNAi created worms with an increased sensitivity to several prooxidants, arsenite, and heat shock. We conclude that the stress-responsive GSTO-1 plays a key role in counteracting environmental stress.

Mechanisms of reversible protein glutathionylation in redox signaling and oxidative stress

Reversible protein S-glutathionylation (protein-SSG) is an important post-translational modification, providing protection of protein cysteines from irreversible oxidation and serving to transduce redox signals. Analogous to phosphatases, glutaredoxin (GRx) enzymes catalyze deglutathionylation of proteins, regulating diverse intracellular signaling pathways. Recently, other enzymes have been reported to exhibit deglutathionylating activity, but their contribution to intracellular protein deglutathionylation is uncertain. Currently, no enzyme has been shown to serve as a catalyst of S-glutathionylation in situ, although potential prototypes are reported, including human GRx1 and the pi isoform of glutathione-S-transferase (GSTpi). Further insight into cellular mechanisms of protein glutathionylation and deglutathionylation will enrich our understanding of redox signal transduction and potentially identify new therapeutic targets for diseases in which oxidative stress perturbs normal redox signaling. Accordingly, this review focuses primarily on mechanisms of catalysis in mammalian systems.

Menadione stress in Saccharomyces cerevisiae strains deficient in the glutathione transferases

Using S. cerevisiae as a eukaryotic cell model we have analyzed the involvement of both glutathione transferase isoforms, Gtt1 and Gtt2, in constitutive resistance and adaptive response to menadione, a quinone which can exert its toxicity as redox cycling and/or electrophiles. The detoxification properties, of these enzymes, have also been analyzed by the appearance of S-conjugates in the media. Direct exposure to menadione (20 mM/60 min) showed to be lethal for cells deficient on both Gtt1 and Gtt2 isoforms. However, after pre-treatment with a low menadione concentration, cells deficient in Gtt2 displayed reduced ability to acquire tolerance when compared with the control and the Gtt1 deficient strains. Analyzing the toxic effects of menadione we observed that the gtt2 mutant showed no reduction in lipid peroxidation levels. Moreover, measuring the levels of intracellular oxidation during menadione stress we have shown that the increase of this oxidative stress parameter was due to the capacity menadione possesses in generating reactive oxygen species (ROS) and that both GSH and Gtt2 isoform were required to enhance ROS production. Furthermore, the efflux of the menadione-GSH conjugate, which is related with detoxification of xenobiotic pathways, was not detected in the gtt2 mutant. Taken together, these results suggest that acquisition of tolerance against stress generated by menadione and the process of detoxification through S-conjugates are dependent upon Gtt2 activity. This assessment was corroborated by the increase of GTT2 expression, and not of GTT1, after menadione treatment.

Glutaredoxins in fungi

Glutaredoxins (GRXs) can be subdivided into two subfamilies: dithiol GRXs with the CPY/FC active site motif, and monothiol GRXs with the CGFS motif. Both subfamilies share a thioredoxin-fold structure. Some monothiol GRXs exist with a single-Grx domain while others have a thioredoxin-like domain (Trx) and one or more Grx domains in tandem. Most fungi have both dithiol and monothiol GRXs with different subcellular locations. GRX-like molecules also exist in fungi that differ by one residue from one of the canonical active site motifs. Additionally, Omega-class glutathione transferases (GSTs) are active as GRXs. Among fungi, the GRXs more extensively studied are those from Saccharomyces cerevisiae. This organism contains two dithiol GRXs (ScGrx1 and ScGrx2) with partially overlapping functions in defence against oxidative stress. In this function, they cooperate with GSTs Gtt1 and Gtt2. While ScGrx1 is cytosolic, two pools exist for ScGrx2, a major one at the cytosol and a minor one at mitochondria. On the other hand, S. cerevisiae cells have two monothiol GRXs with the Trx-Grx structure (ScGrx3 and ScGrx4) that locate at the nucleus and probably regulate the activity of transcription factors such as Aft1, and one monothiol GRX with the Grx structure (ScGrx5) that localizes at the mitochondrial matrix, where it participates in the synthesis of iron-sulphur clusters. The function of yeast Grx5 seems to be conserved along the evolutionary scale.

A peroxisomal glutathione transferase of Saccharomyces cerevisiae is functionally related to sulfur amino acid metabolism

Saccharomyces cerevisiae cells contain three omega-class glutathione transferases with glutaredoxin activity (Gto1, Gto2, and Gto3), in addition to two glutathione transferases (Gtt1 and Gtt2) not classifiable into standard classes. Gto1 is located at the peroxisomes, where it is targeted through a PTS1-type sequence, whereas Gto2 and Gto3 are in the cytosol. Among the GTO genes, GTO2 shows the strongest induction of expression by agents such as diamide, 1-chloro-2,4-dinitrobenzene, tert-butyl hydroperoxide or cadmium, in a manner that is dependent on transcriptional factors Yap1 and/or Msn2/4. Diamide and 1-chloro-2,4-dinitrobenzene (causing depletion of reduced glutathione) also induce expression of GTO1 over basal levels. Phenotypic analyses with single and multiple mutants in the S. cerevisiae glutathione transferase genes show that, in the absence of Gto1 and the two Gtt proteins, cells display increased sensitivity to cadmium. A gto1-null mutant also shows growth defects on oleic acid-based medium, which is indicative of abnormal peroxisomal functions, and altered expression of genes related to sulfur amino acid metabolism. As a consequence, growth of the gto1 mutant is delayed in growth medium without lysine, serine, or threonine, and the mutant cells have low levels of reduced glutathione. The role of Gto1 at the S. cerevisiae peroxisomes could be related to the redox regulation of the Str3 cystathionine beta-lyase protein. This protein is also located at the peroxisomes in S. cerevisiae, where it is involved in transulfuration of cysteine into homocysteine, and requires a conserved cysteine residue for its biological activity.