The ABC transporter Atm1p is required for mitochondrial iron homeostasis

SufU is an essential iron-sulfur cluster scaffold protein in Bacillus subtilis

Bacteria use three distinct systems for iron-sulfur (Fe/S) cluster biogenesis: the ISC, SUF, and NIF machineries. The ISC and SUF systems are widely distributed, and many bacteria possess both of them. In Escherichia coli, ISC is the major and constitutive system, whereas SUF is induced under iron starvation and/or oxidative stress. Genomic analysis of the Fe/S cluster biosynthesis genes in Bacillus subtilis suggests that this bacterium's genome encodes only a SUF system consisting of a sufCDSUB gene cluster and a distant sufA gene. Mutant analysis of the putative Fe/S scaffold genes sufU and sufA revealed that sufU is essential for growth under minimal standard conditions, but not sufA. The drastic growth retardation of a conditional mutant depleted of SufU was coupled with a severe reduction of aconitase and succinate dehydrogenase activities in total-cell lysates, suggesting a crucial function of SufU in Fe/S protein biogenesis. Recombinant SufU was devoid of Fe/S clusters after aerobic purification. Upon in vitro reconstitution, SufU bound an Fe/S cluster with up to approximately 1.5 Fe and S per monomer. The assembled Fe/S cluster could be transferred from SufU to the apo form of isopropylmalate isomerase Leu1, rapidly forming catalytically active [4Fe-4S]-containing holo-enzyme. In contrast to native SufU, its D43A variant carried a Fe/S cluster after aerobic purification, indicating that the cluster is stabilized by this mutation. Further, we show that apo-SufU is an activator of the cysteine desulfurase SufS by enhancing its activity about 40-fold in vitro. SufS-dependent formation of holo-SufU suggests that SufU functions as an Fe/S cluster scaffold protein tightly cooperating with the SufS cysteine desulfurase.

Biophysical characterization of the iron in mitochondria from Atm1p-depleted Saccharomyces cerevisiae

Atm1p is an ABC transporter localized in the mitochondrial inner membrane; it functions to export an unknown species into the cytosol and is involved in cellular iron metabolism. Depletion or deletion of Atm1p causes Fe accumulation in mitochondria and a defect in cytosolic Fe/S cluster assembly but reportedly not a defect in mitochondrial Fe/S cluster assembly. In this study the nature of the accumulated Fe was examined using Mossbauer spectroscopy, EPR, electronic absorption spectroscopy, X-ray absorption spectroscopy, and electron microscopy. The Fe that accumulated in aerobically grown cells was in the form of iron(III) phosphate nanoparticles similar to that which accumulates in yeast frataxin Yfh1p-deleted or yeast ferredoxin Yah1p-depleted cells. Relative to WT mitochondria, Fe/S cluster and heme levels in Atm1p-depleted mitochondria from aerobic cells were significantly diminished. Atm1p depletion also caused a buildup of nonheme Fe(II) ions in the mitochondria and an increase in oxidative damage. Atm1p-depleted mitochondria isolated from anaerobically grown cells exhibited WT levels of Fe/S clusters and hemes, and they did not hyperaccumulate Fe. Atm1p-depleted cells lacked Leu1p activity, regardless of whether they were grown aerobically or anaerobically. These results indicate that Atm1p does not participate in mitochondrial Fe/S cluster assembly and that the species exported by Atm1p is required for cytosolic Fe/S cluster assembly. The Fe/S cluster defect and the Fe-accumulation phenotype, resulting from the depletion of Atm1p in aerobic cells (but not in anaerobic cells), may be secondary effects that are observed only when cells are exposed to oxygen during growth. Reactive oxygen species generated under these conditions might degrade iron-sulfur clusters and lower heme levels in the organelle.

Post-translational modifications of superoxide dismutase

Post-translational modifications of proteins control many biological processes through the activation, inactivation, or gain-of-function of the proteins. Recent developments in mass spectrometry have enabled detailed structural analyses of covalent modifications of proteins and also have shed light on the post-translational modification of superoxide dismutase. In this review, we introduce some covalent modifications of superoxide dismutase, nitration, phosphorylation, glutathionylaion, and glycation. Nitration has been the most extensively analyzed modification both in vitro and in vivo. Reaction of human Cu,Zn superoxide dismutase (SOD) with reactive nitrogen species resulted in nitration of a single tryptophan residue to 6-nitrotryptophan, which could be a new biomarker of a formation of reactive nitrogen species. On the other hand, tyrosine 34 of human MnSOD was exclusively nitrated to 3-nitrotyrosine and almost completely inactivated by the reaction with peroxynitrite. The nitrated MnSOD has been found in many diseases caused by ischemia/reperfusion, inflammation, and others and may have a pivotal role in the pathology of the diseases. Most of the post-translational modifications have given rise to a reduced activity of SOD. Since phosphorylation and nitration of SOD have been shown to have a possible reversible process, these modifications may be related to a redox signaling process in cells. Finally we briefly introduce a metal insertion system of SOD, focusing particularly on the iron misincorporation of nSOD, as a part of post-translational modifications.

An allelic mutant series of ATM3 reveals its key role in the biogenesis of cytosolic iron-sulfur proteins in Arabidopsis

The ATP-binding cassette transporters of mitochondria (ATMs) are highly conserved proteins, but their function in plants is poorly defined. Arabidopsis (Arabidopsis thaliana) has three ATM genes, namely ATM1, ATM2, and ATM3. Using a collection of insertional mutants, we show that only ATM3 has an important function for plant growth. Additional atm3 alleles were identified among sirtinol-resistant lines, correlating with decreased activities of aldehyde oxidases, cytosolic enzymes that convert sirtinol into an auxin analog, and depend on iron-sulfur (Fe-S) and molybdenum cofactor (Moco) as prosthetic groups. In the sirtinol-resistant atm3-3 allele, the highly conserved arginine-612 is replaced by a lysine residue, the negative effect of which could be mimicked in the yeast Atm1p ortholog. Arabidopsis atm3 mutants displayed defects in root growth, chlorophyll content, and seedling establishment. Analyses of selected metal enzymes showed that the activity of cytosolic aconitase (Fe-S) was strongly decreased across the range of atm3 alleles, whereas mitochondrial and plastid Fe-S enzymes were unaffected. Nitrate reductase activity (Moco, heme) was decreased by 50% in the strong atm3 alleles, but catalase activity (heme) was similar to that of the wild type. Strikingly, in contrast to mutants in the yeast and mammalian orthologs, Arabidopsis atm3 mutants did not display a dramatic iron homeostasis defect and did not accumulate iron in mitochondria. Our data suggest that Arabidopsis ATM3 may transport (1) at least two distinct compounds or (2) a single compound required for both Fe-S and Moco assembly machineries in the cytosol, but not iron.

The interaction of mitochondrial iron with manganese superoxide dismutase

Superoxide dismutase 2 (SOD2) is one of the rare mitochondrial enzymes evolved to use manganese as a cofactor over the more abundant element iron. Although mitochondrial iron does not normally bind SOD2, iron will misincorporate into Saccharomyces cerevisiae Sod2p when cells are starved for manganese or when mitochondrial iron homeostasis is disrupted by mutations in yeast grx5, ssq1, and mtm1. We report here that such changes in mitochondrial manganese and iron similarly affect cofactor selection in a heterologously expressed Escherichia coli Mn-SOD, but not a highly homologous Fe-SOD. By x-ray absorption near edge structure and extended x-ray absorption fine structure analyses of isolated mitochondria, we find that misincorporation of iron into yeast Sod2p does not correlate with significant changes in the average oxidation state or coordination chemistry of bulk mitochondrial iron. Instead, small changes in mitochondrial iron are likely to promote iron-SOD2 interactions. Iron binds Sod2p in yeast mutants blocking late stages of iron-sulfur cluster biogenesis (grx5, ssq1, and atm1), but not in mutants defective in the upstream Isu proteins that serve as scaffolds for iron-sulfur biosynthesis. In fact, we observed a requirement for the Isu proteins in iron inactivation of yeast Sod2p. Sod2p activity was restored in mtm1 and grx5 mutants by depleting cells of Isu proteins or using a dominant negative Isu1p predicted to stabilize iron binding to Isu1p. In all cases where disruptions in iron homeostasis inactivated Sod2p, we observed an increase in mitochondrial Isu proteins. These studies indicate that the Isu proteins and the iron-sulfur pathway can donate iron to Sod2p.

Mitochondrial dysfunction leads to nuclear genome instability via an iron-sulfur cluster defect

Mutations and deletions in the mitochondrial genome (mtDNA), as well as instability of the nuclear genome, are involved in multiple human diseases. Here, we report that in Saccharomyces cerevisiae, loss of mtDNA leads to nuclear genome instability, through a process of cell-cycle arrest and selection we define as a cellular crisis. This crisis is not mediated by the absence of respiration, but instead correlates with a reduction in the mitochondrial membrane potential. Analysis of cells undergoing this crisis identified a defect in iron-sulfur cluster (ISC) biogenesis, which requires normal mitochondrial function. We found that downregulation of nonmitochondrial ISC protein biogenesis was sufficient to cause increased genomic instability in cells with intact mitochondrial function. These results suggest mitochondrial dysfunction stimulates nuclear genome instability by inhibiting the production of ISC-containing protein(s), which are required for maintenance of nuclear genome integrity. For a video summary of this article, see the PaperFlick file available with the online Supplemental Data.

Stimulation of the ATPase activity of the yeast mitochondrial ABC transporter Atm1p by thiol compounds

The ATP binding cassette (ABC) transporter Atm1p of the mitochondrial inner membrane performs crucial roles in both the biogenesis of cytosolic/nuclear iron-sulfur proteins and cellular iron homeostasis. Since the function of the mitochondrial iron-sulfur cluster (ISC) assembly machinery is also required for these two processes, Atm1p is thought to translocate a still unknown product of this pathway to the cytosol. Here, we provide a detailed in vitro characterization of Atm1p in order to better understand its function. Atm1p was purified using an expression system in E. coli. The detergent-solubilised protein exhibits a stable ATPase activity. Reconstitution of Atm1p into proteoliposomes allowed us to determine the biochemical characteristics of the ATPase such as: (i) the strong inhibition by the transition state analogue vanadate, (ii) a Km value of 0.1 mM, and (iii) a turnover number of 127 min-1. The ATPase activity of ABC transporters is generally stimulated by their specific substrate. We used this property to define the chemical properties of the substrate transported by Atm1p. ATPase hydrolysis by Atm1p-containing proteoliposomes was specifically increased 3-5-fold by thiol-containing compounds, in particular by micromolar concentrations of cysteine thiol groups in peptides, even though Atm1p is not a general peptide transporter such as yeast Mdl1p or mammalian TAP which share sequence similarity with Atm1p. We speculate that the physiological substrate of Atm1p may contain multiple sulfhydryl groups in a peptidic environment.

Mitochondrial ABC proteins in health and disease

ABC transporters represent one of the largest families of membrane proteins that are found in all three phyla of life. Mitochondria comprise up to four ABC systems, ABCB7/ATM1, ABCB10/MDL1, ABCB8 and ABCB6. These half-transporters, which assemble into homodimeric complexes, are involved in a number of key cellular processes, e.g. biogenesis of cytosolic iron-sulfur clusters, heme biosynthesis, iron homeostasis, multidrug resistance, and protection against oxidative stress. Here, we summarize recent advances and emerging themes in our understanding of how these ABC systems in the inner and outer mitochondrial membrane fulfill their functions in important (patho) physiological processes, including neurodegenerative and hematological disorders.

The ABC-transporter AtmA is involved in nickel and cobalt resistance of Cupriavidus metallidurans strain CH34

Cupriavidus metallidurans CH34 genome contains an ortholog of Atm1p named AtmA (Rmet_0391, YP_582546). In Saccharomyces cerevisiae, the ABC-type transport system Atm1p is involved in export of iron-sulfur clusters from mitochondria into the cytoplasm for assembly of cytoplasmic iron-sulfur containing proteins. An atmA mutant of C. metallidurans was sensitive to nickel and cobalt but not iron cations. AtmA increased also resistance to these cations in Escherichia coli strains that carry deletions of the genes for other nickel and cobalt transport systems. In C. metallidurans, atmA expression was not significantly induced by nickel and cobalt, but repressed by zinc. AtmA was purified as a 70 kDa protein after expression in E. coli. ATPase activity of AtmA was stimulated by nickel and cobalt.

The power plant of the cell is also a smithy: the emerging role of mitochondria in cellular iron homeostasis

Iron is required for a barrage of essential biochemical functions in virtually every species of life. Perturbation of the availability or utilization of iron in these functions or disruption of other components along iron-requiring pathways can not only lead to cellular/organismal insufficiency of respective biochemical end-products but also result in a broad derangement of iron homeostasis. This is largely because of the elaborate regulatory mechanisms that connect cellular iron utilization with uptake and distribution. Such mechanisms are necessitated by the 'double-edged' nature of the metal, whose very property as a useful biological catalyst also makes it able to generate highly toxic compounds. Since the majority of iron is dispatched onto a functional course by mitochondria-localized pathways, these organelles are in an ideal position within the cellular iron anabolic pathways to be a central site for regulation of iron homeostasis. The goal of this article is to provide an overview of how mitochondria acquire and use iron and examine the ramifications of disturbances in these processes on overall cellular iron homeostasis.

Chapter 12 Controlled expression of iron-sulfur cluster assembly components for respiratory chain complexes in mammalian cells

Three of the respiratory chain complexes contain essential iron-sulfur (Fe/S) cluster prosthetic groups. Besides respiration, these ancient inorganic cofactors are also necessary for numerous other fundamental biochemical processes in virtually every known organism. Both the synthesis of Fe/S clusters and their delivery to apoproteins depend on the concerted function of specialized, often dedicated, proteins located in the mitochondria and cytosol of eukaryotes. Impaired function of the mitochondria-located Fe/S cluster (ISC) assembly machinery affects all cellular Fe/S proteins, including enzymes of the respiratory chain, NADH: ubiquinone oxidoreductase (complex I; eight Fe/S clusters), succinate: ubiquinone oxidoreductase (complex II; three Fe/S clusters), and cytochrome bc(1) complex (complex III; one Fe/S cluster). Here, we describe strategies and techniques both to deprive respiratory chain proteins of their Fe/S cofactors and to study changes in activity and composition of these proteins. As examples, we present the results of the depletion of two types of Fe/S biogenesis proteins, huNfs1 and huInd1, in a human tissue culture model. The ISC assembly component huNfs1 is required for biogenesis of all cellular Fe/S proteins, its loss exerting pleiotropic effects, whereas huInd1 is specific for Fe/S cluster maturation of complex I. Disorders in Fe/S cluster assembly are candidate causes for defects in respiratory complex assembly of unknown etiology.

Regulation of mitochondrial iron import through differential turnover of mitoferrin 1 and mitoferrin 2

Mitoferrin 1 and mitoferrin 2 are homologous members of the mitochondrial solute carrier family. Mitoferrin 1 is required for mitochondrial iron delivery in developing erythrocytes. Here we show that mitoferrin 1 and mitoferrin 2 contribute to mitochondrial iron delivery in a variety of cells. Reductions in mitoferrin 1 and/or mitoferrin 2 levels by RNA interference result in decreased mitochondrial iron accumulation, heme synthesis, and iron-sulfur cluster synthesis. The ectopic expression of mitoferrin 1 in nonerythroid cells silenced for mitoferrin 2 or the expression of mitoferrin 2 in cells silenced for mitoferrin 1 restored heme synthesis to "baseline" levels. The ectopic expression of mitoferrin 2, however, did not support hemoglobinization in erythroid cells deficient in mitoferrin 1. Mitoferrin 2 could not restore heme synthesis in developing erythroid cells because of an inability of the protein to accumulate in mitochondria. The half-life of mitoferrin 1 was increased in developing erythroid cells, while the half-life of mitoferrin 2 did not change. These results suggest that mitochondrial iron accumulation is tightly regulated and that controlling mitoferrin levels within the mitochondrial membrane provides a mechanism to regulate mitochondrial iron levels.

Human Nbp35 is essential for both cytosolic iron-sulfur protein assembly and iron homeostasis

The maturation of cytosolic iron-sulfur (Fe/S) proteins in mammalian cells requires components of the mitochondrial iron-sulfur cluster assembly and export machineries. Little is known about the cytosolic components that may facilitate the assembly process. Here, we identified the cytosolic soluble P-loop NTPase termed huNbp35 (also known as Nubp1) as an Fe/S protein, and we defined its role in the maturation of Fe/S proteins in HeLa cells. Depletion of huNbp35 by RNA interference decreased cell growth considerably, indicating its essential function. The deficiency in huNbp35 was associated with an impaired maturation of the cytosolic Fe/S proteins glutamine phosphoribosylpyrophosphate amidotransferase and iron regulatory protein 1 (IRP1), while mitochondrial Fe/S proteins remained intact. Consequently, huNbp35 is specifically involved in the formation of extramitochondrial Fe/S proteins. The impaired maturation of IRP1 upon huNbp35 depletion had profound consequences for cellular iron metabolism, leading to decreased cellular H-ferritin, increased transferrin receptor levels, and higher transferrin uptake. These properties clearly distinguished huNbp35 from its yeast counterpart Nbp35, which is essential for cytosolic-nuclear Fe/S protein assembly but plays no role in iron regulation. huNbp35 formed a complex with its close homologue huCfd1 (also known as Nubp2) in vivo, suggesting the existence of a heteromeric P-loop NTPase complex that is required for both cytosolic Fe/S protein assembly and cellular iron homeostasis.

The role of the iron transporter ABCB7 in refractory anemia with ring sideroblasts

Refractory Anemia with Ring Sideroblasts (RARS) is an acquired myelodysplastic syndrome (MDS) characterized by an excess iron accumulation in the mitochondria of erythroblasts. The pathogenesis of RARS and the cause of this unusual pattern of iron deposition remain unknown. We considered that the inherited X-linked sideroblastic anemia with ataxia (XLSA/A) might be informative for the acquired disorder, RARS. XLSA/A is caused by partial inactivating mutations of the ABCB7 ATP-binding cassette transporter gene, which functions to enable transport of iron from the mitochondria to the cytoplasm. Furthermore, ABCB7 gene silencing in HeLa cells causes an accumulation of iron in the mitochondria. We have studied the role of ABCB7 in RARS by DNA sequencing, methylation studies, and gene expression studies in primary CD34(+) cells and in cultured erythroblasts. The DNA sequence of the ABCB7 gene is normal in patients with RARS. We have investigated ABCB7 gene expression levels in the CD34(+) cells of 122 MDS cases, comprising 35 patients with refractory anemia (RA), 33 patients with RARS and 54 patients with RA with excess blasts (RAEB), and in the CD34(+) cells of 16 healthy controls. We found that the expression levels of ABCB7 are significantly lower in the RARS group. RARS is thus characterized by lower levels of ABCB7 gene expression in comparison to other MDS subtypes. Moreover, we find a strong relationship between increasing percentage of bone marrow ring sideroblasts and decreasing ABCB7 gene expression levels. Erythroblast cell cultures confirm the low levels of ABCB7 gene expression levels in RARS. These data provide an important link between inherited and acquired forms of sideroblastic anemia and indicate that ABCB7 is a strong candidate gene for RARS.

Mitochondrial ATP-binding cassette proteins

The family of ATP-binding cassette (ABC) proteins is among the largest and most diverse in biology. Members of this family are transmembrane proteins found in all organisms and all biologic membranes from the plasma membrane to intracellular organelles such as the Golgi apparatus, lysosomes, peroxisomes, endoplasmic reticulum, and mitochondria. These proteins are very abundant in bacteria, and given the generally accepted origin of mitochondria from an alpha-proteobacterium, it is logical to assume the mitochondria would also contain these proteins. Mitochondria, however, have surprisingly few ABC proteins and they are dissimilar from those of bacteria. Despite their relative paucity, mitochondrial ABC proteins are believed to play a very important role in cellular homeostasis across very diverse species, including yeast, higher plants, mice, and humans. The yeast protein Atm1p plays a critical role in the transport of Fe/S clusters to the cytosol, and a similar function has been attributed to the homologous human proteins MTABC3 and ABC7. Another yeast protein Mdl1p is a high copy suppressor of ATM1, and regulates cellular resistance to oxidative stress and may be involved in peptide transport across the mitochondrial membrane. The human protein mABC1 has recently been identified to be involved in protection of myocardial cells against oxidative stress. Despite their low numbers, mitochondrial ABC proteins are intricately involved in mitochondrial and cellular homeostasis and may be important mediators of cell survival. In this review, we will discuss the structure, function, physiology, and pathophysiology of these mitochondrial ABC proteins.

Human ABCB6 Localizes to Both the Outer Mitochondrial Membrane and the Plasma Membrane

Expression of the ATP-binding cassette transporter ABCB6 has been associated with multiple cellular functions, including resistance to several cytotoxic agents, iron homeostasis, and porphyrin transport. To further elucidate its physiological function and/or role in drug resistance, we determined the subcellular location of ABCB6. Using three novel ABCB6-specific antibodies, Western blot analysis of cells expressing cDNA-derived or endogenous ABCB6 revealed two distinct molecular weight forms. Confocal microscopy indicates that the protein localizes to both mitochondria and the plasma membrane. Differential centrifugation revealed that the lower molecular weight form predominantly resides in the mitochondria, while the larger protein form is more abundant in the plasma membrane. Preliminary studies indicate that ABCB6 is functionally relevant in the plasma membrane, where its expression prevents the accumulation of specific porphyrins in the cell. Digitonin solubilization of mitochondria demonstrated that ABCB6 is present in the outer mitochondrial membrane, while back-titration assays with the ABCB6-specific antibodies reveal that the nucleotide binding domain of ABCB6 is cytoplasmic. These studies are the first to demonstrate that ABCB6 exists in two molecular weight forms, is localized to both the outer mitochondrial membrane and the plasma membrane, and plays a functional role in the plasma membrane.

The system biology of thiol redox system inEscherichia coliand yeast: Differential functions in oxidative stress, iron metabolism and DNA synthesis

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

Functional characterization of AtATM1, AtATM2, and AtATM3, a subfamily of Arabidopsis half-molecule ATP-binding cassette transporters implicated in iron homeostasis

The functional capabilities of one of the smallest subfamilies of ATP-binding cassette transporters from Arabidopsis thaliana, the AtATMs, are described. Designated AtATM1, AtAATM2, and AtATM3, these half-molecule ABC proteins are homologous to the yeast mitochondrial membrane protein ATM1 (ScATM1), which is clearly implicated in the export of mitochondrially synthesized iron/sulfur clusters. Yeast ATM1-deficient (atm1) mutants grow very slowly (have a petite phenotype), are respiration-deficient, accumulate toxic levels of iron in their mitochondria, and show enhanced compensatory high affinity iron uptake. Of the three Arabidopsis ATMs, AtATM3 bears the closest functional resemblance to ScATM1. Heterologously expressed AtATM3 is not only able to complement the yeast atm1 petite phenotype but is also able to suppress the constitutively high capacity for high affinity iron uptake associated with loss of the chromosomal copy of ScATM1, abrogate intra-mitochondrial iron hyperaccumulation, and restore mitochondrial respiratory function and cytochrome c levels. By comparison, AtATM1 only weakly suppresses the atm1 phenotype, and AtATM2 exerts little or no suppressive action but instead is toxic when expressed in this system. The differences between AtATM3 and AtATM1 are maintained after exchanging their target peptides, and these proteins as well as AtATM2 colocalize with the mitochondrial fluor MitoTracker Red when expressed in yeast as GFP fusions. Although its toxicity when heterologously expressed in yeast, except when fused with GFP, precludes the functional analysis of native AtATM2, a common function, mitochondrial export of Fe/S clusters or their precursors for the assembly of cytosolic Fe/S proteins, is inferred for AtATM3 and AtATM1.

Arabidopsis ALS1 encodes a root tip and stele localized half type ABC transporter required for root growth in an aluminum toxic environment

Aluminum toxicity in acid soils severely limits crop productivity through inhibition of root growth and, consequently, shoot development. Several Arabidopsis mutants were previously identified as having roots with Al hypersensitivity, suggesting that these represent deleterious mutations affecting genes required for either Al tolerance or resistance mechanisms. For this report, the als1-1 mutant was chosen for further characterization. The phenotype of als1-1 is most obviously presented in Al challenged roots, as evidenced by exaggerated root growth inhibition in conjunction with increased expression of Al-responsive genes compared to wt. Using a map-based cloning approach, the als1-1 mutation was isolated and found to represent a deleterious amino acid substitution in a previously uncharacterized half type ABC transporter, At5g39040, which is expressed in a non-Al dependent manner in all organs tested. GUS-dependent analyses revealed that ALS1 expression is primarily localized to the root tip and the vasculature throughout the plant. Concomitant with this, an ALS1: GFP fusion accumulates at the vacuolar membrane of root cells, indicating that ALS1 may be important for intracellular movement of some substrate, possibly chelated Al, as part of a mechanism of Al sequestration.

Friedreich's ataxia: coenzyme Q10 and vitamin E therapy

Since the identification of the genetic mutation causing Friedreich's ataxia (FRDA) our understanding of the mechanisms underlying disease pathogenesis have improved markedly. The genetic abnormality results in the deficiency of frataxin, a protein targeted to the mitochondrion. There is extensive evidence that mitochondrial respiratory chain dysfunction, oxidative damage and iron accumulation play significant roles in the disease mechanism. There remains considerable debate as to the normal function of frataxin, but it is likely to be involved in mitochondrial iron handling, antioxidant regulation, and/or iron sulphur centre regulation. Therapeutic avenues for patients with FRDA are beginning to be explored in particular targeting antioxidant protection, enhancement of mitochondrial oxidative phosphorylation, iron chelation and more recently increasing FRDA transcription. The use of quinone therapy has been the most extensively studied to date with clear benefits demonstrated using evaluations of both disease biomarkers and clinical symptoms, and this is the topic that will be covered in this review.

Investigation of iron-sulfur protein maturation in eukaryotes

Iron-sulfur (Fe-S) clusters are cofactors of many proteins that are involved in central biochemical pathways, such as oxidative phosphorylation, photosynthesis, and amino acid biosynthesis. The assembly of these cofactors and the maturation of Fe-S proteins require complex cellular machineries in all kingdoms of life. In eukaryotes, Fe-S protein biogenesis is an essential process, and mitochondria perform a primary role in synthesis. Defects in Fe-S protein maturation in yeast result in respiratory deficiency and auxotrophies for certain amino acids and vitamins that require Fe-S proteins for their biosynthesis. Frequently, heme biosynthesis is also affected. The present compendium describes assays for the analysis of de novo Fe-S cluster and heme formation, cellular iron homeostasis, and the activity of Fe-S cluster- and heme-containing enzymes. These approaches are crucial to elucidate the mechanisms underlying the maturation of Fe-S proteins and may aid in the identification of new members of this evolutionary ancient process.

Abcb7, the gene responsible for X-linked sideroblastic anemia with ataxia, is essential for hematopoiesis

X-linked sideroblastic anemia with ataxia (XLSA/A) is a rare syndromic form of inherited sideroblastic anemia associated with spinocerebellar ataxia, and is due to mutations in the mitochondrial ATP-binding cassette transporter Abcb7. Here, we show that Abcb7 is essential for hematopoiesis and formally demonstrate that XLSA/A is due to partial loss of function mutations in Abcb7 that directly or indirectly inhibit heme biosynthesis.

Iron-sulfur protein biogenesis in eukaryotes: components and mechanisms

Iron-sulfur (Fe/S) clusters require a complex set of proteins to become assembled and incorporated into apoproteins in a living cell. Researchers have described three distinct assembly systems in eukaryotes that are involved in the maturation of cellular Fe/S proteins. Mitochondria are central for biogenesis. They contain the ISC-the iron-sulfur cluster assembly machinery that was inherited from a similar system of eubacteria in evolution and is involved in biogenesis of all cellular Fe/S proteins. The basic principle of mitochondrial (and bacterial) Fe/S protein maturation is the synthesis of the Fe/S cluster on a scaffold protein before the cluster is transferred to apoproteins. Biogenesis of cytosolic and nuclear Fe/S proteins is facilitated by the cytosolic iron-sulfur protein assembly (CIA) apparatus. This process requires the participation of mitochondria that export a still unknown component via the ISC export machinery, including an ABC transporter.

The role of Leishmania enriettii multidrug resistance protein 1 (LeMDR1) in mediating drug resistance is iron-dependent

In parasitic protozoan Leishmania enriettii, the role of a multidrug resistance (mdr) gene LeMDR1 (L. enriettii multidrug resistance 1) in mediating vinblastine resistance has been previously demonstrated by association, transfection and "gene knockout" studies. LeMDR1 has been shown to be located intracellularly and it was proposed to mediate drug resistance by sequestering drugs into intracellular organelles rather than by active efflux. Here we compared LeMDR1 overexpressed cell lines (Vint3 and V160), wild type (Le) and LeMDR1 "double knockout" mutant (LeMDR1-/-) and demonstrated that LeMDR1 gene copy number was associated with (1) higher level of intracellular iron, (2) increased sensitivity to an iron-dependent antibiotic, streptonigrin and (3) increased enzyme activity of an iron-sulfur-containing mitochondrial enzyme, aconitase. This result suggests that the normal function of LeMDR1 is related to mitochondrial iron homeostasis. To test such hypothesis, we have used the LeMDR1-overexpressing mutant V160 and LeMDR1-/- mutant to determine how iron level can affect its resistance level to drugs targeting either cytosol (vinblastine) or mitochondria (rhodamine 123 and pentamidine). It was found that the resistance level of V160 to vinblastine can be increased by iron whereas resistance to both rhodamine 123 and pentamidine can be increased by iron depletion and vice versa. Iron treatment can potentiate rhodamine 123 and pentamidine accumulation whereas iron deprivation can cause the reduction of rhodamine 123 accumulation. Our result highly suggests that LeMDR1's function in mediating drug resistance is iron-dependent.

Mitochondrial involvement in genetically determined transition metal toxicity

Iron that is not specifically chaperoned through its essential functional pathways is damaging to biological systems, in major part by catalyzing the production of reactive oxygen species. Iron serves in several essential roles in the mitochondrion, as an essential cofactor for certain enzymes of electron transport, and through its involvement in the assembly of iron-sulfur clusters and iron-porphyrin (heme) complexes, both processes occurring in the mitochondrion. Therefore, there are mechanisms that deliver iron specifically to mitochondria, although these are not well understood. Under normal circumstances the mitochondrion has levels of stored iron that are higher than other organelles, though lower than in cytosol, while in some disorders of iron metabolism, mitochondrial iron levels exceed those in the cytosol. Under these circumstances of excess iron, protective mechanisms are overwhelmed and mitochondrial damage ensues. This may take the form of acute oxidative stress with structural damage and functional impairment, but also may result in long-term damage to the mitochondrial genome. This review discusses the evidence that mitochondria do indeed accumulate iron in several genetic disorders, and are a direct target for iron toxicity when it is present in excess. We then consider two classes of genetic disorders involving iron and the mitochondrion. The first include defects in genes directly regulating mitochondrial iron metabolism that lead to Friedreich's ataxia and the various sideroblastic anemias, with excessive mitochondrial iron accumulation. Under the second class, we discuss various primary hemochromatoses that lead to direct mitochondrial damage, with reference to mutations in genes encoding HFE, hepcidin, hemojuvelin, transferrin receptor-2, ferroportin, transferrin, and ceruloplasmin.

Characterization of Saccharomyces cerevisiae Atm1p: functional studies of an ABC7 type transporter

Saccharomyces cerevisiae Atm1p has been cloned, over-expressed and purified from a yeast expression system. The sequence includes both the soluble ATPase and transmembrane-spanning domains. With the introduction of an N-terminal Kozak sequence and a C-terminal (His)(6)-tag, a yield of 1 mg of Atm1p was obtained from 3 g wet yeast cells, which is comparable to other membrane-associated proteins isolated from eukaryotic expression systems. The ATPase activity of Atm1p is sensitive to sodium vanadate, a P-type ATPase inhibitor, with an IC(50) of 4 microM. MgADP is a product inhibitor for Atm1p with an IC(50) of 0.9 mM. The Michaelis-Menten constants V(max), K(M) and k(cat) of Atm1p were measured as 8.7+/-0.3 microM/min, 107+/-16 microM and 1.24+/-0.06 min(-1), respectively. A plot of ATPase activity versus concentration of Atm1p exhibits a nonlinear relationship, suggesting an allosteric response and an important role for the transmembrane domain in mediating both ATP hydrolysis and MgADP release. The metal dependence of Atm1p ATPase activity demonstrated a reactivity order of Mg(2+)>Mn(2+)>Co(2+), while each divalent ion was found to be inhibitory at higher concentrations. The activation and inhibitory effect of phospholipids suggest that formation of a lipid-micelle complex is important for enzymatic activity and stability. Structural analysis of Atm1p by CD spectroscopy suggested a similarity of secondary structure to that found for other members of this ABC protein family.

A survey of all 11 ABC transporters in fission yeast: two novel ABC transporters are required for red pigment accumulation in a Schizosaccharomyces pombe adenine biosynthetic mutant

ATP-binding cassette (ABC) proteins transport a wide variety of substrates, including sugars, amino acids, metal ions, lipids, peptides and proteins, across membranes, and most ABC proteins contain transmembrane domains (ABC transporters). Sequencing of theSchizosaccharomyces pombegenome has allowed identification of all genes encoding ABC transporters in fission yeast. To date, six such genes have been characterized, and an additional five genes encoding ABC transporters were identified from the genome sequence. In an attempt to characterize all of the ABC transporters in fission yeast, all 11 genes were disrupted. While all the genes were found to be dispensable for cell viability, some disruptants lacked apparent phenotypes. GFP-tagged ABC transporters were localized to membranes as follows: plasma membrane (2), vacuolar membrane (4), mitochondrial membrane (2), endoplasmic reticulum membrane (2), and endosome and Golgi membranes (1). Two Cluster II. 1 proteins, Abc2p (SPAC3F10.11c) and Abc4p (SPAC30.04c), were found to be localized to vacuolar membranes, and to be responsible for accumulation of a characteristic red pigment in the vacuole of an adenine biosynthetic mutant. The doubly disrupted mutantabc2Δabc4Δ exhibited drug sensitivity, and a decreased accumulation of monochlorobimane, suggesting that both of the proteins encoded by these genes are involved in detoxification of xenobiotics, and vacuolar sequestration of glutathioneS-conjugates.

Heteronuclear multidimensional NMR and homology modelling studies of the C-terminal nucleotide-binding domain of the human mitochondrial ABC transporter ABCB6

Human ATP-binding cassette, sub-family B, member 6 (ABCB6) is a mitochondrial ABC transporter, and presumably contributes to iron homeostasis. Aimed at understanding the structural basis for the conformational changes accompanying the substrate-transportation cycle, we have studied the C-terminal nucleotide-binding domain of ABCB6 (ABCB6-C) in both the nucleotide-free and ADP-bound states by heteronuclear multidimensional NMR and homology modelling. A non-linear sampling scheme was utilised for indirectly acquired 13C and 15N dimensions of all 3D triple-resonance NMR experiments, in order to overcome the instability and the low solubility of ABCB6-C. The backbone resonances for approximately 25% of non-proline residues, which are mostly distributed around the functionally important loops and in the Helical domain, were not observed for nucleotide-free form of ABCB6-C. From the pH, temperature and magnetic field strength dependencies of the resonance intensities, we concluded that this incompleteness in the assignments is mainly due to the exchange between multiple conformations at an intermediate rate on the NMR timescale. These localised conformational dynamics remained in ADP-bound ABCB6-C except for the loops responsible for adenine base and alpha/beta-phosphate binding. These results revealed that the localised dynamic cooperativity, which was recently proposed for a prokaryotic ABC MJ1267, also exists in a higher eukaryotic ABC, and is presumably shared by all members of the ABC family. Since the Helical domain is the putative interface to the transmembrane domain, this cooperativity may explain the coupled functions between domains in the substrate-transportation cycle.

The effects of mitochondrial iron homeostasis on cofactor specificity of superoxide dismutase 2

Many metalloproteins have the capacity to bind diverse metals, but in living cells connect only with their cognate metal cofactor. In eukaryotes, this metal specificity can be achieved through metal-specific metallochaperone proteins. Herein, we describe a mechanism whereby Saccharomyces cerevisiae manganese superoxide dismutase (SOD2) preferentially binds manganese over iron based on the differential bioavailability of these ions within mitochondria. The bulk of mitochondrial iron is normally unavailable to SOD2, but when mitochondrial iron homeostasis is disrupted, for example, by mutations in S. cerevisiae mtm1, ssq1 and grx5, iron accumulates in a reactive form that potently competes with manganese for binding to SOD2, inactivating the enzyme. Studies in mtm1 mutants indicate that iron inactivation of SOD2 involves the Mrs3p/Mrs4p mitochondrial carriers and iron-binding frataxin (Yfh1p). A small pool of SOD2-reactive iron also exists under normal iron homeostasis conditions and binds SOD2 when mitochondrial manganese is low. The ability to control this reactive pool of iron is critical to maintaining SOD2 activity and has important potential implications for oxidative stress in disorders of iron overload.

AtATM3 is involved in heavy metal resistance in Arabidopsis

AtATM3, an ATP-binding cassette transporter of Arabidopsis (Arabidopsis thaliana), is a mitochondrial protein involved in the biogenesis of iron-sulfur clusters and iron homeostasis in plants. Our gene expression analysis showed that AtATM3 is up-regulated in roots of plants treated with cadmium [Cd(II)] or lead (II); hence, we investigated whether this gene is involved in heavy metal tolerance. We found that AtATM3-overexpressing plants were enhanced in resistance to Cd, whereas atatm3 mutant plants were more sensitive to Cd than their wild-type controls. Moreover, atatm3 mutant plants expressing 35S promoter-driven AtATM3 were more resistant to Cd than wild-type plants. Since previous reports often showed that the cytosolic glutathione level is positively correlated with heavy metal resistance, we measured nonprotein thiols (NPSH) in these mutant plants. Surprisingly, we found that atatm3 contained more NPSH than the wild type under normal conditions. AtATM3-overexpressing plants did not differ under normal conditions, but contained less NPSH than wild-type plants when exposed to Cd(II). These results suggest a role for AtATM3 in regulating cellular NPSH level, a hypothesis that was further supported by our gene expression study. Genetic or pharmacological inhibition of glutathione biosynthesis led to the elevated expression of AtATM3, whereas expression of the glutathione synthase gene GSH1 was increased under Cd(II) stress and in the atatm3 mutant. Because the closest homolog of AtATM3 in fission yeast (Schizosaccharomyces pombe), HMT1, is a vacuolar membrane-localized phytochelatin-Cd transporter, it is tempting to speculate that glutathione-Cd(II) complexes formed in the mitochondria are exported by AtATM3. In conclusion, our data show that AtATM3 contributes to Cd resistance and suggest that it may mediate transport of glutamine synthetase-conjugated Cd(II) across the mitochondrial membrane.

The mitochondrial ATP-binding cassette transporter Abcb7 is essential in mice and participates in cytosolic iron–sulfur cluster biogenesis

Proteins with iron-sulfur (Fe-S) clusters participate in multiple metabolic pathways throughout the cell. The mitochondrial ABC half-transporter Abcb7, which is mutated in X-linked sideroblastic anemia with ataxia in humans, is a functional ortholog of yeast Atm1p and is predicted to export a mitochondrially derived metabolite required for cytosolic Fe-S cluster assembly. Using an inducible Cre/loxP system to delete exons 9 and 10 of the Abcb7 gene, we examined the phenotype of mice deficient in Abcb7. We found that Abcb7 was essential in extra-embryonic tissues early in gestation and that the mutant allele exhibits an X-linked parent-of-origin lethality effect. Furthermore, using X-chromosome inactivation assays and tissue-specific deletions, Abcb7 was found to be essential for the development and function of numerous other cell types and tissues. A notable exception to this was liver, where loss of Abcb7 impaired cytosolic Fe-S cluster assembly but was not lethal. In this situation, control of iron regulatory protein 1, a key cytosolic modulator of iron metabolism, which is responsive to the availability of cytosolic Fe-S clusters, was impaired and contributed to the dysregulation of hepatocyte iron metabolism. Altogether, these studies demonstrate the essential nature of Abcb7 in mammals and further substantiate a central role for mitochondria in the biogenesis of cytosolic Fe-S proteins.

Iron-regulated transcription and capsule formation in the fungal pathogen Cryptococcus neoformans

Cryptococcus neoformans is the leading cause of fungal meningitis in humans. Production of a polysaccharide capsule is a key virulence property for the fungus and capsule synthesis is regulated by iron levels. Given that iron acquisition is an important aspect of virulence for many pathogens, we employed serial analysis of gene expression (SAGE) to examine the transcriptome under iron-limiting and iron-replete conditions. Initially, we demonstrated by SAGE and Northern analysis that iron limitation results in an elevated transcript level for the CAP60 gene that is required for capsule production. We also identified genes encoding putative components for iron transport and homeostasis, including the FTR1 (iron permease) gene, with higher transcript levels in the low-iron condition. An FTR1 disruption mutant grows more slowly than wild-type cells in low-iron medium, and shows delayed growth and altered capsule regulation in iron-replete medium. Iron deprivation also resulted in elevated SAGE tags for putative extracellular mannoproteins and the GPI8 gene encoding a glycosylphosphatidylinositol (GPI) transamidase. The GPI8 gene appears to be essential while disruption of the CIG1 gene encoding a mannoprotein resulted in impaired growth in low-iron medium and altered capsule response to the iron-replete condition. Additionally, we found that iron-replete conditions led to elevated transcripts for genes for iron storage, nitrogen metabolism, glycolysis, mitochondrial function, lipid metabolism and calmodulin-calcineurin signalling. Overall, these studies provide the first view of the C. neoformans transcriptional response to different iron levels.

Unique Iron Binding and Oxidation Properties of Human Mitochondrial Ferritin: A Comparative Analysis with Human H-chain Ferritin

Ferritins are ubiquitous iron mineralizing and storage proteins that play an important role in iron homeostasis. Although excess iron is stored in the cytoplasm, most of the metabolically active iron is processed in the mitochondria of the cell. Little is known about how these organelles regulate iron homeostasis and toxicity. The recently discovered human mitochondrial ferritin (MtF), unlike other mammalian ferritins, is a homopolymer of 24 subunits that has a high degree of sequence homology with human H-chain ferritin (HuHF). Parallel experiments with MtF and HuHF reported here reveal striking differences in their iron oxidation and hydrolysis chemistry despite their similar diFe ferroxidase centers. In contrast to HuHF, MtF does not regenerate its ferroxidase activity after oxidation of its initial complement of Fe(II) and generally has considerably slower ferroxidation and mineralization activities as well. MtF exhibits sigmoidal kinetics of mineralization more characteristic of an L-chain than an H-chain ferritin. Site-directed mutagenesis reveals that serine 144, a residue situated near the ferroxidase center in MtF but absent from HuHF, is one player in this impairment of activity. Additionally only one-half of the 24 ferroxidase centers of MtF are functional, further contributing to its lower activity. Stopped-flow absorption spectrometry of Fe(II) oxidation by O(2) in MtF shows the formation of a transient diiron(III) mu-peroxo species (lambda(max) = 650 nm) as observed in HuHF. Also, as for HuHF, minimal hydroxyl radical is produced during the oxidative deposition of iron in MtF using O(2) as the oxidant. However, the 2Fe(II) + H(2)O(2) detoxification reaction found in HuHF does not occur in MtF. The structural differences and the physiological implications of the unique iron oxidation properties of MtF are discussed in light of these results.

Iron trafficking in the mitochondrion: novel pathways revealed by disease

It is well known that iron (Fe) is transported to the mitochondrion for heme synthesis. However, only recently has the importance of this organelle for many other facets of Fe metabolism become widely appreciated. Indeed, this was stimulated by the description of human disease states that implicate mitochondrial Fe metabolism. In particular, studies assessing various diseases leading to mitochondrial Fe loading have produced intriguing findings. For instance, the disease X-linked sideroblastic anemia with ataxia (XLSA/A) is due to a mutation in the ATP-binding cassette protein B7 (ABCB7) transporter that is thought to transfer [Fe-S] clusters from the mitochondrion to the cytoplasm. This and numerous other findings suggest the mitochondrion is a dynamo of Fe metabolism, being vital not only for heme synthesis but also for playing a critical role in the genesis of [Fe-S] clusters. Studies examining the disease Friedreich ataxia have suggested that a mutation in the gene encoding frataxin leads to mitochondrial Fe loading. Apart from these findings, the recently discovered mitochondrial ferritin that may store Fe in ring sideroblasts could also regulate the level of Fe needed for heme and [Fe-S] cluster synthesis. In this review, we suggest a model of mitochondrial Fe processing that may account for the pathology observed in these disease states.

Biogenesis of cytosolic ribosomes requires the essential iron-sulphur protein Rli1p and mitochondria

Mitochondria perform a central function in the biogenesis of cellular iron-sulphur (Fe/S) proteins. It is unknown to date why this biosynthetic pathway is indispensable for life, the more so as no essential mitochondrial Fe/S proteins are known. Here, we show that the soluble ATP-binding cassette (ABC) protein Rli1p carries N-terminal Fe/S clusters that require the mitochondrial and cytosolic Fe/S protein biogenesis machineries for assembly. Mutations in critical cysteine residues of Rli1p abolish association with Fe/S clusters and lead to loss of cell viability. Hence, the essential character of Fe/S clusters in Rli1p explains the indispensable character of mitochondria in eukaryotes. We further report that Rli1p is associated with ribosomes and with Hcr1p, a protein involved in rRNA processing and translation initiation. Depletion of Rli1p causes a nuclear export defect of the small and large ribosomal subunits and subsequently a translational arrest. Thus, ribosome biogenesis and function are intimately linked to the crucial role of mitochondria in the maturation of the essential Fe/S protein Rli1p.

Parasite plastids: approaching the endgame

Considerable work still needs to be done to understand more fully the basic processes going on inside the non-photosynthetic plastid organelle of Plasmodium spp., the causative agent of malaria. Following an explosion of genomic and transcriptional information in recent years, research workers are still analysing these data looking for new material relevant to the plastid. Several metabolic and housekeeping functions based on bacterial biochemistry have been elucidated and this has given impetus to finding lead inhibitors based on established anti-microbials. Structural investigations of plastid-associated enzymes identified as potential targets have begun. This review gives a perspective on the research to date and hopes to emphasize that a practical outcome for the clinic should be an important focus of future efforts. Malaria parasites have become resistant to front-line anti-malarials that are widely used and were formerly dependable. This has become a worrying problem in many regions where malaria is endemic. The time lag between hunting for new inhibitors and their application as pharmaceuticals is so long and costly that a steady stream of new ventures has to be undertaken to give a reasonable chance of finding affordable and appropriate anti-malarials for the future. Attempts to find inhibitors of the plastid organelle of the malaria parasite should be intensified in such programmes.

The mitochondrial ABC transporter Atm1p functions as a homodimer

The ATP-binding cassette (ABC) transporters constitute one of the largest families of proteins in evolution. The ATM1 gene of the yeast Saccharomyces cerevisiae encodes an ABC protein, which is localized to the mitochondrial inner membrane. A deletion of ATM1 results in the accumulation of up to a 30-fold excess of mitochondrial iron, loss of mitochondrial cytochromes and abnormalities of cytosolic iron metabolism. In this study, we have evaluated the role of conserved sequence elements in Atm1p in its function and dimerization in vivo. We report that conserved residues in the Walker A and B motifs of the nucleotide binding domain, which are required for ATP binding and hydrolysis, are essential for Atm1p function. In addition, we provide evidence that ATP binding is important for Atm1p dimerization.

Iron binding and oxidation kinetics in frataxin CyaY of Escherichia coli

Friedreich's ataxia is associated with a deficiency in frataxin, a conserved mitochondrial protein of unknown function. Here, we investigate the iron binding and oxidation chemistry of Escherichia coli frataxin (CyaY), a homologue of human frataxin, with the aim of better understanding the functional properties of this protein. Anaerobic isothermal titration calorimetry (ITC) demonstrates that at least two ferrous ions bind specifically but relatively weakly per CyaY monomer (K(d) approximately 4 microM). Such weak binding is consistent with the hypothesis that the protein functions as an iron chaperone. The bound Fe(II) is oxidized slowly by O(2). However, oxidation occurs rapidly and completely with H(2)O(2) through a non-enzymatic process with a stoichiometry of two Fe(II)/H(2)O(2), indicating complete reduction of H(2)O(2) to H(2)O. In accord with this stoichiometry, electron paramagnetic resonance (EPR) spin trapping experiments indicate that iron catalyzed production of hydroxyl radical from Fenton chemistry is greatly attenuated in the presence of CyaY. The Fe(III) produced from oxidation of Fe(II) by H(2)O(2) binds to the protein with a stoichiometry of six Fe(III)/CyaY monomer as independently measured by kinetic, UV-visible, fluorescence, iron analysis and pH-stat titrations. However, as many as 25-26 Fe(III)/monomer can bind to the protein, exhibiting UV absorption properties similar to those of hydrolyzed polynuclear Fe(III) species. Analytical ultracentrifugation measurements indicate that a tetramer is formed when Fe(II) is added anaerobically to the protein; multiple protein aggregates are formed upon oxidation of the bound Fe(II). The observed iron oxidation and binding properties of frataxin CyaY may afford the mitochondria protection against iron-induced oxidative damage.

Mutations in Saccharomyces cerevisiae Iron-Sulfur Cluster Assembly Genes and Oxidative Stress Relevant to Cu,Zn Superoxide Dismutase

Saccharomyces cerevisiae lacking Cu,Zn superoxide dismutase (SOD1) show several metabolic defects including aerobic blockages in methionine and lysine biosynthesis. We have previously shown that mutations in genes implicated in the formation of iron-sulfur clusters, designated seo (suppressors of endogenous oxidation), reverse the oxygen-dependent methionine and lysine auxotrophies of a sod1Delta strain. We now report the surprising finding that seo mutants do not reduce oxidative damage as shown by the lack of reduction of EPR-detectable "free" iron, which is characteristic of sod1Delta mutants. In fact, they exhibit increased oxidative damage as evidenced by increased accumulation of protein carbonyls. The seo class of mutants overaccumulates mitochondrial iron, and this iron accumulation is critical for suppression of the sod1Delta biosynthetic defects. Blocking overaccumulation of mitochondrial iron abolished the ability of the seo mutants to suppress the sod1Delta auxotrophies. By contrast, increasing the mitochondrial iron content of sod1Delta yeast using high copy MMT1, which encodes a mitochondrial iron transporter, was sufficient to mimic the seo mutants. Our studies indicated that suppression of the sod1Delta methionine auxotrophy was dependent on the pentose phosphate pathway, which is a major source of NADPH production. By comparison, the sod1Delta lysine auxotrophy appears to be reversed in the seo mutants by increased expression of genes in the lysine biosynthetic pathway, perhaps through sensing of mitochondrial damage by the retrograde response.