Importance of Post-translational Modifications in the Interaction of Proteins with Mineral Surfaces: The Case of Arginine Methylation and Silica surfaces.

Understanding the mechanisms involved in the interaction of proteins with inorganic surfaces is of major interest for both basic research and practical applications involving nanotechnology. From the list of cellular proteins with the highest affinity for silica nanoparticles, we highlighted the group of proteins containing arginine-glycine-glycine (RGG) motifs. Biochemical experiments confirmed that RGG motifs interact strongly with the silica surfaces. The affinity of these motifs is further increased when the R residue is asymmetrically, but not symmetrically, dimethylated. Molecular dynamics simulations show that the asymmetrical dimethylation generates an electrostatic asymmetry in the guanidinium group of the R residue, orientating and stabilizing it on the silica surface. The RGG motifs (methylated or not) systematically target the siloxide groups on the silica surface through an ionic interaction, immediately strengthened by hydrogen bonds with proximal silanol and siloxane groups. Given that, in vivo, RGG motifs are often asymmetrically dimethylated by specific cellular methylases, our data add support to the idea that this type of methylation is a key mechanism for cells to regulate the interaction of the RGG proteins with their cellular partners.

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.

Endoplasmic reticulum is a major target of cadmium toxicity in yeast.

Cadmium (Cd(2+)) is a very toxic metal that causes DNA damage, oxidative stress and apoptosis. Despite many studies, the cellular and molecular mechanisms underlying its high toxicity are not clearly understood. We show here that very low doses of Cd(2+) cause ER stress in Saccharomyces cerevisiae as evidenced by the induction of the unfolded protein response (UPR) and the splicing of HAC1 mRNA. Furthermore, mutant strains (Delta ire1 and Delta hac1) unable to induce the UPR are hypersensitive to Cd(2+), but not to arsenite and mercury. The full functionality of the pathways involved in ER stress response is required for Cd(2+) tolerance. The data also suggest that Cd(2+)-induced ER stress and Cd(2+) toxicity are a direct consequence of Cd(2+) accumulation in the ER. Cd(2+) does not inhibit disulfide bond formation but perturbs calcium metabolism. In particular, Cd(2+) activates the calcium channel Cch1/Mid1, which also contributes to Cd(2+) entry into the cell. The results reinforce the interest of using yeast as a cellular model to study toxicity mechanisms in eukaryotic cells.

H2O2 activates the nuclear localization of Msn2 and Maf1 through thioredoxins in Saccharomyces cerevisiae.

The cellular response to hydrogen peroxide (H(2)O(2)) is characterized by a repression of growth-related processes and an enhanced expression of genes important for cell defense. In budding yeast, this response requires the activation of a set of transcriptional effectors. Some of them, such as the transcriptional activator Yap1, are specific to oxidative stress, and others, such as the transcriptional activators Msn2/4 and the negative regulator Maf1, are activated by a wide spectrum of stress conditions. How these general effectors are activated in response to oxidative stress remains an open question. In this study, we demonstrate that the two cytoplasmic thioredoxins, Trx1 and Trx2, are essential to trigger the nuclear accumulation of Msn2/4 and Maf1, specifically under H(2)O(2) treatment. Contrary to the case with many stress conditions previously described for yeast, the H(2)O(2)-induced nuclear accumulation of Msn2 and Maf1 does not correlate with the downregulation of PKA kinase activity. Nevertheless, we show that PP2A phosphatase activity is essential for driving Maf1 dephosphorylation and its subsequent nuclear accumulation in response to H(2)O(2) treatment. Interestingly, under this condition, the lack of PP2A activity has no impact on the subcellular localization of Msn2, demonstrating that the H(2)O(2) signaling pathways share a common route through the thioredoxin system and then diverge to activate Msn2 and Maf1, the final integrators of these pathways.

Peroxiredoxin promotes longevity and H2O2-resistance in yeast through redox-modulation of protein kinase A.

Peroxiredoxins are H2O2 scavenging enzymes that also carry out H2O2 signaling and chaperone functions. In yeast, the major cytosolic peroxiredoxin, Tsa1 is required for both promoting resistance to H2O2 and extending lifespan upon caloric restriction. We show here that Tsa1 effects both these functions not by scavenging H2O2, but by repressing the nutrient signaling Ras-cAMP-PKA pathway at the level of the protein kinase A (PKA) enzyme. Tsa1 stimulates sulfenylation of cysteines in the PKA catalytic subunit by H2O2 and a significant proportion of the catalytic subunits are glutathionylated on two cysteine residues. Redox modification of the conserved Cys243 inhibits the phosphorylation of a conserved Thr241 in the kinase activation loop and enzyme activity, and preventing Thr241 phosphorylation can overcome the H2O2 sensitivity of Tsa1-deficient cells. Results support a model of aging where nutrient signaling pathways constitute hubs integrating information from multiple aging-related conduits, including a peroxiredoxin-dependent response to H2O2.

Development of a new method for absolute protein quantification on 2-D gels.

With the development of systems biology projects aimed at modeling the cell, accurate and absolute measurements of cellular protein concentrations are increasingly important. However, methods for absolute quantification at the proteomic level remain rare. Using the yeast Saccharomyces cerevisiae, we propose a new method based on the radioactive labeling with an (35)S compound and 2-D PAGE. The principle is simple: cells are grown for more than four generations in the presence of a unique sulfur source labeled at a defined specific radioactivity, ensuring that more than 90% of the proteins are labeled at the same specific radioactivity as the sulfur source. After separation of (35)S-labeled proteins on 2-D gels, each protein is counted. The amount of each protein present in the gel is then calculated, from which is deduced the amount of each protein per cell. The method, limited to soluble and abundant proteins visible on 2-D gels, is simple, precise and reproducible and does not require an internal standard. We use it to compare the amounts of proteins in two growth conditions: 100 microM sulfate or 500 microM methionine. Up to now, we only had transcriptional data on the expression of these proteins in both conditions.

Quantitative transcriptome, proteome, and sulfur metabolite profiling of the Saccharomyces cerevisiae response to arsenite.

Arsenic is ubiquitously present in nature, and various mechanisms have evolved enabling cells to evade toxicity and acquire tolerance. Herein, we explored how Saccharomyces cerevisiae (budding yeast) respond to trivalent arsenic (arsenite) by quantitative transcriptome, proteome, and sulfur metabolite profiling. Arsenite exposure affected transcription of genes encoding functions related to protein biosynthesis, arsenic detoxification, oxidative stress defense, redox maintenance, and proteolytic activity. Importantly, we observed that nearly all components of the sulfate assimilation and glutathione biosynthesis pathways were induced at both gene and protein levels. Kinetic metabolic profiling evidenced a significant increase in the pools of sulfur metabolites as well as elevated cellular glutathione levels. Moreover, the flux in the sulfur assimilation pathway as well as the glutathione synthesis rate strongly increased with a concomitant reduction of sulfur incorporation into proteins. By combining comparative genomics and molecular analyses, we pinpointed transcription factors that mediate the core of the transcriptional response to arsenite. Taken together, our data reveal that arsenite-exposed cells channel a large part of assimilated sulfur into glutathione biosynthesis, and we provide evidence that the transcriptional regulators Yap1p and Met4p control this response in concert.

Ionizing radiation induces a Yap1-dependent peroxide stress response in yeast.

Repair of DNA damage is fundamental for cellular tolerance to ionizing radiation (IR) and many IR-induced DNA lesions are thought to occur as a result of oxidative stress. We investigated the physiological effects of IR in Saccharomyces cerevisiae by performing protein expression profiles in cells exposed to electron pulse irradiation. Transient induction of several antioxidant enzymes in wild-type cells, but not in cells lacking the oxidative stress regulator Yap1, indicated that IR exposure causes cellular oxidative stress. Yap1 activation involved oxidation to the intramolecular disulfide bond, a signature of activation by peroxide, and was dependent on the Yap1 peroxide sensor Orp1/Gpx3. H(2)O(2) was produced in the culture medium of irradiated cells and was both necessary and sufficient for IR-induced Yap1 activation. When IR was performed in the presence of N(2)O, obviating H(2)O(2) production and increasing hydroxyl radical ((*)OH) production, the Yap1 response was lost, indicating that Yap1 was unable to respond to (*)OH or (*)OH-induced damage. However, the Yap1 response to IR did not seem to be a primary determinant of cellular IR tolerance. Altogether, these data provide a molecular demonstration that cells experience in vivo peroxide stress during IR and indicate that the H(2)O(2) produced cannot account for IR toxicity.

RNA-binding proteins are a major target of silica nanoparticles in cell extracts.

Upon contact with biological fluids, nanoparticles (NPs) are readily coated by cellular compounds, particularly proteins, which are determining factors for the localization and toxicity of NPs in the organism. Here, we improved a methodological approach to identify proteins that adsorb on silica NPs with high affinity. Using large-scale proteomics and mixtures of soluble proteins prepared either from yeast cells or from alveolar human cells, we observed that proteins with large unstructured region(s) are more prone to bind on silica NPs. These disordered regions provide flexibility to proteins, a property that promotes their adsorption. The statistical analyses also pointed to a marked overrepresentation of RNA-binding proteins (RBPs) and of translation initiation factors among the adsorbed proteins. We propose that silica surfaces, which are mainly composed of Si-O- and Si-OH groups, mimic ribose-phosphate molecules (rich in -O- and -OH) and trap the proteins able to interact with ribose-phosphate containing molecules. Finally, using an in vitro assay, we showed that the sequestration of translation initiation factors by silica NPs results in an inhibition of the in vitro translational activity. This result demonstrates that characterizing the protein corona of various NPs would be a relevant approach to predict their potential toxicological effects.

Glutathione is essential to preserve nuclear function and cell survival under oxidative stress.

Organisms growing in aerobic environments must cope with Reactive Oxygen Species (ROS). Although ROS damage all the cellular macromolecules, they play a central role in a range of biological processes requiring a tight control of redox homeostasis. It is achieved by antioxidant systems involving a large collection of enzymes that scavenge or degrade the ROS produced endogenously during cell growth. In addition to this enzymatic protection against ROS, cells also contain small antioxidant molecules, such as glutathione (GSH). With an intracellular concentration between 1 and 10mM, GSH is the most abundant non-protein thiol in the cell and is considered as the major redox buffer of the cell. To better characterize its essential function during oxidative stress conditions, we studied the physiological response of H2O2-treated yeast cells containing different amounts of GSH. We showed that the transcriptional response of GSH-depleted cells is severely impaired, despite an efficient nuclear accumulation of the transcription factor Yap1. Moreover, oxidative stress generates high genome instability in GSH-depleted cells, but does not activate the checkpoint kinase Rad53. Surprisingly, scarce amounts of intracellular GSH are sufficient to preserve cell viability under H2O2 treatment. In these cells, oxidative stress still causes the accumulation of oxidized proteins and the inactivation of the translational activity, but nuclear DNA and nuclear functions are protected against oxidative injury, as exemplified by low mutation frequency, moderate histone carbonylation, activation of the checkpoint kinase Rad53 and of the H2O2 transcriptional response. We conclude that the essential role of GSH is to preserve nuclear function, allowing cell survival and growth resumption after oxidative stress release. We propose that cytosolic proteins are part of a protective machinery that shields the nucleus by scavenging reactive oxygen species before they can cross the nuclear membrane.

Glutathione is essential to preserve nuclear function and cell survival under oxidative stress.

Glutathione (GSH) is considered the most important redox buffer of the cell. To better characterize its essential function during oxidative stress conditions, we studied the physiological response of H2O2-treated yeast cells containing various amounts of GSH. We showed that the transcriptional response of GSH-depleted cells is severely impaired, despite an efficient nuclear accumulation of the transcription factor Yap1. Moreover, oxidative stress generates high genome instability in GSH-depleted cells, but does not activate the checkpoint kinase Rad53. Surprisingly, scarce amounts of intracellular GSH are sufficient to preserve cell viability under H2O2 treatment. In these cells, oxidative stress still causes the accumulation of oxidized proteins and the inactivation of the translational activity, but nuclear components and activities are protected against oxidative injury. We conclude that the essential role of GSH is to preserve nuclear function, allowing cell survival and growth resumption after oxidative stress release. We propose that cytosolic proteins are part of a protective machinery that shields the nucleus by scavenging reactive oxygen species before they can cross the nuclear membrane.

Structural determinants for protein adsorption/non-adsorption to silica surface.

The understanding of the mechanisms involved in the interaction of proteins with inorganic surfaces is of major interest in both fundamental research and applications such as nanotechnology. However, despite intense research, the mechanisms and the structural determinants of protein/surface interactions are still unclear. We developed a strategy consisting in identifying, in a mixture of hundreds of soluble proteins, those proteins that are adsorbed on the surface and those that are not. If the two protein subsets are large enough, their statistical comparative analysis must reveal the physicochemical determinants relevant for adsorption versus non-adsorption. This methodology was tested with silica nanoparticles. We found that the adsorbed proteins contain a higher number of charged amino acids, particularly arginine, which is consistent with involvement of this basic amino acid in electrostatic interactions with silica. The analysis also identified a marked bias toward low aromatic amino acid content (phenylalanine, tryptophan, tyrosine and histidine) in adsorbed proteins. Structural analyses and molecular dynamics simulations of proteins from the two groups indicate that non-adsorbed proteins have twice as many π-π interactions and higher structural rigidity. The data are consistent with the notion that adsorption is correlated with the flexibility of the protein and with its ability to spread on the surface. Our findings led us to propose a refined model of protein adsorption.

Chromate causes sulfur starvation in yeast.

Chromate is a widespread pollutant as a waste of human activities. However, the mechanisms underlying its high toxicity are not clearly understood. In this work, we used the yeast Saccharomyces cerevisiae to analyse the physiological effects of chromate exposure in a eukaryote cell model. We show that chromate causes a strong decrease of sulfate assimilation and sulfur metabolite pools suggesting that cells experience sulfur starvation. As a consequence, nearly all enzymes of the sulfur pathway are highly induced as well as enzymes of the sulfur-sparing response such as Pdc6, the sulfur-poor pyruvate decarboxylase. The induction of Pdc6 was regulated at the mRNA level and dependent upon Met32, a coactivator of Met4, the transcriptional activator of the sulfur pathway. Finally, we found that chromate enters the cells mainly through sulfate transporters and competitively inhibits sulfate uptake. Also consistent with a competition between the two substrates, sulfate supplementation relieves chromate toxicity. However, the data suggest that the chromate-mediated sulfur depletion is not simply due to this competitive uptake but would also be the consequence of competitive metabolism between the two compounds presumably at another step of the sulfur assimilation pathway.

Combined proteome and metabolite-profiling analyses reveal surprising insights into yeast sulfur metabolism.

Metabolomics is considered as an emerging new tool for functional proteomics in the identification of new protein function or in projects aiming at modeling whole cell metabolism. When combined with proteome studies, metabolite-profiling analyses revealed unanticipated insights into the yeast sulfur pathway. In response to cadmium, the observed overproduction of glutathione, essential for the detoxification of the metal, can be entirely accounted for by a marked drop in sulfur-containing protein synthesis and a redirection of sulfur metabolite fluxes to the glutathione pathway. A kinetic analysis showed sequential and dramatic changes in intermediate sulfur metabolite pools within the first hours of the treatment. Strikingly, whereas proteome and metabolic data were positively correlated under cadmium conditions, proteome and metabolic data were negatively correlated during other growth conditions, i.e. methionine supplementation or sulfate starvation. These differences can be explained by alternative mechanisms in the regulation of Met4, the activator of the sulfur pathway. Whereas Met4 activity is controlled by the cellular cysteine content in response to sulfur source and availability, the present study suggests that Met4 activation under cadmium conditions is cysteine-independent. The results clearly indicate that the metabolic state of a cell cannot be safely predicted based solely on proteomic and/or gene expression data. Combined metabolome and proteome studies are necessary to draw a comprehensive and integrated view of cell metabolism.

Transcriptional, proteomic, and metabolic responses to lithium in galactose-grown yeast cells.

Lithium is highly toxic to yeast when grown in galactose medium mainly because phosphoglucomutase, a key enzyme of galactose metabolism, is inhibited. We studied the global protein and gene expression profiles of Saccharomyces cerevisiae grown in galactose in different time intervals after addition of lithium. These results were related to physiological studies where both secreted and intracellular metabolites were determined. Microarray analysis showed that 664 open reading frames were down-regulated and 725 up-regulated in response to addition of lithium. Genes involved in transcription, translation, and nucleotide metabolism were down-regulated at the transcriptional level, whereas genes responsive to different stresses as well as genes from energy reserve metabolism and monosaccharide metabolism were up-regulated. Compared with the proteomic data, 26% of the down-regulated and 48% of the up-regulated proteins were also identified as being changed on the mRNA level. Functional clusters obtained from proteome data were coincident with transcriptional clusters. Physiological studies showed that acetate, glycerol, and glycogen accumulate in response to lithium, as reflected in expression data, whereas a change from respiro-fermentative to respiratory growth could not be predicted from the expression analyses.

Involvement of superoxide dismutases in the response of Escherichia coli to selenium oxides.

Selenium can provoke contrasting effects on living organisms. It is an essential trace element, and low concentrations have beneficial effects, such as the reduction of the incidence of cancer. However, higher concentrations of selenium salts can be toxic and mutagenic. The bases for both toxicity and protection are not clearly understood. To provide insights into these mechanisms, we analyzed the proteomic response of Escherichia coli cells to selenate and selenite treatment under aerobic conditions. We identified 23 proteins induced by both oxides and ca. 20 proteins specifically induced by each oxide. A striking result was the selenite induction of 8 enzymes with antioxidant properties, particularly the manganese and iron superoxide dismutases (SodA and SodB). The selenium inductions of sodA and sodB were controlled by the transcriptional regulators SoxRS and Fur, respectively. Strains with decreased superoxide dismutase activities were severely impaired in selenium oxide tolerance. Pretreatment with a sublethal selenite concentration triggered an adaptive response dependent upon SoxRS, conferring increased selenite tolerance. Altogether, our data indicate that superoxide dismutase activity is essential for the cellular defense against selenium salts, suggesting that superoxide production is a major mechanism of selenium toxicity under aerobic conditions.

Assessing factors for reliable quantitative proteomics based on two-dimensional gel electrophoresis.

We statistically analysed various factors to get accurate estimates of protein quantities from two-dimensional gels. Yeast proteins were labelled with (35)S or stained with Coomassie Brilliant Blue G-250, and spots were automatically quantified with software packages Kepler, ImageQuaNT, Melanie 3.0 and Progenesis. The different software packages proved to have very similar performances. With (35)S-labelled actin spot as a reference, we studied the staining efficiency of colloidal Coomassie blue as a function of amino acid composition of the protein, and derived an equation to estimate the number of molecules per cell from blue-stained proteins. Absolute quantification of most glycolytic enzymes was carried out in two yeast strains.

Sulfur sparing in the yeast proteome in response to sulfur demand.

Genome-wide studies have recently revealed the unexpected complexity of the genetic response to apparently simple physiological changes. Here, we show that when yeast cells are exposed to Cd(2+), most of the sulfur assimilated by the cells is converted into glutathione, a thiol-metabolite essential for detoxification. Cells adapt to this vital metabolite requirement by modifying globally their proteome to reduce the production of abundant sulfur-rich proteins. In particular, some abundant glycolytic enzymes are replaced by sulfur-depleted isozymes. This global change in protein expression allows an overall sulfur amino acid saving of 30%. This proteomic adaptation is essentially regulated at the mRNA level. The main transcriptional activator of the sulfate assimilation pathway, Met4p, plays an essential role in this sulfur-sparing response.

Identification in Saccharomyces cerevisiae of a new stable variant of alkyl hydroperoxide reductase 1 (Ahp1) induced by oxidative stress.

Yeasts lacking cytoplasmic superoxide dismutase (Cu,Zn-SOD) activity are permanently subjected to oxidative stress. We used two-dimensional PAGE to examine the proteome pattern of Saccharomyces cerevisiae strains lacking Cu,Zn-SOD. We found a new stable form of alkyl hydroperoxide reductase 1 (Ahp1) with a lower isoelectric point. This form was also present in wild type strains after treatment with tert-butyl hydroperoxide. In vitro enzyme assays showed that Ahp1p had lower specific activity in strains lacking Cu,Zn-SOD. We studied three mutants presenting a reduced production of the low pI variant under oxidative stress conditions. Two of the mutants (C62S and S59D) were totally inactive, thus suggesting that the acidic form of Ahp1p may only appear when the enzyme is functional. The other mutant (S59A) was active in vitro and was more resistant to inactivation by tert-butyl hydroperoxide than the wild type enzyme. Furthermore, the inactivation of Ahp1p in vitro is correlated with its conversion to the low pI form. These results suggest that in vivo during some particular oxidative stress (alkyl hydroperoxide treatment or lack of Cu,Zn-SOD activity but not hydrogen peroxide treatment), the catalytic cysteine of Ahp1p is more oxidized than cysteine-sulfenic acid (a natural occurring intermediate of the enzymatic reaction) and that cysteine-sulfinic acid or cysteine-sulfonic acid variant may be inactive.