The monothiol glutaredoxin Grx4 exerts an iron-dependent inhibitory effect on Php4 function

The deletion of ppr2 interferes iron sensing and leads to oxidative stress response in Schizosaccharomyces pombe

Pentatricopeptide repeat proteins are involved in mitochondrial both transcriptional and posttranscriptional regulation. Schizosaccharomyces pombe Ppr2 is a general mitochondrial translation factor that plays a critical role in the synthesis of all mitochondrial DNA-encoded oxidative phosphorylation subunits, which are essential for mitochondrial respiration. Our previous analysis showed that ppr2 deletion resulted in increased expression of iron uptake genes and caused ferroptosis-like cell death in S. pombe. In the present work, we showed that deletion of ppr2 reduced viability on glycerol- and galactose-containing media.Php4 is a transcription repressor that regulates iron homeostasis in fission yeast. We found that in the ppr2 deletion strain, Php4 was constitutively active and accumulated in the nucleus in the stationary phase. We also found that deletion of ppr2 decreased the ferroptosis-related protein Gpx1 in the mitochondria. Overexpression of Gpx1 improves the viability of Δppr2 cells. We showed that the deletion of ppr2 increased the production of ROS, downregulated heme synthesis and iron-sulfur cluster proteins, and induced stress proteins. Finally, we observed the nuclear accumulation of Pap1-GFP and Sty1-GFP, suggesting that Sty1 and Pap1 in response to cellular stress in the ppr2 deletion strain. These results suggest thatppr2 deletion may cause mitochondrial dysfunction, which is likely to lead to iron-sensing defect and iron starvation response, resulting in perturbation of iron homeostasis and increased hydroxyl radical production. The increased hydroxyl radical production triggers cellular responses in theppr2 deletion strain.

E3 ubiquitin ligase Hul6 modulates iron-dependent metabolism by regulating Php4 stability

Schizosaccharomyces pombe Php4 is the regulatory subunit of the CCAAT-binding complexes and plays an important role in the regulation of iron homeostasis and iron-dependent metabolism. Here, we show that Php4 undergoes ubiquitin-dependent degradation in the late logarithmic and stationary phases. The degradation and ubiquitination of Php4 could be attenuated by deletion of hul6, a gene encoding a putative HECT-type E3 ubiquitin ligase. The expression levels of Hul6 and Php4 are oppositely regulated during cell growth. Hul6 interacts with the C-terminal region of Php4. Two lysine residues (K217 and K274) located in the C-terminal region of Php4 are required for its polyubiquitination. Increasing the levels of Php4 by deletion of hul6 or overexpression of php4 decreased expression of Php4 target proteins involved in iron-dependent metabolic pathways such as the tricarboxylic cycle and mitochondrial oxidative phosphorylation, thus causing increased sensitivity to high-iron and reductions in succinate dehydrogenase and mitochondrial complex II activities. Hul6 is located primarily in the mitochondrial outer membrane and most likely targets cytosolic Php4 for ubiquitination and degradation. Taken together, our data suggest that Hul6 regulates iron-dependent metabolism through degradation of Php4 under normal growth conditions. Our results also suggest that Hul6 promotes iron-dependent metabolism to help the cell to adapt to a nutrient-starved growth phase.

Iron homeostasis proteins Grx4 and Fra2 control activity of the Schizosaccharomyces pombe iron repressor Fep1 by facilitating [2Fe-2S] cluster removal

The Bol2 homolog Fra2 and monothiol glutaredoxin Grx4 together play essential roles in regulating iron homeostasis in Schizosaccharomyces pombe. In vivo studies indicate that Grx4 and Fra2 act as coinhibitory partners that inactivate the transcriptional repressor Fep1 in response to iron deficiency. In Saccharomyces cerevisiae, Bol2 is known to form a [2Fe-2S]-bridged heterodimer with the monothiol Grxs Grx3 and Grx4, with the cluster ligands provided by conserved residues in Grx3/4 and Bol2 as well as GSH. In this study, we characterized this analogous [2Fe-2S]-bridged Grx4-Fra2 complex in S. pombe by identifying the specific residues in Fra2 that act as ligands for the Fe-S cluster and are required to regulate Fep1 activity. We present spectroscopic and biochemical evidence confirming the formation of a [2Fe-2S]-bridged Grx4-Fra2 heterodimer with His66 and Cys29 from Fra2 serving as Fe-S cluster ligands in S. pombe. In vivo transcription and growth assays confirm that both His66 and Cys29 are required to fully mediate the response of Fep1 to low iron conditions. Furthermore, we analyzed the interaction between Fep1 and Grx4-Fra2 using CD spectroscopy to monitor changes in Fe-S cluster coordination chemistry. These experiments demonstrate unidirectional [2Fe-2S] cluster transfer from Fep1 to Grx4-Fra2 in the presence of GSH, revealing the Fe-S cluster dependent mechanism of Fep1 inactivation mediated by Grx4 and Fra2 in response to iron deficiency.

Schizosaccharomyces pombe Grx4, Fep1, and Php4: In silico analysis and expression response to different iron concentrations

Due to iron's essential role in cellular metabolism, most organisms must maintain their homeostasis. In this regard, the fission yeast Schizosaccharomyces pombe (sp) uses two transcription factors to regulate intracellular iron levels: spFep1 under iron-rich conditions and spPhp4 under iron-deficient conditions, which are controlled by spGrx4. However, bioinformatics analysis to understand the role of the spGrx4/spFep1/spPhp4 axis in maintaining iron homeostasis in S. pombe is still lacking. Our study aimed to perform bioinformatics analysis on S. pombe proteins and their sequence homologs in Aspergillus flavus (af), Saccharomyces cerevisiae (sc), and Homo sapiens (hs) to understand the role of spGrx4, spFep1, and spPhp4 in maintaining iron homeostasis. The three genes' expression patterns were also examined at various iron concentrations. A multiple sequence alignment analysis of spGrx4 and its sequence homologs revealed a conserved cysteine residue in each PF00085 domain. Blast results showed that hsGLRX3 is most similar to spGrx4. In addition, spFep1 is most closely related in sequence to scDal80, whereas scHap4 is most similar to spFep1. We also found two highly conserved motifs in spFep1 and its sequence homologs that are significant for iron transport systems because they contain residues involved in iron homeostasis. The scHap4 is most similar to spPhp4. Using STRING to analyze protein-protein interactions, we found that spGrx4 interacts strongly with spPhp4 and spFep1. Furthermore, spGrx4, spPhp4, and spFep1 interact with spPhp2, spPhp3, and spPhp5, indicating that the three proteins play cooperative roles in iron homeostasis. At the highest level of Fe, spgrx4 had the highest expression, followed by spfep1, while spphp4 had the lowest expression; a contrast occurred at the lowest level of Fe, where spgrx4 expression remained constant. Our findings support the notion that organisms develop diverse strategies to maintain iron homeostasis.

The basic leucine zipper transcription factor OsbZIP83 and the glutaredoxins OsGRX6 and OsGRX9 facilitate rice iron utilization under the control of OsHRZ ubiquitin ligases

Under low iron availability, plants induce the expression of various genes for iron uptake and translocation. The rice (Oryza sativa) ubiquitin ligases OsHRZ1 and OsHRZ2 cause overall repression of these iron-related genes at the transcript level, but their protein-level regulation is unclear. We conducted a proteome analysis to identify key regulators whose abundance was regulated by OsHRZs at the protein level. In response to iron deficiency or OsHRZ knockdown, many genes showed differential regulation between the transcript and protein levels, including the TGA-type basic leucine zipper transcription factor OsbZIP83. We also identified two glutaredoxins, OsGRX6 and OsGRX9, as OsHRZ-interacting proteins in yeast and plant cells. OsGRX6 also interacted with OsbZIP83. Our in vitro degradation assay suggested that OsbZIP83, OsGRX6 and OsGRX9 proteins are subjected to 26S proteasome- and OsHRZ-dependent degradation. Proteome analysis and our in vitro degradation assay also suggested that OsbZIP83 protein was preferentially degraded under iron-deficient conditions in rice roots. Transgenic rice lines overexpressing OsGRX9 and OsbZIP83 showed improved tolerance to iron deficiency. Expression of iron-related genes was affected in the OsGRX9 and OsGRX6 knockdown lines, suggesting disturbed iron utilization and signaling. OsbZIP83 overexpression lines showed enhanced expression of OsYSL2 and OsNAS3, which are involved in internal iron translocation, in addition to OsGRX9 and genes related to phytoalexin biosynthesis and the salicylic acid pathway. The results suggest that OsbZIP83, OsGRX6 and OsGRX9 facilitate iron utilization downstream of the OsHRZ pathway.

Glutaredoxins and iron-sulfur protein biogenesis at the interface of redox biology and iron metabolism

The physiological roles of the intracellular iron and redox regulatory systems are intimately linked. Iron is an essential trace element for most organisms, yet elevated cellular iron levels are a potent generator and amplifier of reactive oxygen species and redox stress. Proteins binding iron or iron-sulfur (Fe/S) clusters, are particularly sensitive to oxidative damage and require protection from the cellular oxidative stress protection systems. In addition, key components of these systems, most prominently glutathione and monothiol glutaredoxins are involved in the biogenesis of cellular Fe/S proteins. In this review, we address the biochemical role of glutathione and glutaredoxins in cellular Fe/S protein assembly in eukaryotic cells. We also summarize the recent developments in the role of cytosolic glutaredoxins in iron metabolism, in particular the regulation of fungal iron homeostasis. Finally, we discuss recent insights into the interplay of the cellular thiol redox balance and oxygen with that of Fe/S protein biogenesis in eukaryotes.

AtHAP5A regulates iron translocation in iron-deficient Arabidopsis thaliana

Iron (Fe) deficient plants employ multiple strategies to increase root uptake and root-to-shoot translocation of Fe. The identification of genes that are responsible for these processes, and a comprehensive understanding of the regulatory effects of transcriptional networks on their expression, including transcription factors (TFs), is underway in Arabidopsis thaliana. Here, we show that a Histone- or heme-associated proteins (HAP) transcription factor (TF), HAP5A, is necessary for the response to Fe deficiency in Arabidopsis. Its expression was induced under Fe deficiency, and the lack of HAP5A significantly decreased Fe translocation from the root to the shoot, resulting in substantial chlorosis of the newly expanded leaves, compared with the wild-type (WT, Col-0). Further analysis found that the expression of a gene encoding nicotianamine (NA) synthase (NAS1) was dramatically decreased in the hap5a mutant, regardless of the Fe status. Yeast-one-hybrid and ChIP analyses suggested that HAP5A directly binds to the promoter region of NAS1. Moreover, overexpression of NAS1 could rescue the chlorosis phenotype of hap5a in Fe deficient conditions. In summary, a novel pathway was elucidated, showing that NAS1-dependent translocation of Fe from the root to the shoot is controlled by HAP5A in Fe-deficient Arabidopsis thaliana.

Fungal iron homeostasis with a focus on Aspergillus fumigatus

To maintain iron homeostasis, fungi have to balance iron acquisition, storage, and utilization to ensure sufficient supply and to avoid toxic excess of this essential trace element. As pathogens usually encounter iron limitation in the host niche, this metal plays a particular role during virulence. Siderophores are iron-chelators synthesized by most, but not all fungal species to sequester iron extra- and intracellularly. In recent years, the facultative human pathogen Aspergillus fumigatus has become a model for fungal iron homeostasis of siderophore-producing fungal species. This article summarizes the knowledge on fungal iron homeostasis and its links to virulence with a focus on A. fumigatus. It covers mechanisms for iron acquisition, storage, and detoxification, as well as the modes of transcriptional iron regulation and iron sensing in A. fumigatus in comparison to other fungal species. Moreover, potential translational applications of the peculiarities of fungal iron metabolism for treatment and diagnosis of fungal infections is addressed.

Glutaredoxins with iron-sulphur clusters in eukaryotes - Structure, function and impact on disease

Among the thioredoxin superfamily of proteins, the observation that numerous glutaredoxins bind iron-sulphur (Fe/S) clusters is one of the more recent and major developments concerning their functional properties. Glutaredoxins are present in most organisms. All members of the class II subfamily (including most monothiol glutaredoxins), but also some members of the class I (mostly dithiol glutaredoxins) and class III (land plant-specific monothiol or dithiol glutaredoxins) are Fe/S proteins. In glutaredoxins characterised so far, the [2Fe2S] cluster is coordinated by two active-site cysteine residues and two molecules of non-covalently bound glutathione in homo-dimeric complexes bridged by the cluster. In contrast to dithiol glutaredoxins, monothiol glutaredoxins possess no or very little oxidoreductase activity, but have emerged as important players in cellular iron metabolism. In this review we summarise the recent developments of the most prominent Fe/S glutaredoxins in eukaryotes, the mitochondrial single domain monothiol glutaredoxin 5, the chloroplastic single domain monothiol glutaredoxin S14 and S16, the nuclear/cytosolic multi-domain monothiol glutaredoxin 3, and the mitochondrial/cytosolic dithiol glutaredoxin 2.

Iron-sulfur cluster biogenesis, trafficking, and signaling: Roles for CGFS glutaredoxins and BolA proteins

The synthesis and trafficking of iron-sulfur (Fe-S) clusters in both prokaryotes and eukaryotes requires coordination within an expanding network of proteins that function in the cytosol, nucleus, mitochondria, and chloroplasts in order to assemble and deliver these ancient and essential cofactors to a wide variety of Fe-S-dependent enzymes and proteins. This review focuses on the evolving roles of two ubiquitous classes of proteins that operate in this network: CGFS glutaredoxins and BolA proteins. Monothiol or CGFS glutaredoxins possess a Cys-Gly-Phe-Ser active site that coordinates an Fe-S cluster in a homodimeric complex. CGFS glutaredoxins also form [2Fe-2S]-bridged heterocomplexes with BolA proteins, which possess an invariant His and an additional His or Cys residue that serve as cluster ligands. Here we focus on recent discoveries in bacteria, fungi, humans, and plants that highlight the shared and distinct roles of CGFS glutaredoxins and BolA proteins in Fe-S cluster biogenesis, Fe-S cluster storage and trafficking, and Fe-S cluster signaling to transcriptional factors that control iron metabolism--.

Adaptation to iron deficiency in human pathogenic fungi

Iron is an essential micronutrient for virtually all eukaryotic organisms and plays a central role during microbial infections. Invasive fungal diseases are associated with strikingly high rates of mortality, but their impact on human health is usually underestimated. Upon a fungal infection, hosts restrict iron availability in order to limit the growth and virulence of the pathogen. Here, we use two model yeasts, Saccharomyces cerevisiae and Schizosaccharomyces pombe, to delve into the response to iron deficiency of human fungal pathogens, such as Candida glabrata, Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans. Fungi possess common and species-specific mechanisms to acquire iron and to control the response to iron limitation. Upon iron scarcity, fungi activate a wide range of elegant strategies to capture and import exogenous iron, mobilize iron from intracellular stores, and modulate their metabolism to economize and prioritize iron utilization. Hence, iron homeostasis genes represent remarkable virulence factors that can be used as targets for the development of novel antifungal treatments.

Proteomic profiling of the monothiol glutaredoxin Grx3 reveals its global role in the regulation of iron dependent processes

Iron is an essential nutrient required as a cofactor for many biological processes. As a fungal commensal-pathogen of humans, Candida albicans encounters a range of bioavailable iron levels in the human host and maintains homeostasis with a conserved regulatory circuit. How C. albicans senses and responds to iron availability is unknown. In model yeasts, regulation of the iron homeostasis circuit requires monothiol glutaredoxins (Grxs), but their functions beyond the regulatory circuit are unclear. Here, we show Grx3 is required for virulence and growth on low iron for C. albicans. To explore the global roles of Grx3, we applied a proteomic approach and performed in vivo cross-linked tandem affinity purification coupled with mass spectrometry. We identified a large number of Grx3 interacting proteins that function in diverse biological processes. This included Fra1 and Bol2/Fra2, which function with Grxs in intracellular iron trafficking in other organisms. Grx3 interacts with and regulates the activity of Sfu1 and Hap43, components of the C. albicans iron regulatory circuit. Unlike the regulatory circuit, which determines expression or repression of target genes in response to iron availability, Grx3 amplifies levels of gene expression or repression. Consistent with the proteomic data, the grx3 mutant is sensitive to heat shock, oxidative, nitrosative, and genotoxic stresses, and shows growth dependence on histidine, leucine, and tryptophan. We suggest Grx3 is a conserved global regulator of iron-dependent processes occurring within the cell.

Mammalian pathogens occupy a diverse set of niches within the host organism. These niches vary in iron and oxygen availability. As a commensal and pathogen of humans, its ability to regulate iron uptake and utilization in response to bioavailable iron level is critical for its survival in different host environments encompassing a broad range of iron levels. This study aims to understand how C. albicans senses and responds to iron level to regulate multiple aspects of its biology. The cytosolic monothiol glutaredoxin Grx3 is a critical regulator of C. albicans iron homeostasis and virulence. Taking a proteomic approach, we identified a large list of Grx3 associated proteins of diverse functions, including iron-sulfur trafficking, iron homeostasis, metabolism redox homeostasis, protein translation, DNA maintenance and repair. In support of these protein associations, Grx3 is important for all these processes. Thus, Grx3 is a global regulator of iron homeostasis and other iron dependent cellular processes.

Iron-sulfur cluster signaling: The common thread in fungal iron regulation

Iron homeostasis in fungi involves balancing iron uptake and storage with iron utilization to achieve adequate, nontoxic levels of this essential nutrient. Extensive work in the nonpathogenic yeast Saccharomyces cerevisiae and Schizosaccharomyces pombe has uncovered unique iron regulation networks for each organism that control iron metabolism via distinct molecular mechanisms. However, common themes have emerged from these studies. The activities of all fungal iron-sensing transcription factors characterized to date are regulated via iron-sulfur cluster signaling. Furthermore, glutaredoxins often play a key role in relaying the intracellular iron status to these DNA-binding proteins. Recent work with fungal pathogens, including Candida and Aspergillus species and Cryptococcus neoformans, has revealed novel iron regulation mechanisms, yet similar roles for iron-sulfur clusters and glutaredoxins in iron signaling have been confirmed. This review will focus on these recent discoveries regarding iron regulation pathways in both pathogenic and nonpathogenic fungi.

The monothiol glutaredoxin GrxD is essential for sensing iron starvation in Aspergillus fumigatus

Efficient adaptation to iron starvation is an essential virulence determinant of the most common human mold pathogen, Aspergillus fumigatus. Here, we demonstrate that the cytosolic monothiol glutaredoxin GrxD plays an essential role in iron sensing in this fungus. Our studies revealed that (i) GrxD is essential for growth; (ii) expression of the encoding gene, grxD, is repressed by the transcription factor SreA in iron replete conditions and upregulated during iron starvation; (iii) during iron starvation but not iron sufficiency, GrxD displays predominant nuclear localization; (iv) downregulation of grxD expression results in de-repression of genes involved in iron-dependent pathways and repression of genes involved in iron acquisition during iron starvation, but did not significantly affect these genes during iron sufficiency; (v) GrxD displays protein-protein interaction with components of the cytosolic iron-sulfur cluster biosynthetic machinery, indicating a role in this process, and with the transcription factors SreA and HapX, which mediate iron regulation of iron acquisition and iron-dependent pathways; (vi) UV-Vis spectra of recombinant HapX or the complex of HapX and GrxD indicate coordination of iron-sulfur clusters; (vii) the cysteine required for iron-sulfur cluster coordination in GrxD is in vitro dispensable for interaction with HapX; and (viii) there is a GrxD-independent mechanism for sensing iron sufficiency by HapX; (ix) inactivation of SreA suppresses the lethal effect caused by GrxD inactivation. Taken together, this study demonstrates that GrxD is crucial for iron homeostasis in A. fumigatus.

Aspergillus fumigatus is a ubiquitous saprophytic mold and the major causative pathogen causing life-threatening aspergillosis. To improve therapy, there is an urgent need for a better understanding of the fungal physiology. We have previously shown that adaptation to iron starvation is an essential virulence attribute of A. fumigatus. In the present study, we characterized the mechanism employed by A. fumigatus to sense the cellular iron status, which is essential for iron homeostasis. We demonstrate that the transcription factors SreA and HapX, which coordinate iron acquisition, iron consumption and iron detoxification require physical interaction with the monothiol glutaredoxin GrxD to sense iron starvation. Moreover, we show that there is a GrxD-independent mechanism for sensing excess of iron.

The regulation of iron homeostasis in the fungal human pathogen Candida glabrata

Iron is an essential element to most microorganisms, yet an excess of iron is toxic. Hence, living cells have to maintain a tight balance between iron uptake and iron consumption and storage. The control of intracellular iron concentrations is particularly challenging for pathogens because mammalian organisms have evolved sophisticated high-affinity systems to sequester iron from microbes and because iron availability fluctuates among the different host niches. In this review, we present the current understanding of iron homeostasis and its regulation in the fungal pathogen Candida glabrata. This yeast is an emerging pathogen which has become the second leading cause of candidemia, a life-threatening invasive mycosis. C. glabrata is relatively poorly studied compared to the closely related model yeast Saccharomyces cerevisiae or to the pathogenic yeast Candida albicans. Still, several research groups have started to identify the actors of C. glabrata iron homeostasis and its transcriptional and post-transcriptional regulation. These studies have revealed interesting particularities of C. glabrata and have shed new light on the evolution of fungal iron homeostasis.

The Monothiol Glutaredoxin Grx4 Regulates Iron Homeostasis and Virulence in Cryptococcus neoformans

The acquisition of iron and the maintenance of iron homeostasis are important aspects of virulence for the pathogenic fungus Cryptococcus neoformans In this study, we characterized the role of the monothiol glutaredoxin Grx4 in iron homeostasis and virulence in C. neoformans Monothiol glutaredoxins are important regulators of iron homeostasis because of their conserved roles in [2Fe-2S] cluster sensing and trafficking. We initially identified Grx4 as a binding partner of Cir1, a master regulator of iron-responsive genes and virulence factor elaboration in C. neoformans We confirmed that Grx4 binds Cir1 and demonstrated that iron repletion promotes the relocalization of Grx4 from the nucleus to the cytoplasm. We also found that a grx4 mutant lacking the GRX domain displayed iron-related phenotypes similar to those of a cir1Δ mutant, including poor growth upon iron deprivation. Importantly, the grx4 mutant was avirulent in mice, a phenotype consistent with observed defects in the key virulence determinants, capsule and melanin, and poor growth at 37°C. A comparative transcriptome analysis of the grx4 mutant and the WT strain under low-iron and iron-replete conditions confirmed a central role for Grx4 in iron homeostasis. Dysregulation of iron-related metabolism was consistent with grx4 mutant phenotypes related to oxidative stress, mitochondrial function, and DNA repair. Overall, the phenotypes of the grx4 mutant lacking the GRX domain and the transcriptome sequencing (RNA-Seq) analysis of the mutant support the hypothesis that Grx4 functions as an iron sensor, in part through an interaction with Cir1, to extensively regulate iron homeostasis.IMPORTANCE Fungal pathogens cause life-threatening diseases in humans, particularly in immunocompromised people, and there is a tremendous need for a greater understanding of pathogenesis to support new therapies. One prominent fungal pathogen, Cryptococcus neoformans, causes meningitis in people suffering from HIV/AIDS. In the present study, we focused on characterizing mechanisms by which C. neoformans senses iron availability because iron is both a signal and a key nutrient for proliferation of the pathogen in vertebrate hosts. Specifically, we characterized a monothiol glutaredoxin protein, Grx4, that functions as a sensor of iron availability and interacts with regulatory factors to control the ability of C. neoformans to cause disease. Grx4 regulates key virulence factors, and a mutant is unable to cause disease in a mouse model of cryptococcosis. Overall, our study provides new insights into nutrient sensing and the role of iron in the pathogenesis of fungal diseases.

Iron-sensing is governed by mitochondrial, not by cytosolic iron-sulfur cluster biogenesis in Aspergillus fumigatus

Microorganisms have to adapt their metabolism to the requirements of their ecological niche to avoid iron shortage as well as iron toxicity. Therefore, mechanisms have been evolved to tightly regulate iron uptake, consumption, and detoxification, which depend on sensing the cellular iron status. In the facultative anaerobic yeast Saccharomyces cerevisiae, iron-sensing depends on mitochondrial (ISC) but not cytosolic iron-sulfur cluster assembly (CIA), while in mammals further processing of an ISC product via CIA is required for sensing of the cellular iron state. To address the question of how the obligatory aerobic mold Aspergillus fumigatus senses the cellular iron state, mutant strains allowing the downregulation of ISC and CIA were generated. These studies revealed that: (i) Nfs1 (Afu3g14240) and Nbp35 (Afu2g15960), which are involved in ISC and CIA, respectively, are essential for growth; (ii) a decrease in ISC (Nfs1 depletion) but not CIA (Nbp35 depletion) results in a transcriptional iron starvation response, (iii) a decrease in, ISC as well as CIA, increases the chelatable iron pool, accompanied by increased iron toxicity and increased susceptibility to oxidative stress and phleomycin. In agreement with ISC being essential for iron-sensing, a decrease in mitochondrial iron import by deletion of the mitochondrial iron importer MrsA resulted in an iron starvation response. Taken together, these data underline that iron-sensing in A. fumigatus depends on ISC but not CIA. Moreover, depletion of the glutathione pool via generating a mutant lacking γ-glutamylcysteine synthase, GshA (Afu3g13900), caused an iron starvation response, underlining a crucial role of glutathione in iron-sensing in A. fumigatus.

Schizosaccharomyces pombe Grx4 regulates the transcriptional repressor Php4 via [2Fe-2S] cluster binding

The fission yeast Schizosaccharomyces pombe expresses the CCAAT-binding factor Php4 in response to iron deprivation. Php4 forms a transcription complex with Php2, Php3, and Php5 to repress the expression of iron proteins as a means to economize iron usage. Previous in vivo results demonstrate that the function and location of Php4 are regulated in an iron-dependent manner by the cytosolic CGFS type glutaredoxin Grx4. In this study, we aimed to biochemically define these protein-protein and protein-metal interactions. Grx4 was found to bind a [2Fe-2S] cluster with spectroscopic features similar to other CGFS glutaredoxins. Grx4 and Php4 also copurify as a complex with a [2Fe-2S] cluster that is spectroscopically distinct from the cluster on Grx4 alone. In vitro titration experiments suggest that these Fe-S complexes may not be interconvertible in the absence of additional factors. Furthermore, conserved cysteines in Grx4 (Cys172) and Php4 (Cys221 and Cys227) are necessary for Fe-S cluster binding and stable complex formation. Together, these results show that Grx4 controls Php4 function through binding of a bridging [2Fe-2S] cluster.

Lack of a peroxiredoxin suppresses the lethality of cells devoid of electron donors by channelling electrons to oxidized ribonucleotide reductase

The thioredoxin and glutaredoxin pathways are responsible of recycling several enzymes which undergo intramolecular disulfide bond formation as part of their catalytic cycles such as the peroxide scavengers peroxiredoxins or the enzyme ribonucleotide reductase (RNR). RNR, the rate-limiting enzyme of deoxyribonucleotide synthesis, is an essential enzyme relying on these electron flow cascades for recycling. RNR is tightly regulated in a cell cycle-dependent manner at different levels, but little is known about the participation of electron donors in such regulation. Here, we show that cytosolic thioredoxins Trx1 and Trx3 are the primary electron donors for RNR in fission yeast. Unexpectedly, trx1 transcript and Trx1 protein levels are up-regulated in a G1-to-S phase-dependent manner, indicating that the supply of electron donors is also cell cycle-regulated. Indeed, genetic depletion of thioredoxins triggers a DNA replication checkpoint ruled by Rad3 and Cds1, with the final goal of up-regulating transcription of S phase genes and constitutive RNR synthesis. Regarding the thioredoxin and glutaredoxin cascades, one combination of gene deletions is synthetic lethal in fission yeast: cells lacking both thioredoxin reductase and cytosolic dithiol glutaredoxin. We have isolated a suppressor of this lethal phenotype: a mutation at the Tpx1-coding gene, leading to a frame shift and a loss-of-function of Tpx1, the main client of electron donors. We propose that in a mutant strain compromised in reducing equivalents, the absence of an abundant and competitive substrate such as the peroxiredoxin Tpx1 has been selected as a lethality suppressor to favor RNR function at the expense of the non-essential peroxide scavenging function, to allow DNA synthesis and cell growth.

The essential enzyme ribonucleotide reductase (RNR), the rate-limiting enzyme of deoxyribonucleotide synthesis, relies on the thioredoxin and glutaredoxin electron flow cascades for recycling. RNR is tightly regulated in a cell cycle-dependent manner at different levels. Here, we show that cytosolic thioredoxin Trx1 is the primary electron donor for RNR in fission yeast, and that trx1 transcript and protein levels are up-regulated at G1-to-S phase transition. Genetic depletion of thioredoxins triggers the DNA replication checkpoint up-regulating RNR synthesis. Furthermore, deletion of the genes coding for thioredoxin reductase and dithiol glutaredoxin is synthetic lethal, and we show that a loss-of-function mutation at the peroxiredoxin Tpx1-coding gene acts as a genetic suppressor. We propose that in a mutant strain compromised in reducing equivalents, the absence of an abundant and competitive substrate of redoxins, the peroxiredoxin Tpx1, has been selected as a lethality suppressor to favor channeling of electrons to the essential RNR.

Arabidopsis Glutaredoxin S17 Contributes to Vegetative Growth, Mineral Accumulation, and Redox Balance during Iron Deficiency

Iron (Fe) is an essential mineral nutrient and a metal cofactor required for many proteins and enzymes involved in the processes of DNA synthesis, respiration, and photosynthesis. Iron limitation can have detrimental effects on plant growth and development. Such effects are mediated, at least in part, through the generation of reactive oxygen species (ROS). Thus, plants have evolved a complex regulatory network to respond to conditions of iron limitations. However, the mechanisms that couple iron deficiency and oxidative stress responses are not fully understood. Here, we report the discovery that an Arabidopsis thaliana monothiol glutaredoxin S17 (AtGRXS17) plays a critical role in the plants ability to respond to iron deficiency stress and maintain redox homeostasis. In a yeast expression assay, AtGRXS17 was able to suppress the iron accumulation in yeast ScGrx3/ScGrx4 mutant cells. Genetic analysis indicated that plants with reduced AtGRXS17 expression were hypersensitive to iron deficiency and showed increased iron concentrations in mature seeds. Disruption of AtGRXS17 caused plant sensitivity to exogenous oxidants and increased ROS production under iron deficiency. Addition of reduced glutathione rescued the growth and alleviates the sensitivity of atgrxs17 mutants to iron deficiency. These findings suggest AtGRXS17 helps integrate redox homeostasis and iron deficiency responses.

Cytosolic iron chaperones: Proteins delivering iron cofactors in the cytosol of mammalian cells

Eukaryotic cells contain hundreds of metalloproteins that are supported by intracellular systems coordinating the uptake and distribution of metal cofactors. Iron cofactors include heme, iron-sulfur clusters, and simple iron ions. Poly(rC)-binding proteins are multifunctional adaptors that serve as iron ion chaperones in the cytosolic/nuclear compartment, binding iron at import and delivering it to enzymes, for storage (ferritin) and export (ferroportin). Ferritin iron is mobilized by autophagy through the cargo receptor, nuclear co-activator 4. The monothiol glutaredoxin Glrx3 and BolA2 function as a [2Fe-2S] chaperone complex. These proteins form a core system of cytosolic iron cofactor chaperones in mammalian cells.

Arabidopsis Glutaredoxin S17 Contributes to Vegetative Growth, Mineral Accumulation, and Redox Balance during Iron Deficiency

Iron (Fe) is an essential mineral nutrient and a metal cofactor required for many proteins and enzymes involved in the processes of DNA synthesis, respiration, and photosynthesis. Iron limitation can have detrimental effects on plant growth and development. Such effects are mediated, at least in part, through the generation of reactive oxygen species (ROS). Thus, plants have evolved a complex regulatory network to respond to conditions of iron limitations. However, the mechanisms that couple iron deficiency and oxidative stress responses are not fully understood. Here, we report the discovery that an Arabidopsis thaliana monothiol glutaredoxin S17 (AtGRXS17) plays a critical role in the plants ability to respond to iron deficiency stress and maintain redox homeostasis. In a yeast expression assay, AtGRXS17 was able to suppress the iron accumulation in yeast ScGrx3/ScGrx4 mutant cells. Genetic analysis indicated that plants with reduced AtGRXS17 expression were hypersensitive to iron deficiency and showed increased iron concentrations in mature seeds. Disruption of AtGRXS17 caused plant sensitivity to exogenous oxidants and increased ROS production under iron deficiency. Addition of reduced glutathione rescued the growth and alleviates the sensitivity of atgrxs17 mutants to iron deficiency. These findings suggest AtGRXS17 helps integrate redox homeostasis and iron deficiency responses.

A Glutaredoxin·BolA Complex Serves as an Iron-Sulfur Cluster Chaperone for the Cytosolic Cluster Assembly Machinery

Cells contain hundreds of proteins that require iron cofactors for activity. Iron cofactors are synthesized in the cell, but the pathways involved in distributing heme, iron-sulfur clusters, and ferrous/ferric ions to apoproteins remain incompletely defined. In particular, cytosolic monothiol glutaredoxins and BolA-like proteins have been identified as [2Fe-2S]-coordinating complexes in vitro and iron-regulatory proteins in fungi, but it is not clear how these proteins function in mammalian systems or how this complex might affect Fe-S proteins or the cytosolic Fe-S assembly machinery. To explore these questions, we use quantitative immunoprecipitation and live cell proximity-dependent biotinylation to monitor interactions between Glrx3, BolA2, and components of the cytosolic iron-sulfur cluster assembly system. We characterize cytosolic Glrx3·BolA2 as a [2Fe-2S] chaperone complex in human cells. Unlike complexes formed by fungal orthologs, human Glrx3-BolA2 interaction required the coordination of Fe-S clusters, whereas Glrx3 homodimer formation did not. Cellular Glrx3·BolA2 complexes increased 6-8-fold in response to increasing iron, forming a rapidly expandable pool of Fe-S clusters. Fe-S coordination by Glrx3·BolA2 did not depend on Ciapin1 or Ciao1, proteins that bind Glrx3 and are involved in cytosolic Fe-S cluster assembly and distribution. Instead, Glrx3 and BolA2 bound and facilitated Fe-S incorporation into Ciapin1, a [2Fe-2S] protein functioning early in the cytosolic Fe-S assembly pathway. Thus, Glrx3·BolA is a [2Fe-2S] chaperone complex capable of transferring [2Fe-2S] clusters to apoproteins in human cells.

Molecular basis of the regulation of iron homeostasis in fission and filamentous yeasts

When iron load exceeds that needed by fission and filamentous yeasts, iron-regulatory GATA-type transcription factors repress genes encoding iron acquisition systems. In contrast, under iron starvation, optimization of cellular iron utilization is coordinated by a specialized regulatory subunit of the CCAAT-binding factor that fosters repression of genes encoding iron-using proteins. Despite these findings, there is still limited knowledge concerning the mechanisms by which these iron-responsive regulators respond to high- or low-iron availability. To provide a framework for understanding common and distinct properties of iron-dependent transcriptional regulators, a repertoire of their functional domains in different fungal species is presented here. In addition, discovery of interacting partners of these iron-responsive factors contributes to provide additional insight into their properties.

Characterization of the nuclear import mechanism of the CCAAT-regulatory subunit Php4

Php4 is a nucleo-cytoplasmic shuttling protein that accumulates in the nucleus during iron deficiency. When present in the nucleus, Php4 associates with the CCAAT-binding protein complex and represses genes encoding iron-using proteins. Here, we show that nuclear import of Php4 is independent of the other subunits of the CCAAT-binding complex. Php4 nuclear import relies on two functionally independent nuclear localization sequences (NLSs) that are located between amino acid residues 171 to 174 (KRIR) and 234 to 240 (KSVKRVR). Specific substitutions of basic amino acid residues to alanines within these sequences are sufficient to abrogate nuclear targeting of Php4. The two NLSs are biologically redundant and are sufficient to target a heterologous reporter protein to the nucleus. Under low-iron conditions, a functional GFP-Php4 protein is only partly targeted to the nucleus in imp1Δ and sal3Δ mutant cells. We further found that cells expressing a temperature-sensitive mutation in cut15 exhibit increased cytosolic accumulation of Php4 at the nonpermissive temperature. Further analysis by pull-down experiments revealed that Php4 is a cargo of the karyopherins Imp1, Cut15 and Sal3. Collectively, these results indicate that Php4 can be bound by distinct karyopherins, connecting it into more than one nuclear import pathway.

The Janus transcription factor HapX controls fungal adaptation to both iron starvation and iron excess

Balance of physiological levels of iron is essential for every organism. In Aspergillus fumigatus and other fungal pathogens, the transcription factor HapX mediates adaptation to iron limitation and consequently virulence by repressing iron consumption and activating iron uptake. Here, we demonstrate that HapX is also essential for iron resistance via activating vacuolar iron storage. We identified HapX protein domains that are essential for HapX functions during either iron starvation or high-iron conditions. The evolutionary conservation of these domains indicates their wide-spread role in iron sensing. We further demonstrate that a HapX homodimer and the CCAAT-binding complex (CBC) cooperatively bind an evolutionary conserved DNA motif in a target promoter. The latter reveals the mode of discrimination between general CBC and specific HapX/CBC target genes. Collectively, our study uncovers a novel regulatory mechanism mediating both iron resistance and adaptation to iron starvation by the same transcription factor complex with activating and repressing functions depending on ambient iron availability.

Fra2 is a co-regulator of Fep1 inhibition in response to iron starvation

Iron is required for several metabolic functions involved in cellular growth. Although several players involved in iron transport have been identified, the mechanisms by which iron-responsive transcription factors are controlled are still poorly understood. In Schizosaccharomyces pombe, the Fep1 transcription factor represses genes involved in iron acquisition in response to high levels of iron. In contrast, when iron levels are low, Fep1 becomes inactive and loses its ability to associate with chromatin. Although the molecular basis by which Fep1 is inactivated under iron starvation remains unknown, this process requires the monothiol glutaredoxin Grx4. Here, we demonstrate that Fra2 plays a role in the negative regulation of Fep1 activity. Disruption of fra2⁺ (fra2Δ) led to a constitutive repression of the fio1⁺ gene transcription. Fep1 was consistently active and constitutively bound to its target gene promoters in cells lacking fra2⁺. A constitutive activation of Fep1 was also observed in a php4Δ fra2Δ double mutant strain in which the behavior of Fep1 is freed of its transcriptional regulation by Php4. Microscopic analyses of cells expressing a functional Fra2-Myc₁₃ protein revealed that Fra2 localized throughout the cells with a significant proportion of Fra2 being observed within the nuclei. Further analysis by coimmunoprecipitation showed that Fra2, Fep1 and Grx4 are associated in a heteroprotein complex. Bimolecular fluorescence complementation experiments brought further evidence that an interaction between Fep1 and Fra2 occurs in the nucleus. Taken together, results reported here revealed that Fra2 plays a role in the Grx4-mediated pathway that inactivates Fep1 in response to iron deficiency.

Iron uptake and regulation in Schizosaccharomyces pombe

Schizosaccharomyces pombe is a useful model system for understanding many aspects of eukaryotic cell growth. Studies of S. pombe have identified novel genes that function in the regulation of iron homeostasis. In response to high levels of iron, Fep1 represses the expression of several genes involved in the acquisition of iron. When iron levels are limited, optimization of cellular iron utilization is coordinated by Php4, which represses genes encoding iron-using proteins. Results from studies in yeast have shed new light on the role of monothiol glutaredoxins (Grxs) in iron homeostasis. In S. pombe, the Grx4 protein serves as an inhibitory partner for Fep1 in response to iron deficiency, whereas it is required for the inhibition of Php4 under iron-replete conditions.

Crucial function of vertebrate glutaredoxin 3 (PICOT) in iron homeostasis and hemoglobin maturation

Vertebrate glutaredoxin 3 (PICOT) is essential for the maturation of the heme cofactor of hemoglobin through its essential functions in iron homeostasis. The data suggest an evolutionarily conserved role of cytosolic monothiol multidomain Grxs in cellular iron metabolism pathways.

The mechanisms by which eukaryotic cells handle and distribute the essential micronutrient iron within the cytosol and other cellular compartments are only beginning to emerge. The yeast monothiol multidomain glutaredoxins (Grx) 3 and 4 are essential for both transcriptional iron regulation and intracellular iron distribution. Despite the fact that the mechanisms of iron metabolism differ drastically in fungi and higher eukaryotes, the glutaredoxins are conserved, yet their precise function in vertebrates has remained elusive. Here we demonstrate a crucial role of the vertebrate-specific monothiol multidomain Grx3 (PICOT) in cellular iron homeostasis. During zebrafish embryonic development, depletion of Grx3 severely impairs the maturation of hemoglobin, the major iron-consuming process. Silencing of human Grx3 expression in HeLa cells decreases the activities of several cytosolic Fe/S proteins, for example, iron-regulatory protein 1, a major component of posttranscriptional iron regulation. As a consequence, Grx3-depleted cells show decreased levels of ferritin and increased levels of transferrin receptor, features characteristic of cellular iron starvation. Apparently, Grx3-deficient cells are unable to efficiently use iron, despite unimpaired cellular iron uptake. These data suggest an evolutionarily conserved role of cytosolic monothiol multidomain glutaredoxins in cellular iron metabolism pathways, including the biogenesis of Fe/S proteins and hemoglobin maturation.

Loss of vacuolar H+-ATPase (V-ATPase) activity in yeast generates an iron deprivation signal that is moderated by induction of the peroxiredoxin TSA2

Vacuolar H(+)-ATPases (V-ATPases) acidify intracellular organelles and help to regulate overall cellular pH. Yeast vma mutants lack V-ATPase activity and allow exploration of connections between cellular pH, iron, and redox homeostasis common to all eukaryotes. A previous microarray study in a vma mutant demonstrated up-regulation of multiple iron uptake genes under control of Aft1p (the iron regulon) and only one antioxidant gene, the peroxiredoxin TSA2 (Milgrom, E., Diab, H., Middleton, F., and Kane, P. M. (2007) Loss of vacuolar proton-translocating ATPase activity in yeast results in chronic oxidative stress. J. Biol. Chem. 282, 7125-7136). Fluorescent biosensors placing GFP under transcriptional control of either an Aft1-dependent promoter (P(FIT2)-GFP) or the TSA2 promoter (P(TSA2)-GFP) were constructed to monitor transcriptional signaling. Both biosensors were up-regulated in the vma2Δ mutant, and acute V-ATPase inhibition with concanamycin A induced coordinate up-regulation from both promoters. PTSA2-GFP induction was Yap1p-dependent, indicating an oxidative stress signal. Total cell iron measurements indicate that the vma2Δ mutant is iron-replete, despite up-regulation of the iron regulon. Acetic acid up-regulated P(FIT2)-GFP expression in wild-type cells, suggesting that loss of pH control contributes to an iron deficiency signal in the mutant. Iron supplementation significantly decreased P(FIT2)-GFP expression and, surprisingly, restored P(TSA2)-GFP to wild-type levels. A tsa2Δ mutation induced both nuclear localization of Aft1p and P(FIT2)-GFP expression. The data suggest a novel function for Tsa2p as a negative regulator of Aft1p-driven transcription, which is induced in V-ATPase mutants to limit transcription of the iron regulon. This represents a new mechanism bridging the antioxidant and iron-regulatory pathways that is intimately linked to pH homeostasis.

A role for iron-sulfur clusters in the regulation of transcription factor Yap5-dependent high iron transcriptional responses in yeast

Yeast respond to increased cytosolic iron by activating the transcription factor Yap5 increasing transcription of CCC1, which encodes a vacuolar iron importer. Using a genetic screen to identify genes involved in Yap5 iron sensing, we discovered that a mutation in SSQ1, which encodes a mitochondrial chaperone involved in iron-sulfur cluster synthesis, prevented expression of Yap5 target genes. We demonstrated that mutation or reduced expression of other genes involved in mitochondrial iron-sulfur cluster synthesis (YFH1, ISU1) prevented induction of the Yap5 response. We took advantage of the iron-dependent catalytic activity of Pseudaminobacter salicylatoxidans gentisate 1,2-dioxygenase expressed in yeast to measure changes in cytosolic iron. We determined that reductions in iron-sulfur cluster synthesis did not affect the activity of cytosolic gentisate 1,2-dioxygenase. We show that loss of activity of the cytosolic iron-sulfur cluster assembly complex proteins or deletion of cytosolic glutaredoxins did not reduce expression of Yap5 target genes. These results suggest that the high iron transcriptional response, as well as the low iron transcriptional response, senses iron-sulfur clusters.

Monothiol CGFS glutaredoxins and BolA-like proteins: [2Fe-2S] binding partners in iron homeostasis

Monothiol glutaredoxins (Grxs) with a signature CGFS active site and BolA-like proteins have recently emerged as novel players in iron homeostasis. Elegant genetic and biochemical studies examining the functional and physical interactions of CGFS Grxs in the fungi Saccharomyces cerevisiae and Schizosaccharomyces pombe have unveiled their essential roles in intracellular iron signaling, iron trafficking, and the maturation of Fe-S cluster proteins. Biophysical and biochemical analyses of the [2Fe-2S] bridging interaction between CGFS Grxs and a BolA-like protein in S. cerevisiae provided the first molecular-level understanding of the iron regulation mechanism in this model eukaryote and established the ubiquitous CGFS Grxs and BolA-like proteins as novel Fe-S cluster-binding regulatory partners. Parallel studies focused on Escherichia coli and human homologues for CGFS Grxs and BolA-like proteins have supported the studies in yeast and provided additional clues about their involvement in cellular iron metabolism. Herein, we review recent progress in uncovering the cellular and molecular mechanisms by which CGFS Grxs and BolA-like proteins help regulate iron metabolism in both eukaryotic and prokaryotic organisms.