Anamorsin Is a [2Fe-2S] Cluster-Containing Substrate of the Mia40-Dependent Mitochondrial Protein Trapping Machinery

Structural aspects of iron‑sulfur protein biogenesis: An NMR view

Over the last decade, structural aspects involving iron‑sulfur (Fe/S) protein biogenesis have played an increasingly important role in understanding the high mechanistic complexity of mitochondrial and cytosolic machineries maturing Fe/S proteins. In this respect, solution NMR has had a significant impact because of its ability to monitor transient protein-protein interactions, which are abundant in the networks of pathways leading to Fe/S cluster biosynthesis and transfer, as well as thanks to the developments of paramagnetic NMR in both terms of new methodologies and accurate data interpretation. Here, we review the use of solution NMR in characterizing the structural aspects of human Fe/S proteins and their interactions in the framework of Fe/S protein biogenesis. We will first present a summary of the recent advances that have been achieved by paramagnetic NMR and then we will focus our attention on the role of solution NMR in the field of human Fe/S protein biogenesis.

Oxidative protein folding in the intermembrane space of human mitochondria

The mitochondrial intermembrane space hosts a machinery for oxidative protein folding, the mitochondrial disulfide relay. This machinery imports a large number of soluble proteins into the compartment, where they are retained through oxidative folding. Additionally, the disulfide relay enhances the stability of many proteins by forming disulfide bonds. In this review, we describe the mitochondrial disulfide relay in human cells, its components, and their coordinated collaboration in mechanistic detail. We also discuss the human pathologies associated with defects in this machinery and its protein substrates, providing a comprehensive overview of its biological importance and implications for health.

Biochemical and cellular characterization of the CISD3 protein: Molecular bases of cluster release and destabilizing effects of nitric oxide

The NEET proteins, an important family of iron-sulfur (Fe-S) proteins, have generated a strong interest due to their involvement in diverse diseases such as cancer, diabetes, and neurodegenerative disorders. Among the human NEET proteins, CISD3 has been the least studied, and its functional role is still largely unknown. We have investigated the biochemical features of CISD3 at the atomic and in cellulo levels upon challenge with different stress conditions i.e., iron deficiency, exposure to hydrogen peroxide, and nitric oxide. The redox and cellular stability properties of the protein agree on a predominance of reduced form of CISD3 in the cells. Upon the addition of iron chelators, CISD3 loses its Fe-S clusters and becomes unstructured, and its cellular level drastically decreases. Chemical shift perturbation measurements suggest that, upon cluster oxidation, the protein undergoes a conformational change at the C-terminal CDGSH domain, which determines the instability of the oxidized state. This redox-associated conformational change may be the source of cooperative electron transfer via the two [Fe2S2] clusters in CISD3, which displays a single sharp voltammetric signal at -31 mV versus SHE. Oxidized CISD3 is particularly sensitive to the presence of hydrogen peroxide in vitro, whereas only the reduced form is able to bind nitric oxide. Paramagnetic NMR provides clear evidence that, upon NO binding, the cluster is disassembled but iron ions are still bound to the protein. Accordingly, in cellulo CISD3 is unaffected by oxidative stress induced by hydrogen peroxide but it becomes highly unstable in response to nitric oxide treatment.

Plasmodium Iron-Sulfur [Fe-S] cluster assembly protein Dre2 as a plausible target of Artemisinin: Mechanistic insights derived in a prokaryotic heterologous system

Iron-sulfur (Fe-S) cluster containing proteins have been assigned roles in various essential cellular processes, such as regulation of gene expression, electron transfer, sensing of oxygen and balancing free radical chemistry. However, their role as the drug target remains sparse. Recently the screening of protein alkylation targets for artemisinin in Plasmodium falciparum led to identification of Dre2, a protein involved in redox mechanism for the cytoplasmic Fe-S cluster assembly in different organisms. In the present study, to further explore the interaction between artemisinin and Dre2, we have expressed the Dre2 protein of both P. falciparum and P. vivax in E. coli. The opaque brown colour of the IPTG induced recombinant Plasmodium Dre2 bacterial pellet, suggested iron accumulation as confirmed by the ICP-OES analysis. In addition, overexpression of rPvDre2 in E. coli reduced its viability, growth and increased the ROS levels of bacterial cells, which in turn led to an increase in expression of stress response genes of E. coli such as recA, soxS, mazF. Moreover, the overexpression of rDre2 induced cell death could be rescued by treatment with Artemisinin derivatives suggesting their interaction. The interaction between DHA and PfDre2 was later demonstrated by CETSA and microscale thermophoresis. Overall, this study suggests that Dre2 is the probable target of Artemisinin and the antimalarial activity of DHA/Artemether could also be due to yet unidentified molecular mechanism altering the Dre2 activity in addition to inducing DNA and protein damage.

Structural plasticity of NFU1 upon interaction with binding partners: insights into the mitochondrial [4Fe-4S] cluster pathway

In humans, the biosynthesis and trafficking of mitochondrial [4Fe-4S]²⁺ clusters is a highly coordinated process that requires a complex protein machinery. In a mitochondrial pathway among various proposed to biosynthesize nascent [4Fe-4S]²⁺ clusters, two [2Fe-2S]²⁺ clusters are converted into a [4Fe-4S]²⁺ cluster on a ISCA1-ISCA2 complex. Along this pathway, this cluster is then mobilized from this complex to mitochondrial apo recipient proteins with the assistance of accessory proteins. NFU1 is the accessory protein that first receives the [4Fe-4S]²⁺ cluster from ISCA1-ISCA2 complex. A structural view of the protein-protein recognition events occurring along the [4Fe-4S]²⁺ cluster trafficking as well as how the globular N-terminal and C-terminal domains of NFU1 act in such process is, however, still elusive. Here, we applied small-angle X-ray scattering coupled with on-line size-exclusion chromatography and paramagnetic NMR to disclose structural snapshots of ISCA1-, ISCA2- and NFU1-containing apo complexes as well as the coordination of [4Fe-4S]²⁺ cluster bound to the ISCA1-NFU1 complex, which is the terminal stable species of the [4Fe-4S]²⁺ cluster transfer pathway involving ISCA1-, ISCA2- and NFU1 proteins. The structural modelling of ISCA1-ISCA2, ISCA1-ISCA2-NFU1 and ISCA1-NFU1 apo complexes, here reported, reveals that the structural plasticity of NFU1 domains is crucial to drive protein partner recognition and modulate [4Fe-4S]²⁺ cluster transfer from the cluster-assembly site in ISCA1-ISCA2 complex to the cluster-binding site in ISCA1-NFU1 complex. These structures allowed us to provide a first rational for the molecular function of the N-domain of NFU1, which can act as a modulator in the [4Fe-4S]²⁺ cluster transfer.

Oxidization of optic atrophy 1 cysteines occurs during heart ischemia-reperfusion and amplifies cell death by oxidative stress

During cardiac ischemia-reperfusion, excess reactive oxygen species can damage mitochondrial, cellular and organ function. Here we show that cysteine oxidation of the mitochondrial protein Opa1 contributes to mitochondrial damage and cell death caused by oxidative stress. Oxy-proteomics of ischemic-reperfused hearts reveal oxidation of the C-terminal C786 of Opa1 and treatment of perfused mouse hearts, adult cardiomyocytes, and fibroblasts with H₂O₂ leads to the formation of a reduction-sensitive ∼180 KDa Opa1 complex, distinct from the ∼270 KDa one antagonizing cristae remodeling. This Opa1 oxidation process is curtailed by mutation of C786 and of the other 3 Cys residues of its C-terminal domain (Opa1^TetraCys). When reintroduced in Opa1^-/- cells, Opa1^TetraCys is not efficiently processed into short Opa1^TetraCys and hence fails to fuse mitochondria. Unexpectedly, Opa1^TetraCys restores mitochondrial ultrastructure in Opa1^-/- cells and protects them from H₂O₂-induced mitochondrial depolarization, cristae remodeling, cytochrome c release and cell death. Thus, preventing the Opa1 oxidation occurring during cardiac ischemia-reperfusion reduces mitochondrial damage and cell death induced by oxidative stress independent of mitochondrial fusion.

Metal trafficking in the cell: Combining atomic resolution with cellular dimension

Metals are widely present in biological systems as simple ions or complex cofactors, and are involved in a variety of processes essential for life. Their transport inside cells and insertion into the binding sites of the proteins that need metals to function occur through complex and selective pathways involving dedicated multiprotein machineries specifically and transiently interacting with each other, often sharing the coordination of metal ions and/or cofactors. The understanding of these machineries requires integrated approaches, ranging from bioinformatics to experimental investigations, possibly in the cellular context. In this review, we report two case studies where the use of integrated in vitro and in cellulo approaches is necessary to clarify at atomic resolution essential aspects of metal trafficking in cells.

The Intriguing mitoNEET: Functional and Spectroscopic Properties of a Unique [2Fe-2S] Cluster Coordination Geometry

Despite the number of cellular and pathological mitoNEET-related processes, very few details are known about the mechanism of action of the protein. The recently discovered existence of a link between NEET proteins and cancer pave the way to consider mitoNEET and its Fe-S clusters as suitable targets to inhibit cancer cell proliferation. Here, we will review the variety of spectroscopic techniques that have been applied to study mitoNEET in an attempt to explain the drastic difference in clusters stability and reactivity observed for the two redox states, and to elucidate the cellular function of the protein. In particular, the extensive NMR assignment and the characterization of first coordination sphere provide a molecular fingerprint helpful to assist the design of drugs able to impair cellular processes or to directly participate in redox reactions or protein-protein recognition mechanisms.

The Intriguing Role of Iron-Sulfur Clusters in the CIAPIN1 Protein Family

Iron-sulfur (Fe/S) clusters are protein cofactors that play a crucial role in essential cellular functions. Their ability to rapidly exchange electrons with several redox active acceptors makes them an efficient system for fulfilling diverse cellular needs. They include the formation of a relay for long-range electron transfer in enzymes, the biosynthesis of small molecules required for several metabolic pathways and the sensing of cellular levels of reactive oxygen or nitrogen species to activate appropriate cellular responses. An emerging family of iron-sulfur cluster binding proteins is CIAPIN1, which is characterized by a C-terminal domain of about 100 residues. This domain contains two highly conserved cysteine-rich motifs, which are both involved in Fe/S cluster binding. The CIAPIN1 proteins have been described so far to be involved in electron transfer pathways, providing electrons required for the biosynthesis of important protein cofactors, such as Fe/S clusters and the diferric-tyrosyl radical, as well as in the regulation of cell death. Here, we have first investigated the occurrence of CIAPIN1 proteins in different organisms spanning the entire tree of life. Then, we discussed the function of this family of proteins, focusing specifically on the role that the Fe/S clusters play. Finally, we describe the nature of the Fe/S clusters bound to CIAPIN1 proteins and which are the cellular pathways inserting the Fe/S clusters in the two cysteine-rich motifs.

In Cellulo Mössbauer and EPR Studies Bring New Evidence to the Long-Standing Debate on Iron-Sulfur Cluster Binding in Human Anamorsin

Human anamorsin is an iron–sulfur (Fe–S)‐cluster‐binding protein acting as an electron donor in the early steps of cytosolic iron–sulfur protein biogenesis. Human anamorsin belongs to the eukaryotic CIAPIN1 protein family and contains two highly conserved cysteine‐rich motifs, each binding an Fe–S cluster. In vitro works by various groups have provided rather controversial results for the type of Fe–S clusters bound to the CIAPIN1 proteins. In order to unravel the knot on this topic, we used an in cellulo approach combining Mössbauer and EPR spectroscopies to characterize the iron–sulfur‐cluster‐bound form of human anamorsin. We found that the protein binds two [2Fe–2S] clusters at both its cysteine‐rich motifs.

ISCA1 Orchestrates ISCA2 and NFU1 in the Maturation of Human Mitochondrial [4Fe-4S] Proteins

The late-acting steps of the pathway responsible for the maturation of mitochondrial [4Fe-4S] proteins are still elusive. Three proteins ISCA1, ISCA2 and NFU1 were shown to be implicated in the assembly of [4Fe-4S] clusters and their transfer into mitochondrial apo proteins. We present here a NMR-based study showing a detailed molecular model of the succession of events performed in a coordinated manner by ISCA1, ISCA2 and NFU1 to make [4Fe-4S] clusters available to mitochondrial apo proteins. We show that ISCA1 is the key player of the [4Fe-4S] protein maturation process because of its ability to interact with both NFU1 and ISCA2, which, instead do not interact each other. ISCA1 works as the promoter of the interaction between ISCA2 and NFU1 being able to determine the formation of a transient ISCA1-ISCA2-NFU1 ternary complex. We also show that ISCA1, thanks to its specific interaction with the C-terminal cluster-binding domain of NFU1, drives [4Fe-4S] cluster transfer from the site where the cluster is assembled on the ISCA1-ISCA2 complex to a cluster binding site formed by ISCA1 and NFU1 in the ternary ISCA1-ISCA2-NFU1 complex. Such mechanism guarantees that the [4Fe-4S] cluster can be safely moved from where it is assembled on the ISCA1-ISCA2 complex to NFU1, thereby resulting the [4Fe-4S] cluster available for the mitochondrial apo proteins specifically requiring NFU1 for their maturation.

Mechanistic concepts of iron-sulfur protein biogenesis in Biology

Iron-sulfur (Fe/S) proteins are present in virtually all living organisms and are involved in numerous cellular processes such as respiration, photosynthesis, metabolic reactions, nitrogen fixation, radical biochemistry, protein synthesis, antiviral defense, and genome maintenance. Their versatile functions may go back to the proposed role of their Fe/S cofactors in the origin of life as efficient catalysts and electron carriers. More than two decades ago, it was discovered that the in vivo synthesis of cellular Fe/S clusters and their integration into polypeptide chains requires assistance by complex proteinaceous machineries, despite the fact that Fe/S proteins can be assembled chemically in vitro. In prokaryotes, three Fe/S protein biogenesis systems are known; ISC, SUF, and the more specialized NIF. The former two systems have been transferred by endosymbiosis from bacteria to mitochondria and plastids, respectively, of eukaryotes. In their cytosol, eukaryotes use the CIA machinery for the biogenesis of cytosolic and nuclear Fe/S proteins. Despite the structural diversity of the protein constituents of these four machineries, general mechanistic concepts underlie the complex process of Fe/S protein biogenesis. This review provides a comprehensive and comparative overview of the various known biogenesis systems in Biology, and summarizes their common or diverging molecular mechanisms, thereby illustrating both the conservation and diverse adaptions of these four machineries during evolution and under different lifestyles. Knowledge of these fundamental biochemical pathways is not only of basic scientific interest, but is important for the understanding of human 'Fe/S diseases' and can be used in biotechnology.

Cysteine Oxidations in Mitochondrial Membrane Proteins: The Case of VDAC Isoforms in Mammals

Cysteine residues are reactive amino acids that can undergo several modifications driven by redox reagents. Mitochondria are the source of an abundant production of radical species, and it is surprising that such a large availability of highly reactive chemicals is compatible with viable and active organelles, needed for the cell functions. In this work, we review the results highlighting the modifications of cysteines in the most abundant proteins of the outer mitochondrial membrane (OMM), that is, the voltage-dependent anion selective channel (VDAC) isoforms. This interesting protein family carries several cysteines exposed to the oxidative intermembrane space (IMS). Through mass spectrometry (MS) analysis, cysteine posttranslational modifications (PTMs) were precisely determined, and it was discovered that such cysteines can be subject to several oxidization degrees, ranging from the disulfide bridge to the most oxidized, the sulfonic acid, one. The large spectra of VDAC cysteine oxidations, which is unique for OMM proteins, indicate that they have both a regulative function and a buffering capacity able to counteract excess of mitochondrial reactive oxygen species (ROS) load. The consequence of these peculiar cysteine PTMs is discussed.

GLRX3 Acts as a [2Fe-2S] Cluster Chaperone in the Cytosolic Iron-Sulfur Assembly Machinery Transferring [2Fe-2S] Clusters to NUBP1

Human cytosolic monothiol glutaredoxin-3 (GLRX3) is a protein essential for the maturation of cytosolic [4Fe–4S] proteins. We show here that dimeric cluster-bridged GLRX3 transfers its [2Fe–2S]²⁺ clusters to the human P-loop NTPase NUBP1, an essential early component of the cytosolic iron–sulfur assembly (CIA) machinery. Specifically, we observed that [2Fe–2S]²⁺ clusters are transferred from GLRX3 to monomeric apo NUBP1 and reductively coupled to form [4Fe–4S]²⁺ clusters on both N-terminal CX₁₃CX₂CX₅C and C-terminal CPXC motifs of NUBP1 in the presence of glutathione that acts as a reductant. In this process, cluster binding to the C-terminal motif of NUBP1 promotes protein dimerization, while cluster binding to the N-terminal motif does not affect the quaternary structure of NUBP1. The cluster transfer/assembly process is not complete on both N- and C-terminal motifs and indeed requires a reductant stronger than GSH to increase its efficiency. We also showed that the [4Fe–4S]²⁺ cluster formed at the N-terminal motif of NUBP1 is tightly bound, while the [4Fe–4S]²⁺ cluster bound at the C-terminal motif is labile. Our findings provide the first evidence for GLRX3 acting as a [2Fe–2S] cluster chaperone in the early stage of the CIA machinery.

Canonical cytosolic iron-sulfur cluster assembly and non-canonical functions of DRE2 in Arabidopsis

As a component of the Cytosolic Iron-sulfur cluster Assembly (CIA) pathway, DRE2 is essential in organisms from yeast to mammals. However, the roles of DRE2 remain incompletely understood largely due to the lack of viable dre2 mutants. In this study, we successfully created hypomorphic dre2 mutants using the CRISPR/Cas9 technology. Like other CIA pathway mutants, the dre2 mutants have accumulation of DNA lesions and show constitutive DNA damage response. In addition, the dre2 mutants exhibit DNA hypermethylation at hundreds of loci. The mutant forms of DRE2 in the dre2 mutants, which bear deletions in the linker region of DRE2, lost interaction with GRXS17 but have stronger interaction with NBP35, resulting in the CIA-related defects of dre2. Interestingly, we find that DRE2 is also involved in auxin response that may be independent of its CIA role. DRE2 localizes in both the cytoplasm and the nucleus and nuclear DRE2 associates with euchromatin. Furthermore, DRE2 directly associates with multiple auxin responsive genes and maintains their normal expression. Our study highlights the importance of the linker region of DRE2 in coordinating CIA-related protein interactions and identifies the canonical and non-canonical roles of DRE2 in maintaining genome stability, epigenomic patterns, and auxin response.

The Cytosolic Iron-sulfur cluster Assembly (CIA) pathway is essential for the maturation of Fe-S proteins localized in the cytosol and the nucleus. As an important component of the CIA pathway, DRE2 is essential from yeast to mammals. To study the CIA-related functions of DRE2 and further explore novel non-CIA roles of DRE2 in Arabidopsis, we for the first time created two homozygous dre2 hypomorphic mutants using the CRISPR/Cas9 technology. The dre2 mutants exhibit hallmark features of the CIA pathway mutants indicating CIA-dependent functions of DRE2 in Arabidopsis. Unexpectedly, we find that DRE2 participates in auxin response and nuclear DRE2 directly binds multiple auxin responsive genes and regulates their expression, suggesting that DRE2 plays CIA-independent roles. Our findings significantly expand our understanding of the biological functions of DRE2 in eukaryotes.

Protein networks in the maturation of human iron-sulfur proteins

The biogenesis of iron-sulfur (Fe-S) proteins in humans is a multistage process occurring in different cellular compartments. The mitochondrial iron-sulfur cluster (ISC) assembly machinery composed of at least 17 proteins assembles mitochondrial Fe-S proteins. A cytosolic iron-sulfur assembly (CIA) machinery composed of at least 13 proteins has been more recently identified and shown to be responsible for the Fe-S cluster incorporation into cytosolic and nuclear Fe-S proteins. Cytosolic and nuclear Fe-S protein maturation requires not only the CIA machinery, but also the components of the mitochondrial ISC assembly machinery. An ISC export machinery, composed of a protein transporter located in the mitochondrial inner membrane, has been proposed to act in mediating the export process of a still unknown component that is required for the CIA machinery. Several functional and molecular aspects of the protein networks operative in the three machineries are still largely obscure. This Review focuses on the Fe-S protein maturation processes in humans with the specific aim of providing a molecular picture of the currently known protein-protein interaction networks. The human ISC and CIA machineries are presented, and the ISC export machinery is discussed with respect to possible molecules being the substrates of the mitochondrial protein transporter.

Metal cofactors trafficking and assembly in the cell: a molecular view

Metal ions are essential cofactors required by the proteome of organisms from any kingdom of life to correctly exert their functions. Dedicated cellular import, transport and homeostasis systems assure that the needed metal ion is correctly delivered and inserted into the target proteins and avoid the presence of free metal ions in the cell, preventing oxidative damaging. Among metal ions, in eukaryotic organisms copper and iron are required by proteins involved in absolutely essential functions, such as respiration, oxidative stress protection, catalysis, gene expression regulation. Copper and iron binding proteins are localized in essentially all cellular compartments. Copper is physiologically present mainly as individual metal ion. Iron can be present both as individual metal ion or as part of cofactors, such as hemes and iron-sulfur (Fe-S) clusters. Both metal ions are characterized by the ability to cycle between different oxidation states, which enable them to catalyze redox reactions and to participate in electron transfer processes. Here we describe in detail the main processes responsible for the trafficking of copper and iron sulfur clusters, with particular interest for the structural aspects of the maturation of copper and iron-sulfur-binding proteins.

Cysteine residues in mitochondrial intermembrane space proteins: more than just import

The intermembrane space (IMS) is a very small mitochondrial sub-compartment with critical relevance for many cellular processes. IMS proteins fulfil important functions in transport of proteins, lipids, metabolites and metal ions, in signalling, in metabolism and in defining the mitochondrial ultrastructure. Our understanding of the IMS proteome has become increasingly refined although we still lack information on the identity and function of many of its proteins. One characteristic of many IMS proteins are conserved cysteines. Different post-translational modifications of these cysteine residues can have critical roles in protein function, localization and/or stability. The close localization to different ROS-producing enzyme systems, a dedicated machinery for oxidative protein folding, and a unique equipment with antioxidative systems, render the careful balancing of the redox and modification states of the cysteine residues, a major challenge in the IMS. In this review, we discuss different functions of human IMS proteins, the involvement of cysteine residues in these functions, the consequences of cysteine modifications and the consequences of cysteine mutations or defects in the machinery for disulfide bond formation in terms of human health. LINKED ARTICLES: This article is part of a themed section on Chemical Biology of Reactive Sulfur Species. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.4/issuetoc.

NMR as a Tool to Investigate the Processes of Mitochondrial and Cytosolic Iron-Sulfur Cluster Biosynthesis

Iron-sulfur (Fe-S) clusters, the ubiquitous protein cofactors found in all kingdoms of life, perform a myriad of functions including nitrogen fixation, ribosome assembly, DNA repair, mitochondrial respiration, and metabolite catabolism. The biogenesis of Fe-S clusters is a multi-step process that involves the participation of many protein partners. Recent biophysical studies, involving X-ray crystallography, nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS), and small angle X-ray scattering (SAXS), have greatly improved our understanding of these steps. In this review, after describing the biological importance of iron sulfur proteins, we focus on the contributions of NMR spectroscopy has made to our understanding of the structures, dynamics, and interactions of proteins involved in the biosynthesis of Fe-S cluster proteins.

Fe-S cluster assembly in the supergroup Excavata

The majority of established model organisms belong to the supergroup Opisthokonta, which includes yeasts and animals. While enlightening, this focus has neglected protists,  organisms that represent the bulk of eukaryotic diversity and are often regarded as primitive eukaryotes. One of these is the “supergroup” Excavata, which comprises unicellular flagellates of diverse lifestyles and contains species of medical importance, such as Trichomonas, Giardia, Naegleria, Trypanosoma and Leishmania. Excavata exhibits a continuum in mitochondrial forms, ranging from classical aerobic, cristae-bearing mitochondria to mitochondria-related organelles, such as hydrogenosomes and mitosomes, to the extreme case of a complete absence of the organelle. All forms of mitochondria house a machinery for the assembly of Fe–S clusters, ancient cofactors required in various biochemical activities needed to sustain every extant cell. In this review, we survey what is known about the Fe–S cluster assembly in the supergroup Excavata. We aim to bring attention to the diversity found in this group, reflected in gene losses and gains that have shaped the Fe–S cluster biogenesis pathways.

The NMR contribution to protein-protein networking in Fe-S protein maturation

Iron–sulfur proteins were among the first class of metalloproteins that were actively studied using NMR spectroscopy tailored to paramagnetic systems. The hyperfine shifts, their temperature dependencies and the relaxation rates of nuclei of cluster-bound residues are an efficient fingerprint of the nature and the oxidation state of the Fe–S cluster. NMR significantly contributed to the analysis of the magnetic coupling patterns and to the understanding of the electronic structure occurring in [2Fe–2S], [3Fe–4S] and [4Fe–4S] clusters bound to proteins. After the first NMR structure of a paramagnetic protein was obtained for the reduced E. halophila HiPIP I, many NMR structures were determined for several Fe–S proteins in different oxidation states. It was found that differences in chemical shifts, in patterns of unobserved residues, in internal mobility and in thermodynamic stability are suitable data to map subtle changes between the two different oxidation states of the protein. Recently, the interaction networks responsible for maturing human mitochondrial and cytosolic Fe–S proteins have been largely characterized by combining solution NMR standard experiments with those tailored to paramagnetic systems. We show here the contribution of solution NMR in providing a detailed molecular view of “Fe–S interactomics”. This contribution was particularly effective when protein–protein interactions are weak and transient, and thus difficult to be characterized at high resolution with other methodologies.

Iron-sulfur clusters: from metals through mitochondria biogenesis to disease

Iron–sulfur clusters are ubiquitous inorganic co-factors that contribute to a wide range of cell pathways including the maintenance of DNA integrity, regulation of gene expression and protein translation, energy production, and antiviral response. Specifically, the iron–sulfur cluster biogenesis pathways include several proteins dedicated to the maturation of apoproteins in different cell compartments. Given the complexity of the biogenesis process itself, the iron–sulfur research area constitutes a very challenging and interesting field with still many unaddressed questions. Mutations or malfunctions affecting the iron–sulfur biogenesis machinery have been linked with an increasing amount of disorders such as Friedreich’s ataxia and various cardiomyopathies. This review aims to recap the recent discoveries both in the yeast and human iron–sulfur cluster arena, covering recent discoveries from chemistry to disease.

Paramagnetic NMR as a new tool in structural biology

NMR (nuclear magnetic resonance) investigation through the exploitation of paramagnetic effects is passing from an approach limited to few specialists in the field to a generally applicable method that must be considered, especially for the characterization of systems hardly affordable with other techniques. This is mostly due to the fact that paramagnetic data are long range in nature, thus providing information for the structural and dynamic characterization of complex biomolecular architectures in their native environment. On the other hand, this information usually needs to be complemented by data from other sources. Integration of paramagnetic NMR with other techniques, and the development of protocols for a joint analysis of all available data, is fundamental for achieving a comprehensive characterization of complex biological systems. We describe here a few examples of the new possibilities offered by paramagnetic data used in integrated structural approaches.

Perspectives on paramagnetic NMR from a life sciences infrastructure

The effects arising in NMR spectroscopy because of the presence of unpaired electrons, collectively referred to as "paramagnetic NMR" have attracted increasing attention over the last decades. From the standpoint of the structural and mechanistic biology, paramagnetic NMR provides long range restraints that can be used to assess the accuracy of crystal structures in solution and to improve them by simultaneous refinements through NMR and X-ray data. These restraints also provide information on structure rearrangements and conformational variability in biomolecular systems. Theoretical improvements in quantum chemistry calculations can nowadays allow for accurate calculations of the paramagnetic data from a molecular structural model, thus providing a tool to refine the metal coordination environment by matching the paramagnetic effects observed far away from the metal. Furthermore, the availability of an improved technology (higher fields and faster magic angle spinning) has promoted paramagnetic NMR applications in the fast-growing area of biomolecular solid-state NMR. Major improvements in dynamic nuclear polarization have been recently achieved, especially through the exploitation of the Overhauser effect occurring through the contact-driven relaxation mechanism: the very large enhancement of the ¹³C signal observed in a variety of liquid organic compounds at high fields is expected to open up new perspectives for applications of solution NMR.

Leaf Physiological and Proteomic Analysis to Elucidate Silicon Induced Adaptive Response under Salt Stress in Rosa hybrida 'Rock Fire'

Beneficial effects of silicon (Si) on growth and development have been witnessed in several plants. Nevertheless, studies on roses are merely reported. Therefore, the present investigation was carried out to illustrate the impact of Si on photosynthesis, antioxidant defense and leaf proteome of rose under salinity stress. In vitro-grown, acclimatized Rosa hybrida 'Rock Fire' were hydroponically treated with four treatments, such as control, Si (1.8 mM), NaCl (50 mM), and Si+NaCl. After 15 days, the consequences of salinity stress and the response of Si addition were analyzed. Scorching of leaf edges and stomatal damages occurred due to salt stress was ameliorated under Si supplementation. Similarly, reduction of gas exchange, photosynthetic pigments, higher lipid peroxidation rate, and accumulation of reactive oxygen species under salinity stress were mitigated in Si treatment. Lesser oxidative stress observed was correlated with the enhanced activity and expression of antioxidant enzymes, such as superoxide dismutase, catalase, and ascorbate peroxidase in Si+NaCl treatment. Importantly, sodium transportation was synergistically restricted with the stimulated counter-uptake of potassium in Si+NaCl treatment. Furthermore, two-dimensional electrophoresis (2-DE) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) results showed that out of 40 identified proteins, on comparison with control 34 proteins were down-accumulated and six proteins were up-accumulated due to salinity stress. Meanwhile, addition of Si with NaCl treatment enhanced the abundance of 30 proteins and downregulated five proteins. Differentially-expressed proteins were functionally classified into six groups, such as photosynthesis (22%), carbohydrate/energy metabolism (20%), transcription/translation (20%), stress/redox homeostasis (12%), ion binding (13%), and ubiquitination (8%). Hence, the findings reported in this work could facilitate a deeper understanding on potential mechanism(s) adapted by rose due to the exogenous Si supplementation during the salinity stress.

Cytosolic redox components regulate protein homeostasis via additional localisation in the mitochondrial intermembrane space

Oxidative protein folding is confined to the bacterial periplasm, endoplasmic reticulum and the mitochondrial intermembrane space. Maintaining a redox balance requires the presence of reductive pathways. The major thiol‐reducing pathways engage the thioredoxin and the glutaredoxin systems which are involved in removal of oxidants, protein proofreading and folding. Alterations in redox balance likely affect the flux of these redox pathways and are related to ageing and diseases such as neurodegenerative disorders and cancer. Here, we first review the well‐studied oxidative and reductive processes in the bacterial periplasm and the endoplasmic reticulum, and then discuss the less understood process in the mitochondrial intermembrane space, highlighting its importance for the proper function of the cell.

Anamorsin/Ndor1 Complex Reduces [2Fe-2S]-MitoNEET via a Transient Protein-Protein Interaction

Human mitoNEET is a homodimeric protein anchored to the outer mitochondrial membrane and has a C-terminal [2Fe-2S] binding domain located in the cytosol. Recently, human mitoNEET has been shown to be implicated in Fe/S cluster repair of cytosolic iron regulatory protein 1 (IRP1), a key regulator of cellular iron homeostasis in mammalian cells. The Fe/S cluster repair function of mitoNEET is based on an Fe/S redox switch mechanism: under normal cellular conditions, reduced [2Fe-2S]⁺-mitoNEET is present and is inactive as an Fe/S cluster transfer protein; under conditions of oxidative cellular stress, the clusters of mitoNEET become oxidized, and the formed [2Fe-2S]²⁺-mitoNEET species reacts promptly to initiate Fe/S cluster transfer to IRP1, recycling the cytosolic apo-IRP1 into holo-aconitase. Until now, no clear data have been available on which is the system that reduces the mitoNEET clusters back once oxidative stress is not present anymore. In the present work, we used UV-vis and NMR spectroscopies to investigate the electron transfer process between mitoNEET and the cytosolic electron-donor Ndor1/anamorsin complex, a component of the cytosolic iron-sulfur protein assembly (CIA) machinery. The [2Fe-2S] clusters of mitoNEET are reduced via the formation of a transient complex that brings the [2Fe-2S] clusters of mitoNEET close to the redox-active [2Fe-2S] cluster of anamorsin. Our data provide in vitro evidence of a possible direct link between the CIA machinery and the mitoNEET cluster transfer repair pathway. This link might contribute to recovery of CIA machinery efficiency to mature cytosolic and nuclear Fe/S proteins.

Minimal cytosolic iron-sulfur cluster assembly machinery of Giardia intestinalis is partially associated with mitosomes

Iron-sulfur (Fe-S) clusters are essential cofactors that enable proteins to transport electrons, sense signals, or catalyze chemical reactions. The maturation of dozens of Fe-S proteins in various compartments of every eukaryotic cell is driven by several assembly pathways. The ubiquitous cytosolic Fe-S cluster assembly (CIA) pathway, typically composed of eight highly conserved proteins, depends on mitochondrial Fe-S cluster assembly (ISC) machinery. Giardia intestinalis contains one of the smallest eukaryotic genomes and the mitosome, an extremely reduced mitochondrion. Because the only pathway known to be retained within this organelle is the synthesis of Fe-S clusters mediated by ISC machinery, a likely function of the mitosome is to cooperate with the CIA pathway. We investigated the cellular localization of CIA components in G. intestinalis and the origin and distribution of CIA-related components and Tah18-like proteins in other Metamonada. We show that orthologs of Tah18 and Dre2 are missing in these eukaryotes. In Giardia, all CIA components are exclusively cytosolic, with the important exception of Cia2 and two Nbp35 paralogs, which are present in the mitosomes. We propose that the dual localization of Cia2 and Nbp35 proteins in Giardia might represent a novel connection between the ISC and the CIA pathways.

Emerging links between iron-sulfur clusters and 5-methylcytosine base excision repair in plants

Iron-sulfur (Fe-S) clusters are ancient cofactors present in all kingdoms of life. Both the Fe-S cluster assembly machineries and target apoproteins are distributed across different subcellular compartments. The essential function of Fe-S clusters in nuclear enzymes is particularly difficult to study. The base excision repair (BER) pathway guards the integrity of DNA; enzymes from the DEMETER family of DNA glycosylases in plants are Fe-S cluster-dependent and extend the BER repertowere to excision of 5-methylcytosine (5mC). Recent studies in plants genetically link the majority of proteins from the cytosolic Fe-S cluster biogenesis (CIA) pathway with 5mC BER and DNA repair. This link can now be further explored. First, it opens new possibilities for understanding how Fe-S clusters participate in 5mC BER and related processes. I describe DNA-mediated charge transfer, an Fe-S cluster-based mechanism for locating base lesions with high efficiency, which is used by bacterial DNA glycosylases encoding Fe-S cluster binding domains that are also conserved in the DEMETER family. Second, because detailed analysis of the mutant phenotype of CIA proteins relating to 5mC BER revealed that they formed two groups, we may also gain new insights into both the composition of the Fe-S assembly pathway and the biological contexts of Fe-S proteins.

The conserved protein Dre2 uses essential [2Fe–2S] and [4Fe–4S] clusters for its function in cytosolic iron–sulfur protein assembly

The essential protein Dre2 uses iron–sulfur (Fe–S) clusters to transfer electrons for cytosolic Fe–S protein biogenesis. Biochemical, cell biological and spectroscopic approaches demonstrate that recombinant Dre2 binds oxygen-labile [2Fe–2S] and [4Fe–4S] clusters at two conserved C-terminal motifs with four cysteine residues each.

The presence of disulfide bonds reveals an evolutionarily conserved mechanism involved in mitochondrial protein translocase assembly

Disulfide bond formation is crucial for the biogenesis and structure of many proteins that are localized in the intermembrane space of mitochondria. The importance of disulfide bond formation within mitochondrial proteins was extended beyond soluble intermembrane space proteins. Tim22, a membrane protein and core component of the mitochondrial translocase TIM22, forms an intramolecular disulfide bond in yeast. Tim22 belongs to the Tim17/Tim22/Tim23 family of protein translocases. Here, we present evidence of the high evolutionary conservation of disulfide bond formation in Tim17 and Tim22 among fungi and metazoa. Topological models are proposed that include the location of disulfide bonds relative to the predicted transmembrane regions. Yeast and human Tim22 variants that are not oxidized do not properly integrate into the membrane complex. Moreover, the lack of Tim17 oxidation disrupts the TIM23 translocase complex. This underlines the importance of disulfide bond formation for mature translocase assembly through membrane stabilization of weak transmembrane domains.

Oxidative folding in the mitochondrial intermembrane space: A regulated process important for cell physiology and disease

Mitochondria are fundamental organelles with a complex internal architecture that fulfill important diverse functions including iron–sulfur cluster assembly and cell respiration. Intense work for more than 30 years has identified the key protein import components and the pathways involved in protein targeting and assembly. More recently, oxidative folding has been discovered as one important mechanism for mitochondrial proteostasis whilst several human disorders have been linked to this pathway. We describe the molecular components of this pathway in view of their putative redox regulation and we summarize available evidence on the connections of these pathways to human disorders.

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Mitochondria are the cell center of iron–sulfur cluster assembly and cell respiration.

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The MIA pathway has recently been linked to Fe/S pathways, Ca^2 + uptake and apoptosis.

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Mitochondria along with the ER and peroxisomes are major sources of ROS.

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Many diseases have been linked to mitochondrial dysfunction.

Mitochondrial Proteins Containing Coiled-Coil-Helix-Coiled-Coil-Helix (CHCH) Domains in Health and Disease

Members of the coiled-coil-helix-coiled-coil-helix (CHCH) domain-containing protein family that carry (CX9C) type motifs are imported into the mitochondrion with the help of the disulfide relay-dependent MIA import pathway. These evolutionarily conserved proteins are emerging as new cellular factors that control mitochondrial respiration, redox regulation, lipid homeostasis, and membrane ultrastructure and dynamics. We discuss recent insights on the activity of known (CX9C) motif-carrying proteins in mammals and review current data implicating the Mia40/CHCHD4 import machinery in the regulation of their mitochondrial import. Recent findings and the identification of disease-associated mutations in specific (CX9C) motif-carrying proteins have highlighted members of this family of proteins as potential therapeutic targets in a variety of human disorders.

Elucidating the Molecular Function of Human BOLA2 in GRX3-Dependent Anamorsin Maturation Pathway

In eukaryotes, the interaction between members of the monothiol glutaredoxin family and members of the BolA-like protein family has been involved in iron metabolism. To investigate the still unknown functional role of the interaction between human glutaredoxin-3 (GRX3) and its protein partner BOLA2, we characterized at the atomic level the interaction of apo BOLA2 with the apo and holo states of GRX3 and studied the role of BOLA2 in the GRX3-dependent anamorsin maturation pathway. From these studies, it emerged that apo GRX3 and apo BOLA2 form a heterotrimeric complex, composed by two BOLA2 molecules and one GRX3 molecule. This complex is able to bind two [2Fe-2S](2+) clusters, each being bridged between a BOLA2 molecule and a monothiol glutaredoxin domain of GRX3, and to transfer both [2Fe-2S](2+) clusters to apo anamorsin producing its mature holo state. Collectively, the data suggest that the heterotrimeric complex can work as a [2Fe-2S](2+) cluster transfer component in cytosolic Fe/S protein maturation pathways.

Cancer-Related NEET Proteins Transfer 2Fe-2S Clusters to Anamorsin, a Protein Required for Cytosolic Iron-Sulfur Cluster Biogenesis

Iron-sulfur cluster biogenesis is executed by distinct protein assembly systems. Mammals have two systems, the mitochondrial Fe-S cluster assembly system (ISC) and the cytosolic assembly system (CIA), that are connected by an unknown mechanism. The human members of the NEET family of 2Fe-2S proteins, nutrient-deprivation autophagy factor-1 (NAF-1) and mitoNEET (mNT), are located at the interface between the mitochondria and the cytosol. These proteins have been implicated in cancer cell proliferation, and they can transfer their 2Fe-2S clusters to a standard apo-acceptor protein. Here we report the first physiological 2Fe-2S cluster acceptor for both NEET proteins as human Anamorsin (also known as cytokine induced apoptosis inhibitor-1; CIAPIN-1). Anamorsin is an electron transfer protein containing two iron-sulfur cluster-binding sites that is required for cytosolic Fe-S cluster assembly. We show, using UV-Vis spectroscopy, that both NAF-1 and mNT can transfer their 2Fe-2S clusters to apo-Anamorsin with second order rate constants similar to those of other known human 2Fe-2S transfer proteins. A direct protein-protein interaction of the NEET proteins with apo-Anamorsin was detected using biolayer interferometry. Furthermore, electrospray mass spectrometry of holo-Anamorsin prepared by cluster transfer shows that it receives both of its 2Fe-2S clusters from the NEETs. We propose that mNT and NAF-1 can provide parallel routes connecting the mitochondrial ISC system and the CIA. 2Fe-2S clusters assembled in the mitochondria are received by NEET proteins and when needed transferred to Anamorsin, activating the CIA.

N-terminal domains mediate [2Fe-2S] cluster transfer from glutaredoxin-3 to anamorsin

In eukaryotes, cytosolic monothiol glutaredoxins are proteins implicated in intracellular iron trafficking and sensing via their bound [2Fe-2S] clusters. We define a new role of human cytosolic monothiol glutaredoxin-3 (GRX3) in transferring its [2Fe-2S] clusters to human anamorsin, a physical and functional protein partner of GRX3 in the cytosol, whose [2Fe-2S] cluster-bound form is involved in the biogenesis of cytosolic and nuclear Fe-S proteins. Specific protein recognition between the N-terminal domains of the two proteins is the mandatory requisite to promote the [2Fe-2S] cluster transfer from GRX3 to anamorsin.

The MIA pathway: a key regulator of mitochondrial oxidative protein folding and biogenesis

Mitochondria are fundamental intracellular organelles with key roles in important cellular processes like energy production, Fe/S cluster biogenesis, and homeostasis of lipids and inorganic ions. Mitochondrial dysfunction is consequently linked to many human pathologies (cancer, diabetes, neurodegeneration, stroke) and apoptosis. Mitochondrial biogenesis relies on protein import as most mitochondrial proteins (about 10–15% of the human proteome) are imported after their synthesis in the cytosol. Over the last several years many mitochondrial translocation pathways have been discovered. Among them, the import pathway that targets proteins to the intermembrane space (IMS) stands out as it is the only one that couples import to folding and oxidation and results in the covalent modification of the incoming precursor that adopt internal disulfide bonds in the process (the MIA pathway). The discovery of this pathway represented a significant paradigm shift as it challenged the prevailing dogma that the endoplasmic reticulum is the only compartment of eukaryotic cells where oxidative folding can occur.

The concept of the oxidative folding pathway was first proposed on the basis of folding and import data for the small Tim proteins that have conserved cysteine motifs and must adopt intramolecular disulfides after import so that they are retained in the organelle. The introduction of disulfides in the IMS is catalyzed by Mia40 that functions as a chaperone inducing their folding. The sulfhydryl oxidase Erv1 generates the disulfide pairs de novo using either molecular oxygen or, cytochrome c and other proteins as terminal electron acceptors that eventually link this folding process to respiration. The solution NMR structure of Mia40 (and supporting biochemical experiments) showed that Mia40 is a novel type of disulfide donor whose recognition capacity for its substrates relies on a hydrophobic binding cleft found adjacent to a thiol active CPC motif. Targeting of the substrates to this pathway is guided by a novel type of IMS targeting signal called ITS or MISS. This consists of only 9 amino acids, found upstream or downstream of a unique Cys that is primed for docking to Mia40 when the substrate is accommodated in the Mia40 binding cleft. Different routes exist to complete the folding of the substrates and their final maturation in the IMS. Identification of new Mia40 substrates (some even without the requirement of their cysteines) reveals an expanded chaperone-like activity of this protein in the IMS. New evidence on the targeting of redox active proteins like thioredoxin, glutaredoxin, and peroxiredoxin into the IMS suggests the presence of redox-dependent regulatory mechanisms of the protein folding and import process in mitochondria. Maintenance of redox balance in mitochondria is crucial for normal cell physiology and depends on the cross-talk between the various redox signaling processes and the mitochondrial oxidative folding pathway.

Mia40 is a facile oxidant of unfolded reduced proteins but shows minimal isomerase activity

Mia40 participates in oxidative protein folding within the mitochondrial intermembrane space (IMS) by mediating the transfer of reducing equivalents from client proteins to FAD-linked oxidoreductases of the Erv1 family (lfALR in mammals). Here we investigate the specificity of the human Mia40/lfALR system towards non-cognate unfolded protein substrates to assess whether the efficient introduction of disulfides requires a particular amino acid sequence context or the presence of an IMS targeting signal. Reduced pancreatic ribonuclease A (rRNase), avian lysozyme, and riboflavin binding protein are all competent substrates of the Mia40/lfALR system, although they lack those sequence features previously thought to direct disulfide bond formation in cognate IMS substrates. The oxidation of rRNase by Mia40 does not limit overall turnover of unfolded substrate by the Mia40/lfALR system. Mia40 is an ineffective protein disulfide isomerase when its ability to restore enzymatic activity from scrambled RNase is compared to that of protein disulfide isomerase. Mia40's ability to bind amphipathic peptides is evident by avid binding to the isolated B-chain during the insulin reductase assay. In aggregate these data suggest that the Mia40/lfALR system has a broad sequence specificity and that potential substrates may be protected from adventitious oxidation by kinetic sequestration within the mitochondrial IMS.

Mia40 Protein Serves as an Electron Sink in the Mia40-Erv1 Import Pathway

A redox-regulated import pathway consisting of Mia40 and Erv1 mediates the import of cysteine-rich proteins into the mitochondrial intermembrane space. Mia40 is the oxidoreductase that inserts two disulfide bonds into the substrate simultaneously. However, Mia40 has one redox-active cysteine pair, resulting in ambiguity about how Mia40 accepts numerous electrons during substrate oxidation. In this study, we have addressed the oxidation of Tim13 in vitro and in organello. Reductants such as glutathione and ascorbate inhibited both the oxidation of the substrate Tim13 in vitro and the import of Tim13 and Cmc1 into isolated mitochondria. In addition, a ternary complex consisting of Erv1, Mia40, and substrate, linked by disulfide bonds, was not detected in vitro. Instead, Mia40 accepted six electrons from substrates, and this fully reduced Mia40 was sensitive to protease, indicative of conformational changes in the structure. Mia40 in mitochondria from the erv1-101 mutant was also trapped in a completely reduced state, demonstrating that Mia40 can accept up to six electrons as substrates are imported. Therefore, these studies support that Mia40 functions as an electron sink to facilitate the insertion of two disulfide bonds into substrates.

Biogenesis of cytosolic and nuclear iron-sulfur proteins and their role in genome stability

Iron-sulfur (Fe-S) clusters are versatile protein cofactors that require numerous components for their synthesis and insertion into apoproteins. In eukaryotes, maturation of cytosolic and nuclear Fe-S proteins is accomplished by cooperation of the mitochondrial iron-sulfur cluster (ISC) assembly and export machineries, and the cytosolic iron-sulfur protein assembly (CIA) system. Currently, nine CIA proteins are known to specifically assist the two major steps of the biogenesis reaction. They are essential for cell viability and conserved from yeast to man. The essential character of this biosynthetic process is explained by the involvement of Fe-S proteins in central processes of life, e.g., protein translation and numerous steps of nuclear DNA metabolism such as DNA replication and repair. Malfunctioning of these latter Fe-S enzymes leads to genome instability, a hallmark of cancer. This review is focused on the maturation and biological function of cytosolic and nuclear Fe-S proteins, a topic of central interest for both basic and medical research. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.

The diabetes drug target MitoNEET governs a novel trafficking pathway to rebuild an Fe-S cluster into cytosolic aconitase/iron regulatory protein 1

In eukaryotes, mitochondrial iron-sulfur cluster (ISC), export and cytosolic iron-sulfur cluster assembly (CIA) machineries carry out biogenesis of iron-sulfur (Fe-S) clusters, which are critical for multiple essential cellular pathways. However, little is known about their export out of mitochondria. Here we show that Fe-S assembly of mitoNEET, the first identified Fe-S protein anchored in the mitochondrial outer membrane, strictly depends on ISC machineries and not on the CIA or CIAPIN1. We identify a dedicated ISC/export pathway in which augmenter of liver regeneration, a mitochondrial Mia40-dependent protein, is specific to mitoNEET maturation. When inserted, the Fe-S cluster confers mitoNEET folding and stability in vitro and in vivo. The holo-form of mitoNEET is resistant to NO and H2O2 and is capable of repairing oxidatively damaged Fe-S of iron regulatory protein 1 (IRP1), a master regulator of cellular iron that has recently been involved in the mitochondrial iron supply. Therefore, our findings point to IRP1 as the missing link to explain the function of mitoNEET in the control of mitochondrial iron homeostasis.

[2Fe-2S] cluster transfer in iron-sulfur protein biogenesis

Monothiol glutaredoxins play a crucial role in iron-sulfur (Fe/S) protein biogenesis. Essentially all of them can coordinate a [2Fe-2S] cluster and have been proposed to mediate the transfer of [2Fe-2S] clusters from scaffold proteins to target apo proteins, possibly by acting as cluster transfer proteins. The molecular basis of [2Fe-2S] cluster transfer from monothiol glutaredoxins to target proteins is a fundamental, but still unresolved, aspect to be defined in Fe/S protein biogenesis. In mitochondria monothiol glutaredoxin 5 (GRX5) is involved in the maturation of all cellular Fe/S proteins and participates in cellular iron regulation. Here we show that the structural plasticity of the dimeric state of the [2Fe-2S] bound form of human GRX5 (holo hGRX5) is the crucial factor that allows an efficient cluster transfer to the partner proteins human ISCA1 and ISCA2 by a specific protein-protein recognition mechanism. Holo hGRX5 works as a metallochaperone preventing the [2Fe-2S] cluster to be released in solution in the presence of physiological concentrations of glutathione and forming a transient, cluster-mediated protein-protein intermediate with two physiological protein partners receiving the [2Fe-2S] cluster. The cluster transfer mechanism defined here may extend to other mitochondrial [2Fe-2S] target proteins.