Glutathione revisited: a vital function in iron metabolism and ancillary role in thiol-redox control.

Glutathione contributes to thiol-redox control and to extra-mitochondrial iron-sulphur cluster (ISC) maturation. To determine the physiological importance of these functions and sort out those that account for the GSH requirement for viability, we performed a comprehensive analysis of yeast cells depleted of or containing toxic levels of GSH. Both conditions triggered an intense iron starvation-like response and impaired the activity of extra-mitochondrial ISC enzymes but did not impact thiol-redox maintenance, except for high glutathione levels that altered oxidative protein folding in the endoplasmic reticulum. While iron partially rescued the ISC maturation and growth defects of GSH-depleted cells, genetic experiments indicated that unlike thioredoxin, glutathione could not support by itself the thiol-redox duties of the cell. We propose that glutathione is essential by its requirement in ISC assembly, but only serves as a thioredoxin backup in cytosolic thiol-redox maintenance. Glutathione-high physiological levels are thus meant to insulate its cytosolic function in iron metabolism from variations of its concentration during redox stresses, a model challenging the traditional view of it as prime actor in thiol-redox control.

Glutathione degradation is a key determinant of glutathione homeostasis.

Glutathione (GSH) has several important functions in eukaryotic cells, and its intracellular concentration is tightly controlled. Combining mathematical models and (35)S labeling, we analyzed Saccharomyces cerevisiae sulfur metabolism. This led us to the observation that GSH recycling is markedly faster than previously estimated. We set up additional in vivo assays and concluded that under standard conditions, GSH half-life is around 90 min. Sulfur starvation and growth with GSH as the sole sulfur source strongly increase GSH degradation, whereas cadmium (Cd(2+)) treatment inhibits GSH degradation. Whatever the condition tested, GSH is degraded by the cytosolic Dug complex (composed of the three subunits Dug1, Dug2, and Dug3) but not by the γ-glutamyl-transpeptidase, raising the question of the role of this enzyme. In vivo, both DUG2/3 mRNA levels and Dug activity are quickly induced by sulfur deprivation in a Met4-dependent manner. This suggests that Dug activity is mainly regulated at the transcriptional level. Finally, analysis of dug2Δ and dug3Δ mutant cells shows that GSH degradation activity strongly impacts on GSH intracellular concentration and that GSH intracellular concentration does not affect GSH synthesis rate. Altogether, our data led us to reconsider important aspects of GSH metabolism, challenging notions on GSH synthesis and GSH degradation that were considered as established.

The alternative pathway of glutathione degradation is mediated by a novel protein complex involving three new genes in Saccharomyces cerevisiae.

Glutathione (GSH), L-gamma-glutamyl-L-cysteinyl-glycine, is the major low-molecular-weight thiol compound present in almost all eukaryotic cells. GSH degradation proceeds through the gamma-glutamyl cycle that is initiated, in all organisms, by the action of gamma-glutamyl transpeptidase. A novel pathway for the degradation of GSH that requires the participation of three previously uncharacterized genes is described in the yeast Saccharomyces cerevisiae. These genes have been named DUG1 (YFR044c), DUG2 (YBR281c), and DUG3 (YNL191w) (defective in utilization of glutathione). Although dipeptides and tripeptides with a normal peptide bond such as cys-gly or glu-cys-gly required the presence of only a functional DUG1 gene that encoded a protein belonging to the M20A metallohydrolase family, the presence of an unusual peptide bond such as in the dipeptide, gamma-glu-cys, or in GSH, required the participation of the DUG2 and DUG3 gene products as well. The DUG2 gene encodes a protein with a peptidase domain and a large WD40 repeat region, while the DUG3 gene encoded a protein with a glutamine amidotransferase domain. The Dug1p, Dug2p, and Dug3p proteins were found to form a degradosomal complex through Dug1p-Dug2p and Dug2p-Dug3p interactions. A model is proposed for the functioning of the Dug1p/Dug2p/Dug3p proteins as a specific GSH degradosomal complex.

The system biology of thiol redox system in Escherichia coli and yeast: differential functions in oxidative stress, iron metabolism and DNA synthesis.

By its ability to engage in a variety of redox reactions and coordinating metals, cysteine serves as a key residue in mediating enzymatic catalysis, protein oxidative folding and trafficking, and redox signaling. The thiol redox system, which consists of the glutathione and thioredoxin pathways, uses the cysteine residue to catalyze thiol-disulfide exchange reactions, thereby controlling the redox state of cytoplasmic cysteine residues and regulating the biological functions it subserves. Here, we consider the thiol redox systems of Escherichia coli and Saccharomyces cerevisiae, emphasizing the role of genetic approaches in the understanding of the cellular functions of these systems. We show that although prokaryotic and eukaryotic systems have a similar architecture, they profoundly differ in their overall cellular functions.

Utilization of glutathione as an exogenous sulfur source is independent of gamma-glutamyl transpeptidase in the yeast Saccharomyces cerevisiae: evidence for an alternative gluathione degradation pathway.

gamma-Glutamyl transpeptidase (gamma-GT) is the only enzyme known to be responsible for glutathione degradation in living cells. In the present study we provide evidence that the utilization of glutathione can occur in the absence of gamma-GT. When disruptions in the CIS2 gene encoding gamma-GT were created in met15Delta strains, which require organic sulfur sources for growth, the cells were able to grow well with glutathione as the sole sulfur source suggesting that a gamma-GT-independent pathway for glutathione degradation exists in yeast cells. The CIS2 gene was strongly repressed by ammonium and derepressed in glutamate medium, and was found to be regulated by the nitrogen regulatory circuit. The utilization of glutathione as a sulfur source was, however, independent of the nitrogen source in the medium, further underlining that the two degradatory pathways were distinct.

Redox-sensitive YFP sensors monitor dynamic nuclear and cytosolic glutathione redox changes.

Intracellular redox homeostasis is crucial for many cellular functions but accurate measurements of cellular compartment-specific redox states remain technically challenging. To better characterize redox control in the nucleus, we targeted a yellow fluorescent protein-based redox sensor (rxYFP) to the nucleus of the yeast Saccharomyces cerevisiae. Parallel analyses of the redox state of nucleus-rxYFP and cytosol-rxYFP allowed us to monitor distinctively dynamic glutathione (GSH) redox changes within these two compartments under a given condition. We observed that the nuclear GSH redox environment is highly reducing and similar to the cytosol under steady-state conditions. Furthermore, these sensors are able to detect redox variations specific for their respective compartments in glutathione reductase (Glr1) and thioredoxin pathway (Trr1, Trx1, Trx2) mutants that have altered subcellular redox environments. Our mutant redox data provide in vivo evidence that glutathione and the thioredoxin redox systems have distinct but overlapping functions in controlling subcellular redox environments. We also monitored the dynamic response of nucleus-rxYFP and cytosol-rxYFP to GSH depletion and to exogenous low and high doses of H2O2 bursts. These observations indicate a rapid and almost simultaneous oxidation of both nucleus-rxYFP and cytosol-rxYFP, highlighting the robustness of the rxYFP sensors in measuring real-time compartmental redox changes. Taken together, our data suggest that the highly reduced yeast nuclear and cytosolic redox states are maintained independently to some extent and under distinct but subtle redox regulation. Nucleus- and cytosol-rxYFP register compartment-specific localized redox fluctuations that may involve exchange of reduced and/or oxidized glutathione between these two compartments. Finally, we confirmed that GSH depletion has profound effects on mitochondrial genome stability but little effect on nuclear genome stability, thereby emphasizing that the critical requirement for GSH during growth is linked to a mitochondria-dependent process.

Proteome screens for Cys residues oxidation: the redoxome.

The oxidation of the cysteine (Cys) residue to sulfenic (-S-OH), disulfide (-S-S-), or S-nitroso (S-NO) forms are thought to be a posttranslational modifications that regulate protein function. However, despite a few solid examples of its occurrence, thiol-redox regulation of protein function is still debated and often seen as an exotic phenomenon. A systematic and exhaustive characterization of all oxidized Cys residues, an experimental approach called redox proteomics or redoxome analysis, should help establish the physiological scope of Cys residue oxidation and give clues to its mechanisms. Redox proteomics still remains a technical challenge, mainly because of the labile nature of thiol-redox reactions and the lack of tools to directly detect the modified residues. Here we consider recent technical advances in redox proteomics, focusing on a gel-based fluorescent method and on the shotgun OxICAT technique.

Dug1p Is a Cys-Gly peptidase of the gamma-glutamyl cycle of Saccharomyces cerevisiae and represents a novel family of Cys-Gly peptidases.

GSH metabolism in yeast is carried out by the gamma-glutamyl cycle as well as by the DUG complex. One of the last steps in the gamma-glutamyl cycle is the cleavage of Cys-Gly by a peptidase to the constitutent amino acids. Saccharomyces cerevisiae extracts carry Cys-Gly dipeptidase activity, but the corresponding gene has not yet been identified. We describe the isolation and characterization of a novel Cys-Gly dipeptidase, encoded by the DUG1 gene. Dug1p had previously been identified as part of the Dug1p-Dug2p-Dug3p complex that operates as an alternate GSH degradation pathway and has also been suggested to function as a possible di- or tripeptidase based on genetic studies. We show here that Dug1p is a homodimer that can also function in a Dug2-Dug3-independent manner as a dipeptidase with high specificity for Cys-Gly and no activity toward tri- or tetrapeptides in vitro. This activity requires zinc or manganese ions. Yeast cells lacking Dug1p (dug1Delta) accumulate Cys-Gly. Unlike all other Cys-Gly peptidases, which are members of the metallopeptidase M17, M19, or M1 families, Dug1p is the first to belong to the M20A family. We also show that the Dug1p Schizosaccharomyces pombe orthologue functions as the exclusive Cys-Gly peptidase in this organism. The human orthologue CNDP2 also displays Cys-Gly peptidase activity, as seen by complementation of the dug1Delta mutant and by biochemical characterization, which revealed a high substrate specificity and affinity for Cys-Gly. The results indicate that the Dug1p family represents a novel class of Cys-Gly dipeptidases.

Investigations into the polymorphisms at the ECM38 locus of two widely used Saccharomyces cerevisiae S288C strains, YPH499 and BY4742.

The ECM38 gene encodes the gamma-glutamyl transpeptidase enzyme, an enzyme involved in glutathione turnover. The enzyme was found to be present in the S288C strain, BY4742, but absent in another widely used strain congenic to S288C, YPH499. Cloning and sequencing the genes from these yeasts indicated the presence of 11 single nucleotide polymorphisms in the coding region and eight single nucleotide polymorphisms in the promoter region of the ECM38 gene of YPH499 (but none in that of BY4742). One of the SNPs in the ECM38 ORF led to a G --> D conversion in a region conserved in all gamma-GT enzymes and was found to be responsible for the loss of activity in this strain. The presence of gamma-GT activity in other YPH strains led us to trace the origins of the polymorphisms in YPH499. Our results indicated that among the progenitor strains, YPH1 and YPH2, YPH1 carried the polymorphisms seen in YPH499 and also lacked the gamma-GT activity. The implications of these results for the use of these widely used S288C strains and the origin of these single nucleotide polymorphisms are presented.