The Metalloreductase Fre6p in Fe-Efflux from the Yeast Vacuole

Converging Roles of the Metal Transporter SMF11 and the Ferric Reductase FRE1 in Iron Homeostasis of Candida albicans

Pathogenic fungi must appropriately sense the host availability of essential metals such as Fe. In Candida albicans and other yeasts, sensing of Fe involves mitochondrial Fe-S clusters. Yeast mutants for Fe-S cluster assembly sense Fe limitation even when Fe is abundant and hyperaccumulate Fe. We observe this same disrupted Fe sensing with C. albicans mutants of SMF11, a NRAMP transporter of divalent metals. Mutants of smf11 hyperaccumulate both Mn and Fe and the elevated Mn is secondary to Fe overload. As with Fe-S biogenesis mutants, smf11∆/∆ mutants show upregulation of ferric reductases that are normally repressed under high Fe, and Fe import is activated. However, unlike Fe-S biogenesis mutants, smf11∆/∆ mutants show no defects in mitochondrial Fe-S enzymes. Intriguingly, this exact condition of disrupted Fe sensing without inhibiting Fe-S clusters occurs with C. albicans fre1∆/∆ mutants encoding a ferric reductase. Mutants of fre1 and smf11 display similar perturbations in the cell wall, in filamentation and in the ROS burst of morphogenesis, a Fe-dependent process. As with FRE1, SMF11 is important for virulence in a mouse model for disseminated candidiasis. We propose a model in which FRE1 and SMF11 operate outside the mitochondrial Fe-S pathway to donate ferrous Fe for Fe sensing.

The intricate link between iron, mitochondria and azoles in Candida species

Invasive fungal infections are rapidly increasing, and the opportunistic pathogenic Candida species are the fourth most common cause of nosocomial systemic infections. The current antifungal classes, of which azoles are the most widely used, all have shortcomings. Azoles are generally considered fungistatic rather than fungicidal, they do not actively kill fungal cells and therefore resistance against azoles can be rapidly acquired. Combination therapies with azoles provide an interesting therapeutic outlook and agents limiting iron are excellent candidates. We summarize how iron is acquired by the host and transported towards both storage and iron-utilizing organelles. We indicate whether these pathways alter azole susceptibility and/or tolerance, to finally link these transport mechanisms to mitochondrial iron availability. In this review, we highlight putative novel intracellular iron shuffling mechanisms and indicate that mitochondrial iron dynamics in relation to azole treatment and iron limitation is a significant knowledge gap.

Regulatory and pathogenic mechanisms in response to iron deficiency and excess in fungi

Iron is an essential element for all eukaryote organisms because of its redox properties, which are important for many biological processes such as DNA synthesis, mitochondrial respiration, oxygen transport, lipid, and carbon metabolism. For this reason, living organisms have developed different strategies and mechanisms to optimally regulate iron acquisition, transport, storage, and uptake in different environmental responses. Moreover, iron plays an essential role during microbial infections. Saccharomyces cerevisiae has been of key importance for decrypting iron homeostasis and regulation mechanisms in eukaryotes. Specifically, the transcription factors Aft1/Aft2 and Yap5 regulate the expression of genes to control iron metabolism in response to its deficiency or excess, adapting to the cell's iron requirements and its availability in the environment. We also review which iron-related virulence factors have the most common fungal human pathogens (Aspergillus fumigatus, Cryptococcus neoformans, and Candida albicans). These factors are essential for adaptation in different host niches during pathogenesis, including different fungal-specific iron-uptake mechanisms. While being necessary for virulence, they provide hope for developing novel antifungal treatments, which are currently scarce and usually toxic for patients. In this review, we provide a compilation of the current knowledge about the metabolic response to iron deficiency and excess in fungi.

Mössbauer-based molecular-level decomposition of the Saccharomyces cerevisiae ironome, and preliminary characterization of isolated nuclei

One hundred proteins in Saccharomyces cerevisiae are known to contain iron. These proteins are found mainly in mitochondria, cytosol, nuclei, endoplasmic reticula, and vacuoles. Cells also contain non-proteinaceous low-molecular-mass labile iron pools (LFePs). How each molecular iron species interacts on the cellular or systems' level is underdeveloped as doing so would require considering the entire iron content of the cell-the ironome. In this paper, Mössbauer (MB) spectroscopy was used to probe the ironome of yeast. MB spectra of whole cells and isolated organelles were predicted by summing the spectral contribution of each iron-containing species in the cell. Simulations required input from published proteomics and microscopy data, as well as from previous spectroscopic and redox characterization of individual iron-containing proteins. Composite simulations were compared to experimentally determined spectra. Simulated MB spectra of non-proteinaceous iron pools in the cell were assumed to account for major differences between simulated and experimental spectra of whole cells and isolated mitochondria and vacuoles. Nuclei were predicted to contain ∼30 μM iron, mostly in the form of [Fe4S4] clusters. This was experimentally confirmed by isolating nuclei from 57Fe-enriched cells and obtaining the first MB spectra of the organelle. This study provides the first semi-quantitative estimate of all concentrations of iron-containing proteins and non-proteinaceous species in yeast, as well as a novel approach to spectroscopically characterizing LFePs.

Long-term acclimation to cadmium exposure reveals extensive phenotypic plasticity in Chlamydomonas

Increasing industrial and anthropogenic activities are producing and releasing more and more pollutants in the environment. Among them, toxic metals are one of the major threats for human health and natural ecosystems. Because photosynthetic organisms play a critical role in primary productivity and pollution management, investigating their response to metal toxicity is of major interest. Here, the green microalga Chlamydomonas (Chlamydomonas reinhardtii) was subjected to short (3 d) or chronic (6 months) exposure to 50 µM cadmium (Cd), and the recovery from chronic exposure was also examined. An extensive phenotypic characterization and transcriptomic analysis showed that the impact of Cd on biomass production of short-term (ST) exposed cells was almost entirely abolished by long-term (LT) acclimation. The underlying mechanisms were initiated at ST and further amplified after LT exposure resulting in a reversible equilibrium allowing biomass production similar to control condition. This included modification of cell wall-related gene expression and biofilm-like structure formation, dynamics of metal ion uptake and homeostasis, photosynthesis efficiency recovery and Cd acclimation through metal homeostasis adjustment. The contribution of the identified coordination of phosphorus and iron homeostasis (partly) mediated by the main phosphorus homeostasis regulator, Phosphate Starvation Response 1, and a basic Helix-Loop-Helix transcription factor (Cre05.g241636) was further investigated. The study reveals the highly dynamic physiological plasticity enabling algal cell growth in an extreme environment.

AtDTX25, a member of the multidrug and toxic compound extrusion family, is a vacuolar ascorbate transporter that controls intracellular iron cycling in Arabidopsis

Iron (Fe) is an essential element, its transport is regulated by the cell redox balance. In seeds, Fe enters the embryo as Fe²⁺ and is stored in vacuoles as Fe³⁺ . Through its ferric reduction activity, ascorbate plays a major role in Fe redox state and therefore Fe transport within the seed. We searched for ascorbate membrane transporters responsible for controlling Fe reduction by screening the yeast ferric reductase-deficient fre1 strain and isolated AtDTX25, a member of the Multidrug And Toxic compound Extrusion (MATE) family. AtDTX25 was shown to mediate ascorbate efflux when expressed in yeast and Xenopus oocytes, in a pH-dependent manner. In planta, AtDTX25 is highly expressed during germination and encodes a vacuolar membrane protein. Isolated vacuoles from AtDTX25-1 knockout mutant contained less ascorbate and more Fe than wild-type (WT), and mutant seedlings were highly sensitive to Fe deficiency. Iron imaging further showed that the remobilisation of Fe from vacuoles was highly impaired in mutant seedlings. Taken together, our results established AtDTX25 as a vacuolar ascorbate transporter, required during germination to promote the reduction of the pool of stored Fe³⁺ and its remobilisation to feed the developing seedling.

Ferric iron reductases and their contribution to unicellular ferrous iron uptake

Iron is a necessary element for nearly all forms of life, and the ability to acquire this trace nutrient has been identified as a key virulence factor for the establishment of infection by unicellular pathogens. In the presence of O2, iron typically exists in the ferric (Fe3+) oxidation state, which is highly unstable in aqueous conditions, necessitating its sequestration into cofactors and/or host proteins to remain soluble. To counter this insolubility, and to compete with host sequestration mechanisms, many unicellular pathogens will secrete low molecular weight, high-affinity Fe3+ chelators known as siderophores. Once acquired, unicellular pathogens must liberate the siderophore-bound Fe3+ in order to assimilate this nutrient into metabolic pathways. While these organisms may hydrolyze the siderophore backbone to release the chelated Fe3+, this approach is energetically costly. Instead, iron may be liberated from the Fe3+-siderophore complex through reduction to Fe2+, which produces a lower-affinity form of iron that is highly soluble. This reduction is performed by a class of enzymes known as ferric reductases. Ferric reductases are broadly-distributed electron-transport proteins that are expressed by numerous infectious organisms and are connected to the virulence of unicellular pathogens. Despite this importance, ferric reductases remain poorly understood. This review provides an overview of our current understanding of unicellular ferric reductases (both soluble and membrane-bound), with an emphasis on the important but underappreciated connection between ferric-reductase mediated Fe3+ reduction and the transport of Fe2+ via ferrous iron transporters.

Conservation and Loss of a Putative Iron Utilization Gene Cluster among Genotypes of Aspergillus flavus

Iron is an essential component for growth and development. Despite relative abundance in the environment, bioavailability of iron is limited due to oxidation by atmospheric oxygen into insoluble ferric iron. Filamentous fungi have developed diverse pathways to uptake and use iron. In the current study, a putative iron utilization gene cluster (IUC) in Aspergillus flavus was identified and characterized. Gene analyses indicate A. flavus may use reductive as well as siderophore-mediated iron uptake and utilization pathways. The ferroxidation and iron permeation process, in which iron transport depends on the coupling of these two activities, mediates the reductive pathway. The IUC identified in this work includes six genes and is located in a highly polymorphic region of the genome. Diversity among A. flavus genotypes is manifested in the structure of the IUC, which ranged from complete deletion to a region disabled by multiple indels. Molecular profiling of A. flavus populations suggests lineage-specific loss of IUC. The observed variation among A. flavus genotypes in iron utilization and the lineage-specific loss of the iron utilization genes in several A. flavus clonal lineages provide insight on evolution of iron acquisition and utilization within Aspergillus section Flavi. The potential divergence in capacity to acquire iron should be taken into account when selecting A. flavus active ingredients for biocontrol in niches where climate change may alter iron availability.

Mössbauer and LC-ICP-MS investigation of iron trafficking between vacuoles and mitochondria in vma2ΔSaccharomyces cerevisiae

Vacuoles are acidic organelles that store Fe^III polyphosphate, participate in iron homeostasis, and have been proposed to deliver iron to mitochondria for iron–sulfur cluster (ISC) and heme biosynthesis. Vma2Δ cells have dysfunctional V-ATPases, rendering their vacuoles nonacidic. These cells have mitochondria that are iron-dysregulated, suggesting disruption of a putative vacuole-to-mitochondria iron trafficking pathway. To investigate this potential pathway, we examined the iron content of a vma2Δ mutant derived from W303 cells using Mössbauer and EPR spectroscopies and liquid chromatography interfaced with inductively-coupled-plasma mass spectrometry. Relative to WT cells, vma2Δ cells contained WT concentrations of iron but nonheme Fe^II dominated the iron content of fermenting and respiring vma2Δ cells, indicating that the vacuolar Fe^III ions present in WT cells had been reduced. However, vma2Δ cells synthesized WT levels of ISCs/hemes and had normal aconitase activity. The iron content of vma2Δ mitochondria was similar to WT, all suggesting that iron delivery to mitochondria was not disrupted. Chromatograms of cytosolic flow–through solutions exhibited iron species with apparent masses of 600 and 800 Da for WT and vma2∆, respectively. Mutant cells contained high copper concentrations and high concentrations of a species assigned to metallothionein, indicating copper dysregulation. vma2Δ cells from previously studied strain BY4741 exhibited iron-associated properties more consistent with prior studies, suggesting subtle strain differences. Vacuoles with functional V-ATPases appear unnecessary in W303 cells for iron to enter mitochondria and be used in ISC/heme biosynthesis; thus, there appears to be no direct or dedicated vacuole-to-mitochondria iron trafficking pathway. The vma2Δ phenotype may arise from alterations in trafficking of iron directly from cytosol to mitochondria.

Structure and function of the vacuolar Ccc1/VIT1 family of iron transporters and its regulation in fungi

Iron is an essential micronutrient for most living beings since it participates as a redox active cofactor in many biological processes including cellular respiration, lipid biosynthesis, DNA replication and repair, and ribosome biogenesis and recycling. However, when present in excess, iron can participate in Fenton reactions and generate reactive oxygen species that damage cells at the level of proteins, lipids and nucleic acids. Organisms have developed different molecular strategies to protect themselves against the harmful effects of high concentrations of iron. In the case of fungi and plants, detoxification mainly occurs by importing cytosolic iron into the vacuole through the Ccc1/VIT1 iron transporter. New sequenced genomes and bioinformatic tools are facilitating the functional characterization, evolution and ecological relevance of metabolic pathways and homeostatic networks across the Tree of Life. Sequence analysis shows that Ccc1/VIT1 homologs are widely distributed among organisms with the exception of animals. The recent elucidation of the crystal structure of a Ccc1/VIT1 plant ortholog has enabled the identification of both conserved and species-specific motifs required for its metal transport mechanism. Moreover, recent studies in the yeast Saccharomyces cerevisiae have also revealed that multiple transcription factors including Yap5 and Msn2/Msn4 contribute to the expression of CCC1 in high-iron conditions. Interestingly, Malaysian S. cerevisiae strains express a partially functional Ccc1 protein that renders them sensitive to iron. Different regulatory mechanisms have been described for non-Saccharomycetaceae Ccc1 homologs. The characterization of Ccc1/VIT1 proteins is of high interest in the development of biofortified crops and the protection against microbial-derived diseases.

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.

Isolated Saccharomyces cerevisiae vacuoles contain low-molecular-mass transition-metal polyphosphate complexes

Vacuoles play major roles in the trafficking, storage, and homeostasis of metal ions in fungi and plants. In this study, 29 batches of vacuoles were isolated from Saccharomyces cerevisiae. Flow-through solutions (FTS) obtained by passing vacuolar extracts through a 10 kDa cut-off membrane were characterized for metal content using an anaerobic liquid chromatography system interfaced to an online ICP-MS. Nearly all iron, zinc, and manganese ions in these solutions were present as low-molecular-mass (LMM) complexes. Metal-detected peaks with masses between 500-1700 Da dominated; phosphorus-detected peaks generally comigrated. The distribution of metal:polyphosphate complexes was dominated by particular chain-lengths rather than a broad binomial distribution. Similarly treated synthetic FeIII polyphosphate complexes showed similar peaks. Treatment with a phosphatase disrupted the LMM metal-bound species in vacuolar FTSs. These results indicated metal:polyphosphate complexes 6-20 phosphate units in length and coordinated by 1-3 metals on average per chain. The speciation of iron in FTSs from iron-deficient cells was qualitatively similar, but intensities were lower. Under healthy conditions, nearly all copper ions in vacuolar FTSs were present as 1-2 species with masses between 4800-7800 Da. The absence of these high-mass peaks in vacuolar FTS from cup1Δ cells suggests that they were due to metallothionein, Cup1. Disrupting copper homeostasis increased the amount of LMM copper:polyphosphate complexes in vacuoles (masses between 1500-1700 Da). Potentially dangerous LMM copper species in the cytosol of metallothionein-deficient cells may traffic into vacuoles for sequestration and detoxification.

Reduction-dependent siderophore assimilation in a model pennate diatom

Diatoms can access inorganic iron with remarkable efficiency, but this process is contingent on carbonate ion concentration. As ocean acidification reduces carbonate concentration, inorganic iron uptake may be discouraged in favor of carbonate-independent uptake. We report details of an iron assimilation process that needs no carbonate but requires exogenous compounds produced by cooccurring organisms. We show this process to be critical for diatom growth at high siderophore concentrations, but ineffective at acquiring iron from low-affinity organic chelators or lithogenic particulates. Understanding the caveats associated with iron source preference in diatoms will help predict the impacts of climate change on microbial community structure in high-nitrate low-chlorophyll ecosystems.

Iron uptake by diatoms is a biochemical process with global biogeochemical implications. In large regions of the surface ocean diatoms are both responsible for the majority of primary production and frequently experiencing iron limitation of growth. The strategies used by these phytoplankton to extract iron from seawater constrain carbon flux into higher trophic levels and sequestration into sediments. In this study we use reverse genetic techniques to target putative iron-acquisition genes in the model pennate diatom Phaeodactylum tricornutum. We describe components of a reduction-dependent siderophore acquisition pathway that relies on a bacterial-derived receptor protein and provides a viable alternative to inorganic iron uptake under certain conditions. This form of iron uptake entails a close association between diatoms and siderophore-producing organisms during low-iron conditions. Homologs of these proteins are found distributed across diatom lineages, suggesting the significance of siderophore utilization by diatoms in the marine environment. Evaluation of specific proteins enables us to confirm independent iron-acquisition pathways in diatoms and characterize their preferred substrates. These findings refine our mechanistic understanding of the multiple iron-uptake systems used by diatoms and help us better predict the influence of iron speciation on taxa-specific iron bioavailability.

A comprehensive mechanistic model of iron metabolism in Saccharomyces cerevisiae

The ironome of budding yeast (circa 2019) consists of approximately 139 proteins and 5 nonproteinaceous species. These proteins were grouped according to location in the cell, type of iron center(s), and cellular function. The resulting 27 groups were used, along with an additional 13 nonprotein components, to develop a mesoscale mechanistic model that describes the import, trafficking, metallation, and regulation of iron within growing yeast cells. The model was designed to be simultaneously mutually autocatalytic and mutually autoinhibitory - a property called autocatinhibitory that should be most realistic for simulating cellular biochemical processes. The model was assessed at the systems' level. General conclusions are presented, including a new perspective on understanding regulatory mechanisms in cellular systems. Some unsettled issues are described. This model, once fully developed, has the potential to mimic the phenotype (at a coarse-grain level) of all iron-related genetic mutations in this simple and well-studied eukaryote.

Molecular strategies to increase yeast iron accumulation and resistance

All eukaryotic organisms rely on iron as an essential micronutrient for life because it participates as a redox-active cofactor in multiple biological processes. However, excess iron can generate reactive oxygen species that damage cellular macromolecules. The low solubility of ferric iron under physiological conditions increases the prevalence of iron deficiency anemia. A common strategy to treat iron deficiency consists of dietary iron supplementation. The baker's yeast Saccharomyces cerevisiae is used as a model eukaryotic organism, but also as a feed supplement. In response to iron deficiency, the yeast Aft1 transcription factor activates cellular iron acquisition. However, when constitutively active, Aft1 inhibits growth probably due to iron toxicity. In this report, we have studied the consequences of using hyperactive AFT1 alleles, including AFT1-1UP, to increase yeast iron accumulation. We first characterized the iron sensitivity of cells expressing different constitutively active AFT1 alleles. We rescued the high iron sensitivity conferred by the AFT1 alleles by deleting the sphingolipid signaling kinase YPK1. We observed that the deletion of YPK1 exerts different effects on iron accumulation depending on the AFT1 allele and the environmental iron. Moreover, we determined that the impairment of the high-affinity iron transport system partially rescues the high iron toxicity of AFT1-1UP-expressing cells. Finally, we observed that AFT1-1UP inhibits oxygen consumption through activation of the RNA-binding protein Cth2. Deletion of CTH2 partially rescues the AFT1-1UP negative respiratory effect. Collectively, these results contribute to understand how the Aft1 transcription factor functions and the multiple consequences derived from its constitutive activation.

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.

Alteration of the Risk of Oral Pre-Cancer and Cancer in North India Population by CYP1A1 Polymorphism Genotypes and Haplotype

Background:

The aim of this study was to evaluate any association between CYP1A1 (T6235C and C4887A, A4889G) gene polymorphisms and the risk of oral pre-cancer and cancer.

Methods:

In the present study, 250 patients with oral pre-cancer and/or cancer and 250 healthy controls were genotyped for CYP1A1 T6235C, C4887A and A4889G polymorphisms by the PCR-RFLP method.

Results:

None of the CYP1A1 polymorphisms were associated with the risk of either oral cancer or pre cancer. Nor were any links with clinical parameters of oral cancer found. However, among the consumers of areca nut/pan masala the TC, CA and AG genotypes respectively for the CYP1A1 T6235C,C4887Aand A4889G polymorphisms were significantly more frequent in controls compared to cases (p values for cases vs. controls of 0.0032, 0.0019 and 0.0009, respectively). Similarly, compared to the haplotype TCA, TAG constituted by CYP1A1 T6235C and C4887A and A4889G was more common in controls (6.88%) than in cases (4.07%).

Conclusion:

Our results suggest that genotypes regarding CYP1A1 polymorphisms may modulate the risk of oral cancer and pre-cancer among the areca nut/pan masala consumers. The haplotype may also exert an influence in our north Indian population.

A Vacuolar Membrane Ferric-Chelate Reductase, OsFRO1, Alleviates Fe Toxicity in Rice (Oryza sativa L.)

Ferric reductase oxidase (FRO), the enzyme that reduced ferric iron [Fe (III)] into ferrous iron [Fe (II)], is known to play important roles in Fe absorption and homeostasis in plants that utilize a strategy I mechanism to obtain iron. Rice can use both strategies I and II for Fe uptake depending on the growth conditions. FRO is encoded by two genes in rice genome. Amino acid sequence alignment shows that OsFRO1 contains all necessary predicted motifs for a functional FRO enzyme, whereas OsFRO2 lacks a complete transmembrane domain at the N-terminal. Transient expression of OsFRO1: GFP protein fusion revealed that OsFRO1 is localized to the vacuolar membrane in rice protoplast. OsFRO1 is primarily expressed in leaves and transcript abundance was decreased under excess Fe conditions. Transgenic plants overexpressing OsFRO1 were more sensitive to Fe toxicity, in contrast RNA interference lines showed more tolerance to Fe excess stress. Furthermore, RNAi lines showed decreased Fe concentrations compared to wild type plants under Fe excess condition. Together these data show that OsFRO1 is involved in reducing ferric Fe into ferrous Fe in the vacuole, and makes the vacuolar stored Fe available to the cytoplasm through Fe (II) or chelated Fe (II) transporters. Under Fe excess condition, the downregulation of OsFRO1 in the RNAi plants reduced the amount of Fe (II) available for cytoplasm, to alleviate Fe excess toxicity. This indicates that OsFRO1 plays an important role to maintain Fe homeostasis between the cytoplasm and vacuole in rice.

Iron toxicity in yeast: transcriptional regulation of the vacuolar iron importer Ccc1

All eukaryotes require the transition metal, iron, a redox active element that is an essential cofactor in many metabolic pathways, as well as an oxygen carrier. Iron can also react to generate oxygen radicals such as hydroxyl radicals and superoxide anions, which are highly toxic to cells. Therefore, organisms have developed intricate mechanisms to acquire iron as well as to protect themselves from the toxic effects of excess iron. In fungi and plants, iron is stored in the vacuole as a protective mechanism against iron toxicity. Iron storage in the vacuole is mediated predominantly by the vacuolar metal importer Ccc1 in yeast and the homologous transporter VIT1 in plants. Transcription of yeast CCC1 expression is tightly controlled primarily by the transcription factor Yap5, which sits on the CCC1 promoter and activates transcription through the binding of Fe-S clusters. A second mechanism that regulates CCC1 transcription is through the Snf1 signaling pathway involved in low-glucose sensing. Snf1 activates stress transcription factors Msn2 and Msn4 to mediate CCC1 transcription. Transcriptional regulation by Yap5 and Snf1 are completely independent and provide for a graded response in Ccc1 expression. The identification of multiple independent transcriptional pathways that regulate the levels of Ccc1 under high iron conditions accentuates the importance of protecting cells from the toxic effects of high iron.

Heme Assimilation in Schizosaccharomyces pombe Requires Cell-surface-anchored Protein Shu1 and Vacuolar Transporter Abc3

The Schizosaccharomyces pombe shu1⁺ gene encodes a cell-surface protein required for assimilation of exogenous heme. In this study, shaving experiments showed that Shu1 is released from membrane preparations when spheroplast lysates are incubated with phosphoinositide-specific phospholipase C (PI-PLC). Shu1 cleavability by PI-PLC and its predicted hydropathy profile strongly suggested that Shu1 is a glycosylphosphatidylinositol-anchored protein. When heme biosynthesis is selectively blocked in hem1Δ mutant cells, the heme analog zinc mesoporphyrin IX (ZnMP) first accumulates into vacuoles and then subsequently, within the cytoplasm in a rapid and Shu1-dependent manner. An HA₄-tagged shu1⁺ allele that retained wild-type function localizes to the cell surface in response to low hemin concentrations, but under high hemin concentrations, Shu1-HA₄ re-localizes to the vacuolar membrane. Inactivation of abc3⁺, encoding a vacuolar membrane transporter, results in hem1Δ abc3Δ mutant cells being unable to grow in the presence of hemin as the sole iron source. In hem1Δ abc3Δ cells, ZnMP accumulates primarily in vacuoles and does not sequentially accumulate in the cytosol. Consistent with a role for Abc3 as vacuolar hemin exporter, results with hemin-agarose pulldown assays showed that Abc3 binds to hemin. In contrast, an Abc3 mutant in which an inverted Cys-Pro motif had been replaced with Ala residues fails to bind hemin with high affinity. Taken together, these results show that Shu1 undergoes rapid hemin-induced internalization from the cell surface to the vacuolar membrane and that the transporter Abc3 participates in the mobilization of stored heme from the vacuole to the cytosol.

The Organization of Controller Motifs Leading to Robust Plant Iron Homeostasis

Iron is an essential element needed by all organisms for growth and development. Because iron becomes toxic at higher concentrations iron is under homeostatic control. Plants face also the problem that iron in the soil is tightly bound to oxygen and difficult to access. Plants have therefore developed special mechanisms for iron uptake and regulation. During the last years key components of plant iron regulation have been identified. How these components integrate and maintain robust iron homeostasis is presently not well understood. Here we use a computational approach to identify mechanisms for robust iron homeostasis in non-graminaceous plants. In comparison with experimental results certain control arrangements can be eliminated, among them that iron homeostasis is solely based on an iron-dependent degradation of the transporter IRT1. Recent IRT1 overexpression experiments suggested that IRT1-degradation is iron-independent. This suggestion appears to be misleading. We show that iron signaling pathways under IRT1 overexpression conditions become saturated, leading to a breakdown in iron regulation and to the observed iron-independent degradation of IRT1. A model, which complies with experimental data places the regulation of cytosolic iron at the transcript level of the transcription factor FIT. Including the experimental observation that FIT induces inhibition of IRT1 turnover we found a significant improvement in the system’s response time, suggesting a functional role for the FIT-mediated inhibition of IRT1 degradation. By combining iron uptake with storage and remobilization mechanisms a model is obtained which in a concerted manner integrates iron uptake, storage and remobilization. In agreement with experiments the model does not store iron during its high-affinity uptake. As an iron biofortification approach we discuss the possibility how iron can be accumulated even during high-affinity uptake.

A vacuolar iron-transporter homologue acts as a detoxifier in Plasmodium

Iron is an essential micronutrient but is also highly toxic. In yeast and plant cells, a key detoxifying mechanism involves iron sequestration into intracellular storage compartments, mediated by members of the vacuolar iron-transporter (VIT) family of proteins. Here we study the VIT homologue from the malaria parasites Plasmodium falciparum (PfVIT) and Plasmodium berghei (PbVIT). PfVIT-mediated iron transport in a yeast heterologous expression system is saturable (Km ∼ 14.7 μM), and selective for Fe(2+) over other divalent cations. PbVIT-deficient P. berghei lines (Pbvit(-)) show a reduction in parasite load in both liver and blood stages of infection in mice. Moreover, Pbvit(-) parasites have higher levels of labile iron in blood stages and are more sensitive to increased iron levels in liver stages, when compared with wild-type parasites. Our data are consistent with Plasmodium VITs playing a major role in iron detoxification and, thus, normal development of malaria parasites in their mammalian host.

Average oxidation state of carbon in proteins

The formal oxidation state of carbon atoms in organic molecules depends on the covalent structure. In proteins, the average oxidation state of carbon (Z(C)) can be calculated as an elemental ratio from the chemical formula. To investigate oxidation-reduction (redox) patterns, groups of proteins from different subcellular locations and phylogenetic groups were selected for comparison. Extracellular proteins of yeast have a relatively high oxidation state of carbon, corresponding with oxidizing conditions outside of the cell. However, an inverse relationship between Z(C) and redox potential occurs between the endoplasmic reticulum and cytoplasm. This trend provides support for the hypothesis that protein transport and turnover are ultimately coupled to the maintenance of different glutathione redox potentials in subcellular compartments. There are broad changes in Z(C) in whole-genome protein compositions in microbes from different environments, and in Rubisco homologues, lower Z(C) tends to occur in organisms with higher optimal growth temperature. Energetic costs calculated from thermodynamic models are consistent with the notion that thermophilic organisms exhibit molecular adaptation to not only high temperature but also the reducing nature of many hydrothermal fluids. Further characterization of the material requirements of protein metabolism in terms of the chemical conditions of cells and environments may help to reveal other linkages among biochemical processes with implications for changes on evolutionary time scales.

Lysosome-related organelles as mediators of metal homeostasis

Metal ion assimilation is essential for all forms of life. However, organisms must properly control the availability of these nutrients within the cell to avoid inactivating proteins by mismetallation. To safeguard against an imbalance between supply and demand in eukaryotes, intracellular compartments contain metal transporters that load and unload metals. Although the vacuoles of Saccharomyces cerevisiae and Arabidopsis thaliana are well established locales for the storage of copper, zinc, iron, and manganese, related compartments are emerging as important mediators of metal homeostasis. Here we describe these compartments and review their metal transporter complement.

A novel role of the ferric reductase Cfl1 in cell wall integrity, mitochondrial function, and invasion to host cells in Candida albicans

Candida albicans is an important opportunistic pathogen, causing both superficial mucosal infections and life-threatening systemic diseases. Iron acquisition is an important factor for pathogen-host interaction and also a significant element for the pathogenicity of this organism. Ferric reductases, which convert ferric iron into ferrous iron, are important components of the high-affinity iron uptake system. Sequence analyses have identified at least 17 putative ferric reductase genes in C. albicans genome. CFL1 was the first ferric reductase identified in C. albicans. However, little is known about its roles in C. albicans physiology and pathogenicity. In this study, we found that disruption of CFL1 led to hypersensitivity to chemical and physical cell wall stresses, activation of the cell wall integrity (CWI) pathway, abnormal cell wall composition, and enhanced secretion, indicating a defect in CWI in this mutant. Moreover, this mutant showed abnormal mitochondrial activity and morphology, suggesting a link between ferric reductases and mitochondrial function. In addition, this mutant displayed decreased ability of adhesion to both the polystyrene microplates and buccal epithelial cells and invasion of host epithelial cells. These findings revealed a novel role of C. albicans Cfl1 in maintenance of CWI, mitochondrial function, and interaction between this pathogen and the host.

Mössbauer, EPR, and modeling study of iron trafficking and regulation in Δccc1 and CCC1-up Saccharomyces cerevisiae

Strains lacking and overexpressing the vacuolar iron (Fe) importer CCC1 were characterized using Mössbauer and EPR spectroscopies. Vacuolar Fe import is impeded in Δccc1 cells and enhanced in CCC1-up cells, causing vacuolar Fe in these strains to decline and accumulate, respectively, relative to WT cells. Cytosolic Fe levels should behave oppositely. The Fe content of Δccc1 cells grown under low-Fe conditions was similar to that in WT cells. Most Fe was mitochondrial with some nonheme high spin (NHHS) Fe^II present. Δccc1 cells grown with increasing Fe concentration in the medium contained less total Fe, less vacuolar HS Fe^III, and more NHHS Fe^II than in comparable WT cells. As the Fe concentration in the growth medium increased, the concentration of HS Fe^III in Δccc1 cells increased to just 60% of WT levels, while NHHS Fe^II increased to twice WT levels, suggesting that the NHHS Fe^II was cytosolic. Δccc1 cells suffered more oxidative damage than WT cells, suggesting that the accumulated NHHS Fe^II promoted Fenton chemistry. The Fe concentration in CCC1-up cells was higher than in WT cells; the extra Fe was present as NHHS Fe^II and Fe^III and as Fe^III oxyhydroxide nanoparticles. These cells contained less mitochondrial Fe and exhibited less ROS damage than Δccc1 cells. CCC1-up cells were adenine-deficient on minimal medium; supplementing with adenine caused a decline of NHHS Fe^II suggesting that some of the NHHS Fe^II that accumulated in these cells was associated with adenine deficiency rather than the overexpression of CCC1. A mathematical model was developed that simulated changes in Fe distributions. Simulations suggested that only a modest proportion of the observed NHHS Fe^II in both strains was the cytosolic form of Fe that is sensed by the Fe import regulatory system. The remainder is probably generated by the reduction of the vacuolar NHHS Fe^III species.

Novel role of the Candida albicans ferric reductase gene CFL1 in iron acquisition, oxidative stress tolerance, morphogenesis and virulence

Ferric reductase catalyzes the reduction of ferric iron into ferrous iron and plays an essential role in high-affinity iron acquisition. In this study, we found that the cfl1Δ/Δ (orf19.1263) mutant was not defective in iron acquisition. However, deletion of CFL1 increased cellular iron accumulation by elevating surface ferric reductase activity in Candida albicans, revealing that there existed functional redundancy and/or a compensatory upregulation mechanism among ferric reductase genes. The absence of CFL1 resulted in increased expression levels of other alternative ferric reductase genes, including FRP1, CFL2 and FRE10. In addition, CFL1 played an important role in the response to different oxidative stresses. Further research revealed that the cfl1Δ/Δ mutant exhibited higher levels of both ROS production and SOD activity under oxidative conditions. Moreover, deletion of CFL1 led to a profound defect in filamentous development in an iron-independent manner at both 30 and 37 °C. The cfl1Δ/Δ mutant exhibited highly attenuated virulence and reduced fungal burdens in the mouse systemic infection model, indicating that CFL1 might be a potential target for antifungal drug development. In summary, our results provide new insights into the roles of ferric reductase gene in C. albicans.

The diverse roles of FRO family metalloreductases in iron and copper homeostasis

Iron and copper are essential for plants and are important for the function of a number of protein complexes involved in photosynthesis and respiration. As the molecular mechanisms that control uptake, trafficking and storage of these nutrients emerge, the importance of metalloreductase-catalyzed reactions in iron and copper metabolism has become clear. This review focuses on the ferric reductase oxidase (FRO) family of metalloreductases in plants and highlights new insights into the roles of FRO family members in metal homeostasis. Arabidopsis FRO2 was first identified as the ferric chelate reductase that reduces ferric iron-chelates at the root surface-rhizosphere interface. The resulting ferrous iron is subsequently transported across the plasma membrane of root epidermal cells by the ferrous iron transporter, IRT1. Recent work has shown that two other members of the FRO family (FRO4 and FRO5) function redundantly to reduce copper to facilitate its uptake from the soil. In addition, FROs appear to play important roles in subcellular compartmentalization of iron as FRO7 is known to contribute to delivery of iron to chloroplasts while mitochondrial family members FRO3 and FRO8 are hypothesized to influence mitochondrial metal ion homeostasis. Finally, recent studies have underscored the importance of plasma membrane-localized ferric reductase activity in leaves for photosynthetic efficiency. Taken together, these studies highlight a number of diverse roles for FROs in both iron and copper metabolism in plants.

Regulation of cation balance in Saccharomyces cerevisiae

All living organisms require nutrient minerals for growth and have developed mechanisms to acquire, utilize, and store nutrient minerals effectively. In the aqueous cellular environment, these elements exist as charged ions that, together with protons and hydroxide ions, facilitate biochemical reactions and establish the electrochemical gradients across membranes that drive cellular processes such as transport and ATP synthesis. Metal ions serve as essential enzyme cofactors and perform both structural and signaling roles within cells. However, because these ions can also be toxic, cells have developed sophisticated homeostatic mechanisms to regulate their levels and avoid toxicity. Studies in Saccharomyces cerevisiae have characterized many of the gene products and processes responsible for acquiring, utilizing, storing, and regulating levels of these ions. Findings in this model organism have often allowed the corresponding machinery in humans to be identified and have provided insights into diseases that result from defects in ion homeostasis. This review summarizes our current understanding of how cation balance is achieved and modulated in baker's yeast. Control of intracellular pH is discussed, as well as uptake, storage, and efflux mechanisms for the alkali metal cations, Na(+) and K(+), the divalent cations, Ca(2+) and Mg(2+), and the trace metal ions, Fe(2+), Zn(2+), Cu(2+), and Mn(2+). Signal transduction pathways that are regulated by pH and Ca(2+) are reviewed, as well as the mechanisms that allow cells to maintain appropriate intracellular cation concentrations when challenged by extreme conditions, i.e., either limited availability or toxic levels in the environment.

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.

Evolution of the ferric reductase domain (FRD) superfamily: modularity, functional diversification, and signature motifs

A heme-containing transmembrane ferric reductase domain (FRD) is found in bacterial and eukaryotic protein families, including ferric reductases (FRE), and NADPH oxidases (NOX). The aim of this study was to understand the phylogeny of the FRD superfamily. Bacteria contain FRD proteins consisting only of the ferric reductase domain, such as YedZ and short bFRE proteins. Full length FRE and NOX enzymes are mostly found in eukaryotic cells and all possess a dehydrogenase domain, allowing them to catalyze electron transfer from cytosolic NADPH to extracellular metal ions (FRE) or oxygen (NOX). Metazoa possess YedZ-related STEAP proteins, possibly derived from bacteria through horizontal gene transfer. Phylogenetic analyses suggests that FRE enzymes appeared early in evolution, followed by a transition towards EF-hand containing NOX enzymes (NOX5- and DUOX-like). An ancestral gene of the NOX(1-4) family probably lost the EF-hands and new regulatory mechanisms of increasing complexity evolved in this clade. Two signature motifs were identified: NOX enzymes are distinguished from FRE enzymes through a four amino acid motif spanning from transmembrane domain 3 (TM3) to TM4, and YedZ/STEAP proteins are identified by the replacement of the first canonical heme-spanning histidine by a highly conserved arginine. The FRD superfamily most likely originated in bacteria.

The ins and outs of algal metal transport

Metal transporters are a central component in the interaction of algae with their environment. They represent the first line of defense to cellular perturbations in metal concentration, and by analyzing algal metal transporter repertoires, we gain insight into a fundamental aspect of algal biology. The ability of individual algae to thrive in environments with unique geochemistry, compared to non-algal species commonly used as reference organisms for metal homeostasis, provides an opportunity to broaden our understanding of biological metal requirements, preferences and trafficking. Chlamydomonas reinhardtii is the best developed reference organism for the study of algal biology, especially with respect to metal metabolism; however, the diversity of algal niches necessitates a comparative genomic analysis of all sequenced algal genomes. A comparison between known and putative proteins in animals, plants, fungi and algae using protein similarity networks has revealed the presence of novel metal metabolism components in Chlamydomonas including new iron and copper transporters. This analysis also supports the concept that, in terms of metal metabolism, algae from similar niches are more related to one another than to algae from the same phylogenetic clade. This article is part of a Special Issue entitled: Cell Biology of Metals.

Elemental economy: microbial strategies for optimizing growth in the face of nutrient limitation

Microorganisms play a dominant role in the biogeochemical cycling of nutrients. They are rightly praised for their facility for fixing both carbon and nitrogen into organic matter, and microbial driven processes have tangibly altered the chemical composition of the biosphere and its surrounding atmosphere. Despite their prodigious capacity for molecular transformations, microorganisms are powerless in the face of the immutability of the elements. Limitations for specific elements, either fleeting or persisting over eons, have left an indelible trace on microbial genomes, physiology, and their very atomic composition. We here review the impact of elemental limitation on microbes, with a focus on selected genetic model systems and representative microbes from the ocean ecosystem. Evolutionary adaptations that enhance growth in the face of persistent or recurrent elemental limitations are evident from genome and proteome analyses. These range from the extreme (such as dispensing with a requirement for a hard to obtain element) to the extremely subtle (changes in protein amino acid sequences that slightly, but significantly, reduce cellular carbon, nitrogen, or sulfur demand). One near-universal adaptation is the development of sophisticated acclimation programs by which cells adjust their chemical composition in response to a changing environment. When specific elements become limiting, acclimation typically begins with an increased commitment to acquisition and a concomitant mobilization of stored resources. If elemental limitation persists, the cell implements austerity measures including elemental sparing and elemental recycling. Insights into these fundamental cellular properties have emerged from studies at many different levels, including ecology, biological oceanography, biogeochemistry, molecular genetics, genomics, and microbial physiology. Here, we present a synthesis of these diverse studies and attempt to discern some overarching themes.

Mössbauer and EPR study of iron in vacuoles from fermenting Saccharomyces cerevisiae

Vacuoles were isolated from fermenting yeast cells grown on minimal medium supplemented with 40 μM (57)Fe. Absolute concentrations of Fe, Cu, Zn, Mn, Ca, and P in isolated vacuoles were determined by ICP-MS. Mössbauer spectra of isolated vacuoles were dominated by two spectral features: a mononuclear magnetically isolated high-spin (HS) Fe(III) species coordinated primarily by hard/ionic (mostly or exclusively oxygen) ligands and superparamagnetic Fe(III) oxyhydroxo nanoparticles. EPR spectra of isolated vacuoles exhibited a g(ave) ~ 4.3 signal typical of HS Fe(III) with E/D ~ 1/3. Chemical reduction of the HS Fe(III) species was possible, affording a Mössbauer quadrupole doublet with parameters consistent with O/N ligation. Vacuolar spectral features were present in whole fermenting yeast cells; however, quantitative comparisons indicated that Fe leaches out of vacuoles during isolation. The in vivo vacuolar Fe concentration was estimated to be ~1.2 mM while the Fe concentration of isolated vacuoles was ~220 μM. Mössbauer analysis of Fe(III) polyphosphate exhibited properties similar to those of vacuolar Fe. At the vacuolar pH of 5, Fe(III) polyphosphate was magnetically isolated, while at pH 7, it formed nanoparticles. This pH-dependent conversion was reversible. Fe(III) polyphosphate could also be reduced to the Fe(II) state, affording similar Mössbauer parameters to that of reduced vacuolar Fe. These results are insufficient to identify the exact coordination environment of the Fe(III) species in vacuoles, but they suggest a complex closely related to Fe(III) polyphosphate. A model for Fe trafficking into/out of yeast vacuoles is proposed.

The metalloreductase FreB is involved in adaptation of Aspergillus fumigatus to iron starvation

Aspergillus fumigatus employs two high affinity iron uptake mechanisms, siderophore mediated iron uptake and reductive iron assimilation (RIA). The A. fumigatus genome encodes 15 putative metalloreductases (MR) but the ferrireductases involved in RIA remained elusive so far. Expression of the MR FreB was found to be transcriptionally repressed by iron via SreA, a repressor of iron acquisition during iron sufficiency, indicating a role in iron metabolism. FreB-inactivation by gene deletion was phenotypically largely inconspicuous unless combined with inactivation of the siderophore system, which then decreased growth rate, surface ferrireductase activity and oxidative stress resistance during iron starvation. This study also revealed that loss of copper-independent siderophore-mediated iron uptake increases sensitivity of A. fumigatus to copper starvation due to copper-dependence of RIA.

Cap2-HAP complex is a critical transcriptional regulator that has dual but contrasting roles in regulation of iron homeostasis in Candida albicans

Iron homeostasis is highly regulated in organisms across evolutionary time scale as iron is essential for various cellular processes. In a computational screen, we identified the Yap/bZIP domain family in Candida clade genomes. Cap2/Hap43 is essential for C. albicans growth under iron-deprivation conditions and for virulence in mouse. Cap2 has an amino-terminal bipartite domain comprising a fungal-specific Hap4-like domain and a bZIP domain. Our mutational analyses showed that both the bZIP and Hap4-like domains perform critical and independent functions for growth under iron-deprivation conditions. Transcriptome analysis conducted under iron-deprivation conditions identified about 16% of the C. albicans ORFs that were differentially regulated in a Cap2-dependent manner. Microarray data also suggested that Cap2 is required to mobilize iron through multiple mechanisms; chiefly by activation of genes in three iron uptake pathways and repression of iron utilizing and iron storage genes. The expression of HAP2, HAP32, and HAP5, core components of the HAP regulatory complex was induced in a Cap2-dependent manner indicating a feed-forward loop. In a feed-back loop, Cap2 repressed the expression of Sfu1, a negative regulator of iron uptake genes. Cap2 was coimmunoprecipitated with Hap5 from cell extracts prepared from iron-deprivation conditions indicating an in vivo association. ChIP assays demonstrated Hap32-dependent recruitment of Hap5 to the promoters of FRP1 (Cap2-induced) and ACO1 (Cap2-repressed). Together our data indicates that the Cap2-HAP complex functions both as a positive and a negative regulator to maintain iron homeostasis in C. albicans.

Allocation of Fe and ferric chelate reductase activities in mesophyll cells of barley and sorghum under Fe-deficient conditions

Although the photosynthetic apparatus requires large amounts of Fe, the adaptive mechanisms of mesophyll cells for Fe acquisition under Fe-deficient conditions are unknown. Barley and sorghum, which are tolerant and susceptible to Fe deficiency, respectively, have similar Fe and chlorophyll contents in their leaves. However, the Fe-deficient barley photosynthetic apparatus was functional while that of sorghum was not. We show that barley preferentially allocates Fe to thylakoid membranes under Fe-deficient conditions. On the other hand, in sorghum, the proportion of leaf Fe allocated to thylakoids was not altered by Fe deficiency. The relationship between the maintenance of photosynthesis and light-dependent ferric chelate reductase activity on plasma membranes and chloroplast envelopes is also discussed.

Transition metal homeostasis: from yeast to human disease

Transition metal ions are essential nutrients to all forms of life. Iron, copper, zinc, manganese, cobalt and nickel all have unique chemical and physical properties that make them attractive molecules for use in biological systems. Many of these same properties that allow these metals to provide essential biochemical activities and structural motifs to a multitude of proteins including enzymes and other cellular constituents also lead to a potential for cytotoxicity. Organisms have been required to evolve a number of systems for the efficient uptake, intracellular transport, protein loading and storage of metal ions to ensure that the needs of the cells can be met while minimizing the associated toxic effects. Disruptions in the cellular systems for handling transition metals are observed as a number of diseases ranging from hemochromatosis and anemias to neurodegenerative disorders including Alzheimer's and Parkinson's disease. The yeast Saccharomyces cerevisiae has proved useful as a model organism for the investigation of these processes and many of the genes and biological systems that function in yeast metal homeostasis are conserved throughout eukaryotes to humans. This review focuses on the biological roles of iron, copper, zinc, manganese, nickel and cobalt, the homeostatic mechanisms that function in S. cerevisiae and the human diseases in which these metals have been implicated.

Biophysical investigation of the iron in Aft1-1(up) and Gal-YAH1 Saccharomyces cerevisiae

Aft1p is a major iron regulator in budding yeast Saccharomyces cerevisiae. It indirectly senses cytosolic Fe status and responds by activating or repressing iron regulon genes. Aft1p within the Aft1-1(up) strain has a single amino acid mutation which causes it to constitutively activate iron regulon genes regardless of cellular Fe status. This leads to elevated Fe uptake under both low and high Fe growth conditions. Ferredoxin Yah1p is involved in Fe/S cluster assembly, and Aft1p-targeted iron regulon genes are also upregulated in Yah1p-depleted cells. In this study Mössbauer, EPR, and UV-vis spectroscopies were used to characterize the Fe distribution in Aft1-1(up) and Yah1p-depleted cells. Aft1-1(up) cells grown in low Fe medium contained more Fe than did WT cells. A basal level of Fe in both WT and Aft1-1(up) cells was located in mitochondria, primarily in the form of Fe/S clusters and heme centers. The additional Fe in Aft1-1(up) cells was present as mononuclear HS Fe(III) species. These species are in a nonmitochondrial location, assumed here to be vacuolar. Aft1-1(up) cells grown in high Fe medium contained far more Fe than found in WT cells. The extra Fe was present as HS Fe(III) ions, probably stored in vacuoles, and as Fe(III) phosphate nanoparticles, located in mitochondria. Yah1p-deficent cells also accumulated nanoparticles in their mitochondria, but they did not contain HS Fe(III) species. Results are interpreted by a proposed model involving three homeostatic regulatory systems, including the Aft1 system, a vacuolar iron regulatory system, and a mitochondrial Fe regulatory system.

Redox cycling in iron uptake, efflux, and trafficking

Aerobic organisms are faced with a dilemma. Environmental iron is found primarily in the relatively inert Fe(III) form, whereas the more metabolically active ferrous form is a strong pro-oxidant. This conundrum is solved by the redox cycling of iron between Fe(III) and Fe(II) at every step in the iron metabolic pathway. As a transition metal ion, iron can be "metabolized" only by this redox cycling, which is catalyzed in aerobes by the coupled activities of ferric iron reductases (ferrireductases) and ferrous iron oxidases (ferroxidases).

A nonheme high-spin ferrous pool in mitochondria isolated from fermenting Saccharomyces cerevisiae

Mössbauer spectroscopy was used to detect pools of Fe in mitochondria from fermenting yeast cells, including those consisting of nonheme high-spin (HS) Fe(II) species, Fe(III) nanoparticles, and mononuclear HS Fe(III) species. At issue was whether these species were located within mitochondria or on their exterior. None could be removed by washing mitochondria extensively with ethylene glycol tetraacetic acid or bathophenanthroline sulfonate (BPS), Fe(II) chelators that do not appear to penetrate mitochondrial membranes. However, when mitochondrial samples were sonicated, BPS coordinated the Fe(II) species, forming a low-spin Fe(II) complex. This treatment also diminished the levels of both Fe(III) species, suggesting that all of these Fe species are encapsulated by mitochondrial membranes and are protected from chelation until membranes are disrupted. 1,10-Phenanthroline is chemically similar to BPS but is membrane soluble; it coordinated nonheme HS Fe(II) in unsonicated mitochondria. Further, the HS Fe(III) species and nanoparticles were not reduced by dithionite until the detergent deoxycholate was added to disrupt membranes. There was no correlation between the percentage of nonheme HS Fe(II) species in mitochondrial samples and the level of contaminating proteins. These results collectively indicate that the observed Fe species are contained within mitochondria. Mossbauer spectra of whole cells were dominated by HS Fe(III) features; the remainder displayed spectral features typical of isolated mitochondria, suggesting that the Fe in fermenting yeast cells can be coarsely divided into two categories: mitochondrial Fe and (mostly) HS Fe(III) ions in one or more non-mitochondrial locations.

abc3+ encodes an iron-regulated vacuolar ABC-type transporter in Schizosaccharomyces pombe

Studies have shown the fundamental contribution of the yeast vacuole as a site for storage and detoxification of metals. Whereas the transmembrane proteins responsible for iron transport into and out of the vacuole have been identified in Saccharomyces cerevisiae, less information is available concerning the mobilization of vacuolar iron stores in Schizosaccharomyces pombe. In this study, we report the identification of a gene designated abc3(+) that encodes a protein which exhibits sequence homology with the ABCC subfamily of ATP-binding cassette transporters. The transcription of abc3(+) is induced by low concentrations of iron but repressed by high levels of iron. The iron-mediated repression of abc3(+) required a functional fep1(+) gene. Chromatin immunoprecipitation assays showed that Fep1 associates with the abc3(+) promoter in vivo, in an iron-dependent manner. Microscopic analyses revealed that a functional Abc3-green fluorescent protein localizes to the membrane vacuole when iron levels were low. Abc3 was required for growth in low-iron medium in the absence of the transport system mediated by Fio1 and Fip1. abc3Delta cells exhibited increased levels of expression of the frp1(+)-encoded ferric reductase, suggesting a loss of Fep1 repression and, consequently, the activation of Fep1-regulated genes. When abc3(+) was expressed using the nmt1(+) promoter system, its induction led to a reduced transcriptional activity of the frp1(+) gene. Because S. pombe does not possess vacuolar membrane-localized orthologs to S. cerevisiae Fth1, Fet5, and Smf3, our findings suggested that Abc3 may be responsible for mobilizing stored iron from the vacuole to the cytosol in response to iron deficiency.