IscA/SufA paralogues are required for the [4Fe-4S] cluster assembly in enzymes of multiple physiological pathways in Escherichia coli under aerobic growth conditions

Mechanism of mitochondrial [2Fe-2S] cluster biosynthesis

Iron‑sulfur (Fe-S) clusters constitute ancient cofactors that accompany a versatile range of fundamental biological reactions across eukaryotes and prokaryotes. Several cellular pathways exist to coordinate iron acquisition and sulfur mobilization towards a scaffold protein during the tightly regulated synthesis of Fe-S clusters. The mechanism of mitochondrial eukaryotic [2Fe-2S] cluster synthesis is coordinated by the Iron-Sulfur Cluster (ISC) machinery and its aberrations herein have strong implications to the field of disease and medicine which is therefore of particular interest. Here, we describe our current knowledge on the step-by-step mechanism leading to the production of mitochondrial [2Fe-2S] clusters while highlighting the recent developments in the field alongside the challenges that are yet to be overcome.

CyaY and TusA regulate ISC- and SUF-mediated l-cysteine desulfurase activity

CyaY, the frataxin homolog of Escherichia coli, plays an important role in ISC iron-sulfur cluster assembly through interactions with the cysteine desulfurase IscS, which regulate the supply of sulfur. IscS is not exclusive for ISC Fe-S cluster assembly, as it functions as a hub for the supply of sulfur to a number of other sulfur-requiring pathways, such as for the biosynthesis of Moco and thiolated tRNAs. How the balance of sulfur supply to the various competing pathways is achieved is not fully understood, but a network of protein-protein interactions plays a key role. For example, IscU and TusA compete for binding to IscS and thus for sulfur supply to ISC and Moco/tRNA biosynthesis. Here, we show that TusA can displace CyaY from IscS and can form hetero-complexes involving IscS, CyaY and TusA. Displacement of CyaY from IscS raised the question of whether it can interact with the SUF pathway. The SUF cysteine desulfurase SufS functions as a complex with SufE. Native mass spectrometry studies showed that the SufS dimer can bind up to four SufE molecules, two at high affinity, and two at low affinity, sites. Titration of SufSE (or SufS alone) with CyaY demonstrated binding, probably at the lower affinity site in competition with SufE. Binding of CyaY dramatically reduced the activity of SufSE in vitro, and over-expression of CyaY also significantly affected total cellular desulfurase activity and Fe-S cluster assembly, with the greatest effect observed in mutant strains in which SufS was the principal desulfurase. These data point to a physiological role for CyaY in regulating the desulfurase activity of IscS and SufS and, hence, both the E.coli iron-sulfur assembly systems. They also demonstrate that TusA can displace the regulatory CyaY protein from IscS-CyaY complexes, facilitating sulfur delivery from IscS to other essential cellular processes, and increasing the likelihood of SufSE-CyaY interactions.

Recent progress of methods for cuproptosis detection

Varying from other identified cell death pathways, cuproptosis is a new type of regulated cell death characterized by excess Cu ions, abnormal aggregation of lipoylated proteins in TCA cycle, loss of Fe-S cluster proteins, upregulation of HSP70, leading to proteotoxic and oxidative stress. Cuproptosis is highly concerned by scientific community and as the field of cuproptosis further develops, remarkable progress has been made in the verification and mechanism of cuproptosis, and methods used to detect cuproptosis have been continuously improved. According to the characteristic changes of cuproptosis, techniques based on cell death verification, Cu content, morphology, molecular biology of protein levels of cuproptosis-related molecules and biochemical pathways of cuproptosis-related enzyme activity and metabolites of oxidative stress, lipoic acid, TCA cycle, Fe-S cluster proteins, oxidative phosphorylation, cell respiration intensity have been subject to cuproptosis verification and research. In order to further deepen the understanding of detecting cuproptosis, the principle and application of common cuproptosis detection methods are reviewed and categorized in cellular phenomena and molecular mechanism in terms of cell death, Cu content, morphology, molecular biology, biochemical pathways with a flow chart. All the indicating results have been displayed in response to the markers of cuproptosis, their advantages and limitations are summaried, and comparison of cuproptosis and ferroptosis detection is performed in this study. Our collection of methods for cuproptosis detection will provide a great basis for cuproptosis verification and research in the future.

Multimodal Spectroscopic Analysis of the Fe-S Clusters of the as-Isolated Escherichia coli SufBC2D Complex

Iron-sulfur (Fe-S) clusters are essential inorganic cofactors dedicated to a wide range of biological functions, including electron transfer and catalysis. Specialized multiprotein machineries present in all types of organisms support their biosynthesis. These machineries encompass a scaffold protein, on which Fe-S clusters are assembled before being transferred to cellular targets. Here, we describe the first characterization of the native Fe-S cluster of the anaerobically purified SufBC2D scaffold from Escherichia coli by XAS and Mössbauer, UV-visible absorption, and EPR spectroscopies. Interestingly, we propose that SufBC2D harbors two iron-sulfur-containing species, a [2Fe-2S] cluster and an as-yet unidentified species. Mutagenesis and biochemistry were used to propose amino acid ligands for the [2Fe-2S] cluster, supporting the hypothesis that both SufB and SufD are involved in the Fe-S cluster ligation. The [2Fe-2S] cluster can be transferred to ferredoxin in agreement with the SufBC2D scaffold function. These results are discussed in the context of Fe-S cluster biogenesis.

Iron-sulfur cluster assembly scaffold protein IscU is required for activation of ferric uptake regulator (Fur) in Escherichiacoli

It was generally postulated that when intracellular free iron content is elevated in bacteria, the ferric uptake regulator (Fur) binds its corepressor a mononuclear ferrous iron to regulate intracellular iron homeostasis. However, the proposed iron-bound Fur had not been identified in any bacteria. In previous studies, we have demonstrated that Escherichia coli Fur binds a [2Fe-2S] cluster in response to elevation of intracellular free iron content and that binding of the [2Fe-2S] cluster turns on Fur as an active repressor to bind a specific DNA sequence known as the Fur-box. Here we find that the iron-sulfur cluster assembly scaffold protein IscU is required for the [2Fe-2S] cluster assembly in Fur, as deletion of IscU inhibits the [2Fe-2S] cluster assembly in Fur and prevents activation of Fur as a repressor in E. coli cells in response to elevation of intracellular free iron content. Additional studies reveal that IscU promotes the [2Fe-2S] cluster assembly in apo-form Fur and restores its Fur-box binding activity in vitro. While IscU is also required for the [2Fe-2S] cluster assembly in the Haemophilus influenzae Fur in E. coli cells, deletion of IscU does not significantly affect the [2Fe-2S] cluster assembly in the E. coli ferredoxin and siderophore-reductase FhuF. Our results suggest that IscU may have a unique role for the [2Fe-2S] cluster assembly in Fur and that regulation of intracellular iron homeostasis is closely coupled with iron-sulfur cluster biogenesis in E. coli.

The C-terminal domain of the ferric uptake regulator (Fur) binds a [2Fe-2S] cluster to sense the intracellular free iron content in Escherichia coli

Escherichia coli ferric uptake regulator (Fur) binds a [2Fe-2S] cluster, not a mononuclear iron, when the intracellular free iron content is elevated in E. coli cells. Here we report that the C-terminal domain (residues 83-148) of E. coli Fur (Fur-CTD) is sufficient to bind the [2Fe-2S] cluster in response to elevation of the intracellular free iron content in E. coli cells. Deletion of gene fur in E. coli cells increases the intracellular free iron content and promotes the [2Fe-2S] cluster binding in the Fur-CTD in the cells grown in LB medium under aerobic growth conditions. When the Fur-CTD is expressed in wild type E. coli cells grown in M9 medium supplemented with increasing concentrations of iron, the Fur-CTD also progressively binds a [2Fe-2S] cluster with a maximum occupancy of about 36%. Like the E. coli Fur-CTD, the CTD of the Haemophilus influenzae Fur can also bind a [2Fe-2S] cluster in wild type E. coli cells grown in M9 medium supplemented with increasing concentrations of iron, indicating that binding of the [2Fe-2S] cluster in the C-terminal domain is highly conserved among Fur proteins. The results suggest that the Fur-CTD can be used as a physiological probe to assess the intracellular free iron content in bacteria.

The Mycobacterium smegmatis HesB Protein, MSMEG_4272, Is Required for In Vitro Growth and Iron Homeostasis

A-type carrier (ATC) proteins are proposed to function in the biogenesis of Fe-S clusters, although their exact role remains controversial. The genome of Mycobacterium smegmatis encodes a single ATC protein, MSMEG_4272, which belongs to the HesB/YadR/YfhF family of proteins. Attempts to generate an MSMEG_4272 deletion mutant by two-step allelic exchange were unsuccessful, suggesting that the gene is essential for in vitro growth. CRISPRi-mediated transcriptional knock-down of MSMEG_4272 resulted in a growth defect under standard culture conditions, which was exacerbated in mineral-defined media. The knockdown strain displayed reduced intracellular iron levels under iron-replete conditions and increased susceptibility to clofazimine, 2,3-dimethoxy-1,4-naphthoquinone (DMNQ), and isoniazid, while the activity of the Fe-S containing enzymes, succinate dehydrogenase, and aconitase were not affected. This study suggests that MSMEG_4272 plays a role in the regulation of intracellular iron levels and is required for in vitro growth of M. smegmatis, particularly during exponential growth.

Copper exerts cytotoxicity through inhibition of iron-sulfur cluster biogenesis on ISCA1/ISCA2/ISCU assembly proteins

Copper is an essential mineral nutrient that provides the cofactors for some key enzymes. However, excess copper is paradoxically cytotoxic. Wilson's disease is an autosomal recessive hereditary disease characterized by pathological copper accumulation in many organs, with high mortality and disability. Nevertheless, many questions about the molecular mechanism in Wilson's disease remain unknown and there is an imperative need to address these questions to better exploit therapeutic strategy. In this study, we constructed the mouse model of Wilson's disease, ATP7A^-/- immortalized lymphocyte cell line and ATP7B knockdown cells to explore whether copper could impair iron-sulfur cluster biogenesis in eukaryotic mitochondria. Through a series of cellular, molecular, and pharmacological analyses, we demonstrated that copper could suppress the assembly of Fe-S cluster, decrease the activity of the Fe-S enzyme and disorder the mitochondrial function both in vivo and in vitro. Mechanistically, we found that human ISCA1, ISCA2 and ISCU proteins have a strong copper-binding activity, which would hinder the process of iron-sulfur assembly. Of note, we proposed a novel mechanism of action to explain the toxicity of copper by providing evidence that iron-sulfur cluster biogenesis may be a primary target of copper toxicity both in cells and mouse models. In summary, the current work provides an in-depth study on the mechanism of copper intoxication and describes a framework for the further understanding of impaired Fe-S assembly in the pathological processes of Wilson's diseases, which helps to develop latent therapeutic strategies for the management of copper toxicity.

Ferric uptake regulator (Fur) binds a [2Fe-2S] cluster to regulate intracellular iron homeostasis in Escherichia coli

Intracellular iron homeostasis in bacteria is primarily regulated by Ferric uptake regulator (Fur). It has been postulated that when intracellular free iron content is elevated, Fur binds ferrous iron to down-regulate the genes for iron uptake. However, the iron-bound Fur had not been identified in any bacteria until we recently found that Escherichia coli Fur binds a [2Fe-2S] cluster, but not a mononuclear iron, in E. coli mutant cells that hyperaccumulate intracellular free iron. Here we report that E. coli Fur also binds a [2Fe-2S] cluster in wild type E. coli cells grown in M9 medium supplemented with increasing concentrations of iron under aerobic growth conditions. Additionally, we find that binding of the [2Fe-2S] cluster in Fur turns on its binding activity for specific DNA sequences known as the Fur-box, and that removal of the [2Fe-2S] cluster from Fur eliminates its Fur-box binding activity. Mutation of the conserved cysteine residues Cys-93 and Cys-96 to Ala in Fur results in the Fur mutants that fail to bind the [2Fe-2S] cluster, have a diminished binding activity for the Fur-box in vitro, and are inactive to complement the function of Fur in vivo. Our results suggest that Fur binds a [2Fe-2S] cluster to regulate intracellular iron homeostasis in response to elevation of intracellular free iron content in E. coli cells.

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.

Ferric uptake regulators (Fur) from Vibrio cholerae and Helicobacter pylori bind a [2Fe-2S] cluster in response to elevation of intracellular free iron content

Intracellular iron homeostasis in bacteria is primarily regulated by ferric uptake regulator (Fur). Since its discovery, Fur has been assumed to bind ferrous iron and regulate expression of target genes. However, the iron-bound Fur has never been isolated from any bacteria. In previous studies, we have shown that Escherichia coli Fur and Haemophilus influenzae Fur bind a [2Fe-2S] cluster via the conserved Cys-93 and Cys-96 when expressed in the E. coli mutant cells in which intracellular free iron content is elevated. Here we report that Fur homologs from Vibrio cholerae and Helicobacter pylori which contain Cys-93 and Cys-96 can also bind a [2Fe-2S] cluster. On the other hand, Fur homolog from Magnetospirillum gryphiswaldense MSR-1 which has no cysteine residues fails to bind any [2Fe-2S] clusters. Interestingly, different Fur proteins with the conserved Cys-93 and Cys-96 have distinct binding activities for the [2Fe-2S] cluster, with H. influenzae Fur having the highest, followed by E. coli Fur, V. cholera Fur, and H. pylori Fur. Binding of the [2Fe-2S] cluster in the Fur proteins is significantly decreased when expressed in wild-type E. coli cells, indicating that binding of the [2Fe-2S] clusters in Fur proteins is regulated by the levels of intracellular free iron content. Finally, unlike the [2Fe-2S] clusters in E. coli ferredoxin, the [2Fe-2S] clusters in the Fur proteins are not stable and quickly release ferrous iron when the clusters are reduced, suggesting that Fur may undergo reversible binding of the [2Fe-2S] cluster in response to intracellular free iron content in bacteria.

Transcriptomic analysis reveals the inhibition of reproduction in rice brown planthopper, Nilaparvata lugens, after silencing the gene of MagR (IscA1)

MagR (IscA1) is a member of the iron-sulphur cluster assembly proteins, which plays vital roles in many physiological processes, such as energy metabolism, electron transfer, iron homeostasis, heme biosynthesis and physiologically magnetic response. Its deletion leads to the loss of mitochondrial DNA, inactivation of iron-sulphur proteins and abnormal embryonic development in organisms. However, the physiological roles of MagR in insects are unclear. This study characterized the effects and molecular regulatory mechanism of MagR gene silencing on the reproduction of brachypterous female adults of Nilaparvata lugens. After silencing the MagR gene using RNAi approach, the duration of reproductive period was shortened and the fecundity and hatchability reduced significantly. A total of 479 differentially expressed genes (DEGs) were identified for female adults after 2 days of dsRNA injection through RNA-sequencing technology, including 352 significantly upregulated DEGs and 127 significantly downregulated DEGs, among which 44 DEGs were considered the key genes involved in the effects of NlMagR silencing on the reproduction, revealing the regulatory mechanism of MagR at RNA transcription level and providing a new strategy for the control of N. lugens.

Structural Basis of RICs Iron Donation for Iron-Sulfur Cluster Biogenesis

Escherichia coli YtfE is a di-iron protein of the widespread Repair of Iron Centers proteins (RIC) family that has the capacity to donate iron, which is a crucial component of the biogenesis of the ubiquitous family of iron-sulfur proteins. In this work we identify in E. coli a previously unrecognized link between the YtfE protein and the major bacterial system for iron-sulfur cluster (ISC) assembly. We show that YtfE establishes protein-protein interactions with the scaffold IscU, where the transient cluster is formed, and the cysteine desulfurase IscS. Moreover, we found that promotion by YtfE of the formation of an Fe-S cluster in IscU requires two glutamates, E125 and E159 in YtfE. Both glutamates form part of the entrance of a protein channel in YtfE that links the di-iron center to the surface. In particular, E125 is crucial for the exit of iron, as a single mutation to leucine closes the channel rendering YtfE inactive for the build-up of Fe-S clusters. Hence, we provide evidence for the key role of RICs as bacterial iron donor proteins involved in the biogenesis of Fe-S clusters.

Identification of an Intermediate Form of Ferredoxin That Binds Only Iron Suggests That Conversion to Holo-Ferredoxin Is Independent of the ISC System in Escherichia coli

Escherichia coli [2Fe-2S]-ferredoxin and other ISC proteins encoded by the iscRSUA-hscBA-fdx-iscX (isc) operon are responsible for the assembly of iron-sulfur clusters. It is proposed that ferredoxin (Fdx) donates electrons from its reduced [2Fe-2S] center to iron-sulfur cluster biogenesis reactions. However, the underlying mechanisms of the [2Fe-2S] cluster assembly in Fdx remain elusive. Here, we report that Fdx preferentially binds iron, but not the [2Fe-2S] cluster, under cold stress conditions (≤16°C). The iron binding in Fdx is characterized by a unique absorption peak at 320 nm based on UV-visible spectroscopy. In addition, the iron-binding form of Fdx could be converted to the [2Fe-2S] cluster-bound form after transferring cold-stressed cells to normal cultivation temperatures above 25°C. In vitro experiments also revealed that Fdx could utilize bound iron to assemble the [2Fe-2S] cluster by itself. Furthermore, inactivation of the genes encoding IscS, IscU, and IscA did not limit [2Fe-2S] cluster assembly in Fdx, which was also observed by inactivating the isc or suf operon, indicating that iron-sulfur cluster biogenesis in Fdx arose from a unique pathway in E. coli Our results suggest that the intracellular assembly of [2Fe-2S] clusters in Fdx is susceptible to environmental temperatures. The iron binding form of Fdx (Fe-Fdx) is a precursor during its maturation to a cluster binding form ([2Fe-2S]-Fdx), and reassembly of the [2Fe-2S] clusters during temperature increases is not strictly reliant on other specific iron donors and scaffold proteins within the Isc or Suf system.IMPORTANCE Fdx is an electron carrier that is required for the maturation of many other iron-sulfur proteins. Its function strictly depends on its [2Fe-2S] center that bonds with the cysteinyl S atoms of four cysteine residues within Fdx. However, the assembly mechanism of the [2Fe-2S] clusters in Fdx remains controversial. This study reports that Fdx fails to form its [2Fe-2S] cluster under cold stress conditions but instead binds a single Fe atom at the cluster binding site. Moreover, when temperatures increase, Fdx can assemble clusters by itself from its iron-only binding form in E. coli cells. The possibility remains that Fdx can effectively accept clusters from multiple sources. Nevertheless, our results suggest that Fdx has a strong iron binding activity that contributes to the assembly of its own [2Fe-2S] cluster and that Fdx acts as a temperature sensor to regulate Isc system-mediated iron-sulfur cluster biogenesis.

A-type carrier proteins are involved in [4Fe-4S] cluster insertion into the radical SAM protein MoaA for the synthesis of active molybdoenzymes

Iron sulfur (Fe-S) clusters are important biological cofactors present in proteins with crucial biological functions, from photosynthesis to DNA repair, gene expression and bioenergetic processes. For the insertion of Fe-S clusters into proteins, A-type carrier proteins have been identified. So far, three of them were characterized in detail in Escherichia coli, namely IscA, SufA and ErpA, which were shown to partially replace each other in their roles in [4Fe-4S] cluster insertion into specific target proteins. To further expand the knowledge of [4Fe-4S] cluster insertion into proteins, we analyzed the complex Fe-S cluster dependent network for the synthesis of the molybdenum cofactor (Moco) and the expression of genes encoding nitrate reductase in E. coli Our studies include the identification of the A-type carrier proteins ErpA and IscA involved in [4Fe-4S] cluster insertion into the S-adenosyl-methionine dependent radical SAM protein MoaA. We show that ErpA and IscA can partially replace each other in their role to provide [4Fe-4S] clusters for MoaA. Since most genes expressing molybdoenzymes are regulated by the transcriptional regulator for fumarate and nitrate reduction (FNR) under anaerobic conditions, we also identified the proteins that are crucial to obtain an active FNR under conditions of nitrate respiration. We show that ErpA is essential for the FNR-dependent expression of the narGHJI operon, a role that cannot be compensated by IscA under the growth conditions tested. SufA does not have a role in Fe-S cluster insertion into MoaA or FNR under anaerobic growth of nitrate respiration, based on low gene expression levels.IMPORTANCEUnderstanding the assembly of iron-sulfur (Fe-S) proteins is relevant to many fields, including nitrogen fixation, photosynthesis, bioenergetics and gene regulation. Still remaining critical gaps in our knowledge are how Fe-S clusters are transferred to their target proteins and how the specificity in this process is achieved, since different forms of Fe-S clusters need to be delivered to structurally highly diverse target proteins. Numerous Fe-S carrier proteins have been identified in prokaryotes like Escherichia coli, including ErpA, IscA, SusA and NfuA. In addition, the diverse Fe-S cluster delivery proteins and their target proteins underlie a complex regulatory network of expression, to ensure that both proteins are synthesized under particular growth conditions.

Do FeS clusters rule bacterial iron regulation?

For decades, the bacterial ferric uptake regulator (Fur) has been thought to respond to ferrous iron to transcriptionally regulate genes required for balancing iron uptake, storage, and utilization. Because iron binding to Fur has never been confirmed in vivo, the physiological iron-sensing mechanism remains an open question. Fontenot et al. now show that Fur purified from Escherichia coli binds an all-Cys-coordinated [2Fe-2S] cluster. This finding opens the exciting possibility that Fur may join numerous well-studied bacterial, fungal, and mammalian proteins that use FeS clusters for cellular iron regulation.

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.

Ferric uptake regulator (Fur) reversibly binds a [2Fe-2S] cluster to sense intracellular iron homeostasis in Escherichia coli

The ferric uptake regulator (Fur) is a global transcription factor that regulates intracellular iron homeostasis in bacteria. The current hypothesis states that when the intracellular "free" iron concentration is elevated, Fur binds ferrous iron, and the iron-bound Fur represses the genes encoding for iron uptake systems and stimulates the genes encoding for iron storage proteins. However, the "iron-bound" Fur has never been isolated from any bacteria. Here we report that the Escherichia coli Fur has a bright red color when expressed in E. coli mutant cells containing an elevated intracellular free iron content because of deletion of the iron-sulfur cluster assembly proteins IscA and SufA. The acid-labile iron and sulfide content analyses in conjunction with the EPR and Mössbauer spectroscopy measurements and the site-directed mutagenesis studies show that the red Fur protein binds a [2Fe-2S] cluster via conserved cysteine residues. The occupancy of the [2Fe-2S] cluster in Fur protein is ∼31% in the E. coli iscA/sufA mutant cells and is decreased to ∼4% in WT E. coli cells. Depletion of the intracellular free iron content using the membrane-permeable iron chelator 2,2´-dipyridyl effectively removes the [2Fe-2S] cluster from Fur in E. coli cells, suggesting that Fur senses the intracellular free iron content via reversible binding of a [2Fe-2S] cluster. The binding of the [2Fe-2S] cluster in Fur appears to be highly conserved, because the Fur homolog from Hemophilus influenzae expressed in E. coli cells also reversibly binds a [2Fe-2S] cluster to sense intracellular iron homeostasis.

The Requirement of Inorganic Fe-S Clusters for the Biosynthesis of the Organometallic Molybdenum Cofactor

Iron-sulfur (Fe-S) clusters are essential protein cofactors. In enzymes, they are present either in the rhombic [2Fe-2S] or the cubic [4Fe-4S] form, where they are involved in catalysis and electron transfer and in the biosynthesis of metal-containing prosthetic groups like the molybdenum cofactor (Moco). Here, we give an overview of the assembly of Fe-S clusters in bacteria and humans and present their connection to the Moco biosynthesis pathway. In all organisms, Fe-S cluster assembly starts with the abstraction of sulfur from l-cysteine and its transfer to a scaffold protein. After formation, Fe-S clusters are transferred to carrier proteins that insert them into recipient apo-proteins. In eukaryotes like humans and plants, Fe-S cluster assembly takes place both in mitochondria and in the cytosol. Both Moco biosynthesis and Fe-S cluster assembly are highly conserved among all kingdoms of life. Moco is a tricyclic pterin compound with molybdenum coordinated through its unique dithiolene group. Moco biosynthesis begins in the mitochondria in a Fe-S cluster dependent step involving radical/S-adenosylmethionine (SAM) chemistry. An intermediate is transferred to the cytosol where the dithiolene group is formed, to which molybdenum is finally added. Further connections between Fe-S cluster assembly and Moco biosynthesis are discussed in detail.

Identification of IscU residues critical for de novo iron-sulfur cluster assembly

IscU is a central component of the ISC machinery and serves as a scaffold for the de novo assembly of iron-sulfur (Fe-S) clusters prior to their delivery to target apo-Fe-S proteins. However, the molecular mechanism is not yet fully understood. In this study, we have conducted mutational analysis of E. coli IscU using the recently developed genetic complementation system of a mutant that can survive without Fe-S clusters. The Fe-S cluster ligands (C37, C63, H105, C106) and the proximal D39 and K103 residues are essential for in vivo function of IscU and could not be substituted with any other amino acids. Furthermore, we found that substitution of Y3, a strictly conserved residue among IscU homologs, abolished in vivo functions. Surprisingly, a second-site suppressor mutation in IscS (A349V) reverted the defect caused by IscU Y3 substitutions. Biochemical analysis revealed that IscU Y3 was crucial for functional interaction with IscS and sulfur transfer between the two proteins. Our findings suggest that the critical role of IscU Y3 is linked to the conformational dynamics of the flexible loop of IscS, which is required for the ingenious sulfur transfer to IscU.

A conserved motif liganding the [4Fe-4S] cluster in [4Fe-4S] fumarases prevents irreversible inactivation of the enzyme during hydrogen peroxide stress

Organisms have evolved two different classes of the ubiquitous enzyme fumarase: the [4Fe–4S] cluster-containing class I enzymes are oxidant-sensitive, whereas the class II enzymes are iron-free and therefore oxidant-resistant. When hydrogen peroxide (H₂O₂) attacks the most-studied [4Fe–4S] fumarases, only the cluster is damaged, and thus the cell can rapidly repair the enzyme. However, this study shows that when elevated levels of H₂O₂ oxidized the class I fumarase of the obligate anaerobe Bacteroides thetaiotaomicron (Bt-Fum), a hydroxyl-like radical species was produced that caused irreversible covalent damage to the polypeptide. Unlike the fumarase of oxygen-tolerant bacteria, Bt-Fum lacks a key cysteine residue in the typical “CX_nCX₂C″ motif that ligands [4Fe–4S] clusters. Consequently H₂O₂ can access and oxidize an iron atom other than the catalytic one in its cluster. Phylogenetic analysis showed that certain clades of bacteria may have evolved the full “CX_nCX₂C″ motif to shield the [4Fe–4S] cluster of fumarase. This effect was reproduced by the construction of a chimeric enzyme. These data demonstrate the irreversible oxidation of Fe–S cluster enzymes and may recapitulate evolutionary steps that occurred when microorganisms originally confronted oxidizing environments. It is also suggested that, if H₂O₂ is generated within the colon as a consequence of inflammation or the action of lactic acid bacteria, the inactivation of fumarase could potentially impair the central fermentation pathway of Bacteroides species and contribute to gut dysbiosis.

The l-Thr Kinase/l-Thr-Phosphate Decarboxylase (CobD) Enzyme from Methanosarcina mazei Gö1 Contains Metallocenters Needed for Optimal Activity

The MM2060 (cobD) gene from Methanosarcina mazei strain Gö1 encodes a protein (MmCobD) with l-threonine kinase (PduX) and l-threonine-O-3-phosphate decarboxylase (CobD) activities. In addition to the unexpected l-Thr kinase activity, MmCobD has an extended carboxy-terminal (C-terminal) region annotated as a putative metal-binding zinc finger-like domain. Here, we demonstrate that the C-terminus of MmCobD is a ferroprotein containing ∼25 non-heme iron atoms per monomer of protein. The absence of the C-terminus substantially reduces, but does not abolish, enzymatic activities in vitro and in vivo. Single-residue substitutions of C-terminal putative Fe-binding cysteinyl and histidinyl residues resulted in the loss of Fe and changes in enzyme activity levels. Salmonella enterica ΔpduX and ΔcobD strains were used as heterologous hosts to assess coenzyme B12 biosynthesis as a function of 17 MmCobD variants tested. Some of the latter displayed 5-fold higher enzymatic activity in vitro and enhanced the growth rate of the S. enterica strains that synthesized them. Most of the MmCobD variants tested were up to 6-fold less active in vitro and supported slow growth rates of the S. enterica strains that synthesized them; some substitutions abolished enzyme activity. MmCobD exhibited an ultraviolet-visible absorption spectrum consistent with [4Fe-4S] clusters that appeared to be susceptible to oxidation by H2O2 and reduction by sodium dithionite. The presence of FeS clusters in MmCobD was corroborated by electron paramagnetic resonance and magnetic circular dichroism studies. Collectively, our results suggest that MmCobD contains one or more diamagnetic [4Fe-4S]2+ center(s) that may play a structural or regulatory role.

Recent Advances in the [Fe-S] Cluster Biogenesis (SUF) Pathway Functional in the Apicoplast of Plasmodium

Iron-sulfur [Fe-S] clusters are one of the most ancient, ubiquitous, structurally and functionally versatile natural biosynthetic prosthetic groups required by various proteins involved in important metabolic processes. Genome mining and localization studies in Plasmodium have shown two evolutionarily distinct biogenesis pathways: the ISC pathway in mitochondria and the SUF pathway in the apicoplast. In recent years, the myriad efforts made to elucidate the SUF pathway have deciphered the role of various proteins involved in the pathway and their importance for the parasite life cycle in both asexual and sexual stages. This review aims to discuss recent research in the apicoplast [Fe-S] biogenesis pathway from Plasmodium to enhance our current understanding of parasite biology with an overall aim to identify gaps to strengthen our fight against malaria.

B. subtilis as a Model for Studying the Assembly of Fe-S Clusters in Gram-Positive Bacteria

Complexes of iron and sulfur (Fe-S clusters) are widely distributed in nature and participate in essential biochemical reactions. The biological formation of Fe-S clusters involves dedicated pathways responsible for the mobilization of sulfur, the assembly of Fe-S clusters, and the transfer of these clusters to target proteins. Genomic analysis of Bacillus subtilis and other Gram-positive bacteria indicated the presence of only one Fe-S cluster biosynthesis pathway, which is distinct in number of components and organization from previously studied systems. B. subtilis has been used as a model system for the characterization of cysteine desulfurases responsible for sulfur mobilization reactions in the biogenesis of Fe-S clusters and other sulfur-containing cofactors. Cysteine desulfurases catalyze the cleavage of the C-S bond from the amino acid cysteine and subsequent transfer of sulfur to acceptor molecules. These reactions can be monitored by the rate of alanine formation, the first product in the reaction, and sulfide formation, a byproduct of reactions performed under reducing conditions. The assembly of Fe-S clusters on protein scaffolds and the transfer of these clusters to target acceptors are determined through a combination of spectroscopic methods probing the rate of cluster assembly and transfer. This chapter provides a description of reactions promoting the assembly of Fe-S clusters in bacteria as well as methods used to study functions of each biosynthetic component and identify mechanistic differences employed by these enzymes across different pathways.

Function and maturation of the Fe-S center in dihydroxyacid dehydratase from Arabidopsis

Dihydroxyacid dehydratase (DHAD) is the third enzyme required for branched-chain amino acid biosynthesis in bacteria, fungi, and plants. DHAD enzymes contain two distinct types of active-site Fe-S clusters. The best characterized examples are Escherichia coli DHAD, which contains an oxygen-labile [Fe₄S₄] cluster, and spinach DHAD, which contains an oxygen-resistant [Fe₂S₂] cluster. Although the Fe-S cluster is crucial for DHAD function, little is known about the cluster-coordination environment or the mechanism of catalysis and cluster biogenesis. Here, using the combination of UV-visible absorption and circular dichroism and resonance Raman and electron paramagnetic resonance, we spectroscopically characterized the Fe-S center in DHAD from Arabidopsis thaliana (At). Our results indicated that AtDHAD can accommodate [Fe₂S₂] and [Fe₄S₄] clusters. However, only the [Fe₂S₂] cluster-bound form is catalytically active. We found that the [Fe₂S₂] cluster is coordinated by at least one non-cysteinyl ligand, which can be replaced by the thiol group(s) of dithiothreitol. In vitro cluster transfer and reconstitution reactions revealed that [Fe₂S₂] cluster-containing NFU2 protein is likely the physiological cluster donor for in vivo maturation of AtDHAD. In summary, AtDHAD binds either one [Fe₄S₄] or one [Fe₂S₂] cluster, with only the latter being catalytically competent and capable of substrate and product binding, and NFU2 appears to be the physiological [Fe₂S₂] cluster donor for DHAD maturation. This work represents the first in vitro characterization of recombinant AtDHAD, providing new insights into the properties, biogenesis, and catalytic role of the active-site Fe-S center in a plant DHAD.

Iron-sulfur clusters biogenesis by the SUF machinery: close to the molecular mechanism understanding

Iron–sulfur clusters (Fe–S) are amongst the most ancient and versatile inorganic cofactors in nature which are used by proteins for fundamental biological processes. Multiprotein machineries (NIF, ISC, SUF) exist for Fe–S cluster biogenesis which are mainly conserved from bacteria to human. SUF system (sufABCDSE operon) plays a general role in many bacteria under conditions of iron limitation or oxidative stress. In this mini-review, we will summarize the current understanding of the molecular mechanism of Fe–S biogenesis by SUF. The advances in our understanding of the molecular aspects of SUF originate from biochemical, biophysical and recent structural studies. Combined with recent in vivo experiments, the understanding of the Fe–S biogenesis mechanism considerably moved forward.

Mammalian Fe-S proteins: definition of a consensus motif recognized by the co-chaperone HSC20

Iron-sulfur (Fe-S) clusters are inorganic cofactors that are fundamental to several biological processes in all three kingdoms of life. In most organisms, Fe-S clusters are initially assembled on a scaffold protein, ISCU, and subsequently transferred to target proteins or to intermediate carriers by a dedicated chaperone/co-chaperone system. The delivery of assembled Fe-S clusters to recipient proteins is a crucial step in the biogenesis of Fe-S proteins, and, in mammals, it relies on the activity of a multiprotein transfer complex that contains the chaperone HSPA9, the co-chaperone HSC20 and the scaffold ISCU. How the transfer complex efficiently engages recipient Fe-S target proteins involves specific protein interactions that are not fully understood. This mini review focuses on recent insights into the molecular mechanism of amino acid motif recognition and discrimination by the co-chaperone HSC20, which guides Fe-S cluster delivery.

ISCA1 is essential for mitochondrial Fe4S4 biogenesis in vivo

Mammalian A-type proteins, ISCA1 and ISCA2, are evolutionarily conserved proteins involved in iron–sulfur cluster (Fe–S) biogenesis. Recently, it was shown that ISCA1 and ISCA2 form a heterocomplex that is implicated in the maturation of mitochondrial Fe₄S₄ proteins. Here we report that mouse ISCA1 and ISCA2 are Fe₂S₂-containing proteins that combine all features of Fe–S carrier proteins. We use biochemical, spectroscopic and in vivo approaches to demonstrate that despite forming a complex, ISCA1 and ISCA2 establish discrete interactions with components of the late Fe–S machinery. Surprisingly, knockdown experiments in mouse skeletal muscle and in primary cultures of neurons suggest that ISCA1, but not ISCA2, is required for mitochondrial Fe₄S₄ proteins biogenesis. Collectively, our data suggest that cellular processes with different requirements for ISCA1, ISCA2 and ISCA1–ISCA2 complex seem to exist.

The mitochondrial proteins ISCA1 and ISCA2 form a complex that is involved in the biogenesis of Fe–S clusters. Here the authors report that ISCA1 and ISCA2 interact differently with proteins of the Fe–S machinery and that under certain conditions, ISCA2 seems dispensable for Fe–S biogenesis.

Real-Time Kinetic Probes Support Monothiol Glutaredoxins As Intermediate Carriers in Fe-S Cluster Biosynthetic Pathways

Iron-sulfur (Fe-S) clusters are protein cofactors that are required for many essential cellular functions. Fe-S clusters are synthesized and inserted into target proteins by an elaborate biosynthetic process. The insensitivity of most Fe-S assembly and transfer assays requires high concentrations for components and places major limits on reaction complexity. Recently, fluorophore labels were shown to be effective at reporting cluster content for Fe-S proteins. Here, the incorporation of this labeling approach allowed the design and interrogation of complex Fe-S cluster biosynthetic reactions that mimic in vivo conditions. A bacterial Fe-S assembly complex, composed of the cysteine desulfurase IscS and scaffold protein IscU, was used to generate [2Fe-2S] clusters for transfer to mixtures of putative intermediate carrier and acceptor proteins. The focus of this study was to test whether the monothiol glutaredoxin, Grx4, functions as an obligate [2Fe-2S] carrier protein in the Fe-S cluster distribution network. Interestingly, [2Fe-2S] clusters generated by the IscS-IscU complex transferred to Grx4 at rates comparable to previous assays using uncomplexed IscU as a cluster source in chaperone-assisted transfer reactions. Further, we provide evidence that [2Fe-2S]-Grx4 delivers clusters to multiple classes of Fe-S targets via direct ligand exchange in a process that is both dynamic and reversible. Global fits of cluster transfer kinetics support a model in which Grx4 outcompetes terminal target proteins for IscU-bound [2Fe-2S] clusters and functions as an intermediate cluster carrier. Overall, these studies demonstrate the power of chemically conjugated fluorophore reporters for unraveling mechanistic details of biological metal cofactor assembly and distribution networks.

A Sinorhizobium meliloti RpoH-Regulated Gene Is Involved in Iron-Sulfur Protein Metabolism and Effective Plant Symbiosis under Intrinsic Iron Limitation

In Sinorhizobium meliloti, RpoH-type sigma factors have a global impact on gene expression during heat shock and play an essential role in symbiosis with leguminous plants. Using mutational analysis of a set of genes showing highly RpoH-dependent expression during heat shock, we identified a gene indispensable for effective symbiosis. This gene, designated sufT, was located downstream of the sufBCDS homologs that specify the iron-sulfur (Fe/S) cluster assembly pathway. The identified transcription start site was preceded by an RpoH-dependent promoter consensus sequence. SufT was related to a conserved protein family of unknown molecular function, of which some members are involved in Fe/S cluster metabolism in diverse organisms. A sufT mutation decreased bacterial growth in both rich and minimal media, tolerance to stresses such as iron starvation, and activities of some Fe/S cluster-dependent enzymes. These results support the involvement of SufT in SUF (sulfur mobilization) system-mediated Fe/S protein metabolism. Furthermore, we isolated spontaneous pseudorevertants of the sufT mutant with partially recovered growth; each of them had a mutation in rirA This gene encodes a global iron regulator whose loss increases the intracellular iron content. Deletion of rirA in the original sufT mutant improved growth and restored Fe/S enzyme activities and effective symbiosis. These results suggest that enhanced iron availability compensates for the lack of SufT in the maintenance of Fe/S proteins.

IMPORTANCE

Although RpoH-type sigma factors of the RNA polymerase are present in diverse proteobacteria, their role as global regulators of protein homeostasis has been studied mainly in the enteric gammaproteobacterium Escherichia coli In the soil alphaproteobacterium Sinorhizobium meliloti, the rpoH mutations have a strong impact on symbiosis with leguminous plants. We found that sufT is a unique member of the S. meliloti RpoH regulon; sufT contributes to Fe/S protein metabolism and effective symbiosis under intrinsic iron limitation exerted by RirA, a global iron regulator. Our study provides insights into the RpoH regulon function in diverse proteobacteria adapted to particular ecological niches and into the mechanism of conserved Fe/S protein biogenesis.

Synthesis, delivery and regulation of eukaryotic heme and Fe-S cluster cofactors

In humans, the bulk of iron in the body (over 75%) is directed towards heme- or Fe-S cluster cofactor synthesis, and the complex, highly regulated pathways in place to accomplish biosynthesis have evolved to safely assemble and load these cofactors into apoprotein partners. In eukaryotes, heme biosynthesis is both initiated and finalized within the mitochondria, while cellular Fe-S cluster assembly is controlled by correlated pathways both within the mitochondria and within the cytosol. Iron plays a vital role in a wide array of metabolic processes and defects in iron cofactor assembly leads to human diseases. This review describes progress towards our molecular-level understanding of cellular heme and Fe-S cluster biosynthesis, focusing on the regulation and mechanistic details that are essential for understanding human disorders related to the breakdown in these essential pathways.

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.

Novel features of the ISC machinery revealed by characterization of Escherichia coli mutants that survive without iron-sulfur clusters

Biological assembly of iron-sulfur (Fe-S) clusters is mediated by complex systems consisting of multiple proteins. Escherichia coli possesses two distinct systems called the ISC and SUF machineries encoded by iscSUA-hscBA-fdx-iscX and sufABCDSE respectively. Deletion of both pathways results in absence of the biosynthetic apparatus for Fe-S clusters, and consequent lethality, which has hampered detailed genetic studies. Here we report that modification of the isoprenoid biosynthetic pathway can offset the indispensability of the Fe-S cluster biosynthetic systems and show that the resulting Δisc Δsuf double mutants can grow without detectable Fe-S cluster-containing proteins. We also constructed a series of mutants in which each isc gene was disrupted in the deletion background of sufABCDSE. Phenotypic analysis of the mutants revealed that Fdx, an essential electron-transfer Fe-S protein in the ISC machinery, is dispensable under anaerobic conditions, which is similar to the situation with IscA. Furthermore, we found that several suppressor mutations in IscU, an Fe-S scaffold protein responsible for the de novo Fe-S cluster assembly, could bypass the essential role of the chaperone system HscA and HscB. These findings pave the way toward a detailed molecular analysis to understand the mechanisms involved in Fe-S cluster biosynthesis.

The deca-GX3 proteins Yae1-Lto1 function as adaptors recruiting the ABC protein Rli1 for iron-sulfur cluster insertion

Cytosolic and nuclear iron-sulfur (Fe-S) proteins are involved in many essential pathways including translation and DNA maintenance. Their maturation requires the cytosolic Fe-S protein assembly (CIA) machinery. To identify new CIA proteins we employed systematic protein interaction approaches and discovered the essential proteins Yae1 and Lto1 as binding partners of the CIA targeting complex. Depletion of Yae1 or Lto1 results in defective Fe-S maturation of the ribosome-associated ABC protein Rli1, but surprisingly no other tested targets. Yae1 and Lto1 facilitate Fe-S cluster assembly on Rli1 in a chain of binding events. Lto1 uses its conserved C-terminal tryptophan for binding the CIA targeting complex, the deca-GX₃ motifs in both Yae1 and Lto1 facilitate their complex formation, and Yae1 recruits Rli1. Human YAE1D1 and the cancer-related ORAOV1 can replace their yeast counterparts demonstrating evolutionary conservation. Collectively, the Yae1-Lto1 complex functions as a target-specific adaptor that recruits apo-Rli1 to the generic CIA machinery.

DOI:http://dx.doi.org/10.7554/eLife.08231.001

Many proteins depend on small molecules called cofactors to be able to perform their roles in cells. One class of proteins—the iron-sulfur proteins—contain cofactors that are made of clusters of iron and sulfide ions. In yeast, humans and other eukaryotes, the clusters are assembled and incorporated into their target proteins by a group of assembly factors called the CIA machinery.

Several components of the CIA machinery have previously been identified and most of them appear to be core components that are needed to assemble many different proteins in cells. Since these iron-sulfur proteins are involved in important processes such as the production of proteins and the maintenance of DNA, losing of any of these CIA proteins tends to be lethal to the organism.

Paul et al. used several ‘proteomic’ techniques to study the assembly of iron-sulfur proteins in yeast and identified two new proteins called Yae1 and Lto1 that are involved in this process. Unlike other CIA proteins, Yae1 and Lto1 are only required for the assembly of just one particular iron-sulfur protein called Rli1, which is essential for the production of proteins. Most newly made iron-sulfur proteins can bind directly to a group of CIA proteins called the CIA targeting complex, but Rli1 cannot. The experiments show that Lto1 binds to both the CIA targeting complex and to Yae1, which in turn recruits the Rli1 to the CIA complex.

Paul et al. also show that humans have proteins that are very similar to Yae1 and Lto1. Inserting the human counterparts of Yae1 and Lto1 into yeast lacking these proteins could fully restore the assembly of iron-sulfur clusters into Rli1. This suggests that Yae1 and Lto1 proteins evolved in the common ancestors of fungi and humans and have changed little since. Taken together, Paul et al.'s findings reveal that Yae1 and Lto1 act as adaptors that link the rest of the CIA machinery to their specific target protein Rli1 in yeast and humans. A future challenge is to find out the three-dimensional structures of Yae1 and Lto1 to better understand how these proteins work and interact.

DOI:http://dx.doi.org/10.7554/eLife.08231.002

Deletion of the Proposed Iron Chaperones IscA/SufA Results in Accumulation of a Red Intermediate Cysteine Desulfurase IscS in Escherichia coli

In Escherichia coli, sulfur in iron-sulfur clusters is primarily derived from L-cysteine via the cysteine desulfurase IscS. However, the iron donor for iron-sulfur cluster assembly remains elusive. Previous studies have shown that, among the iron-sulfur cluster assembly proteins in E. coli, IscA has a unique and strong iron-binding activity and that the iron-bound IscA can efficiently provide iron for iron-sulfur cluster assembly in proteins in vitro, indicating that IscA may act as an iron chaperone for iron-sulfur cluster biogenesis. Here we report that deletion of IscA and its paralog SufA in E. coli cells results in the accumulation of a red-colored cysteine desulfurase IscS under aerobic growth conditions. Depletion of intracellular iron using a membrane-permeable iron chelator, 2,2'-dipyridyl, also leads to the accumulation of red IscS in wild-type E. coli cells, suggesting that the deletion of IscA/SufA may be emulated by depletion of intracellular iron. Purified red IscS has an absorption peak at 528 nm in addition to the peak at 395 nm of pyridoxal 5'-phosphate. When red IscS is oxidized by hydrogen peroxide, the peak at 528 nm is shifted to 510 nm, which is similar to that of alanine-quinonoid intermediate in cysteine desulfurases. Indeed, red IscS can also be produced in vitro by incubating wild-type IscS with excess L-alanine and sulfide. The results led us to propose that deletion of IscA/SufA may disrupt the iron delivery for iron-sulfur cluster biogenesis, therefore impeding sulfur delivery by IscS, and result in the accumulation of red IscS in E. coli cells.

Fluorescent probes for tracking the transfer of iron-sulfur cluster and other metal cofactors in biosynthetic reaction pathways

Iron-sulfur (Fe-S) clusters are protein cofactors that are constructed and delivered to target proteins by elaborate biosynthetic machinery. Mechanistic insights into these processes have been limited by the lack of sensitive probes for tracking Fe-S cluster synthesis and transfer reactions. Here we present fusion protein- and intein-based fluorescent labeling strategies that can probe Fe-S cluster binding. The fluorescence is sensitive to different cluster types ([2Fe-2S] and [4Fe-4S] clusters), ligand environments ([2Fe-2S] clusters on Rieske, ferredoxin (Fdx), and glutaredoxin), and cluster oxidation states. The power of this approach is highlighted with an extreme example in which the kinetics of Fe-S cluster transfer reactions are monitored between two Fdx molecules that have identical Fe-S spectroscopic properties. This exchange reaction between labeled and unlabeled Fdx is catalyzed by dithiothreitol (DTT), a result that was confirmed by mass spectrometry. DTT likely functions in a ligand substitution reaction that generates a [2Fe-2S]-DTT species, which can transfer the cluster to either labeled or unlabeled Fdx. The ability to monitor this challenging cluster exchange reaction indicates that real-time Fe-S cluster incorporation can be tracked for a specific labeled protein in multicomponent assays that include several unlabeled Fe-S binding proteins or other chromophores. Such advanced kinetic experiments are required to untangle the intricate networks of transfer pathways and the factors affecting flux through branch points. High sensitivity and suitability with high-throughput methodology are additional benefits of this approach. We anticipate that this cluster detection methodology will transform the study of Fe-S cluster pathways and potentially other metal cofactor biosynthetic pathways.

Assembly of Fe/S proteins in bacterial systems: Biochemistry of the bacterial ISC system

Iron/sulfur clusters are key cofactors in proteins involved in a large number of conserved cellular processes, including gene expression, DNA replication and repair, ribosome biogenesis, tRNA modification, central metabolism and respiration. Fe/S proteins can perform a wide range of functions, from electron transfer to redox and non-redox catalysis. In all living organisms, Fe/S proteins are first synthesized in an apo-form. However, as the Fe/S prosthetic group is required for correct folding and/or protein stability, Fe/S clusters are inserted co-translationally or immediately after translation by specific assembly machineries. These systems have been extensively studied over the last decade, both in prokaryotes and eukaryotes. The present review covers the basic principles of the bacterial housekeeping Fe/S biogenesis ISC system, and related recent molecular advances. Some of the most exciting recent highlights relating to this system include structural and functional characterization of binary and ternary complexes involved in Fe/S cluster formation on the scaffold protein IscU. These advances enhance our understanding of the Fe/S cluster assembly mechanism by revealing essential interactions that could never be determined with isolated proteins and likely are closer to an in vivo situation. Much less is currently known about the molecular mechanism of the Fe/S transfer step, but a brief account of the protein-protein interactions involved is given. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.

Recent advances in the Suf Fe-S cluster biogenesis pathway: Beyond the Proteobacteria

Fe-S clusters play critical roles in cellular function throughout all three kingdoms of life. Consequently, Fe-S cluster biogenesis systems are present in most organisms. The Suf (sulfur formation) system is the most ancient of the three characterized Fe-S cluster biogenesis pathways, which also include the Isc and Nif systems. Much of the first work on the Suf system took place in Gram-negative Proteobacteria used as model organisms. These early studies led to a wealth of biochemical, genetic, and physiological information on Suf function. From those studies we have learned that SufB functions as an Fe-S scaffold in conjunction with SufC (and in some cases SufD). SufS and SufE together mobilize sulfur for cluster assembly and SufA traffics the complete Fe-S cluster from SufB to target apo-proteins. However, recent progress on the Suf system in other organisms has opened up new avenues of research and new hypotheses about Suf function. This review focuses primarily on the most recent discoveries about the Suf pathway and where those new models may lead the field. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.

Iron-sulfur cluster biogenesis in mammalian cells: New insights into the molecular mechanisms of cluster delivery

Iron-sulfur (Fe-S) clusters are ancient, ubiquitous cofactors composed of iron and inorganic sulfur. The combination of the chemical reactivity of iron and sulfur, together with many variations of cluster composition, oxidation states and protein environments, enables Fe-S clusters to participate in numerous biological processes. Fe-S clusters are essential to redox catalysis in nitrogen fixation, mitochondrial respiration and photosynthesis, to regulatory sensing in key metabolic pathways (i.e. cellular iron homeostasis and oxidative stress response), and to the replication and maintenance of the nuclear genome. Fe-S cluster biogenesis is a multistep process that involves a complex sequence of catalyzed protein-protein interactions and coupled conformational changes between the components of several dedicated multimeric complexes. Intensive studies of the assembly process have clarified key points in the biogenesis of Fe-S proteins. However several critical questions still remain, such as: what is the role of frataxin? Why do some defects of Fe-S cluster biogenesis cause mitochondrial iron overload? How are specific Fe-S recipient proteins recognized in the process of Fe-S transfer? This review focuses on the basic steps of Fe-S cluster biogenesis, drawing attention to recent advances achieved on the identification of molecular features that guide selection of specific subsets of nascent Fe-S recipients by the cochaperone HSC20. Additionally, it outlines the distinctive phenotypes of human diseases due to mutations in the components of the basic pathway. 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.

Copper binding in IscA inhibits iron-sulphur cluster assembly in Escherichia coli

Among the iron-sulphur cluster assembly proteins encoded by gene cluster iscSUA-hscBA-fdx in Escherichia coli, IscA has a unique and strong iron binding activity and can provide iron for iron-sulphur cluster assembly in proteins in vitro. Deletion of IscA and its paralogue SufA results in an E. coli mutant that fails to assemble [4Fe-4S] clusters in proteins under aerobic conditions, suggesting that IscA has a crucial role for iron-sulphur cluster biogenesis. Here we report that among the iron-sulphur cluster assembly proteins, IscA also has a strong and specific binding activity for Cu(I) in vivo and in vitro. The Cu(I) centre in IscA is stable and resistant to oxidation under aerobic conditions. Mutation of the conserved cysteine residues that are essential for the iron binding in IscA abolishes the copper binding activity, indicating that copper and iron may share the same binding site in the protein. Additional studies reveal that copper can compete with iron for the metal binding site in IscA and effectively inhibits the IscA-mediated [4Fe-4S] cluster assembly in E. coli cells. The results suggest that copper may not only attack the [4Fe-4S] clusters in dehydratases, but also block the [4Fe-4S] cluster assembly in proteins by targeting IscA in cells.

Reprint of: Iron/sulfur proteins biogenesis in prokaryotes: formation, regulation and diversity

Iron/sulfur centers are key cofactors of proteins intervening in multiple conserved cellular processes, such as gene expression, DNA repair, RNA modification, central metabolism and respiration. Mechanisms allowing Fe/S centers to be assembled, and inserted into polypeptides have attracted much attention in the last decade, both in eukaryotes and prokaryotes. Basic principles and recent advances in our understanding of the prokaryotic Fe/S biogenesis ISC and SUF systems are reviewed in the present communication. Most studies covered stem from investigations in Escherichia coli and Azotobacter vinelandii. Remarkable insights were brought about by complementary structural, spectroscopic, biochemical and genetic studies. Highlights of the recent years include scaffold mediated assembly of Fe/S cluster, A-type carriers mediated delivery of clusters and regulatory control of Fe/S homeostasis via a set of interconnected genetic regulatory circuits. Also, the importance of Fe/S biosynthesis systems in mediating soft metal toxicity was documented. A brief account of the Fe/S biosynthesis systems diversity as present in current databases is given here. Moreover, Fe/S biosynthesis factors have themselves been the object of molecular tailoring during evolution and some examples are discussed here. An effort was made to provide, based on the E. coli system, a general classification associating a given domain with a given function such as to help next search and annotation of genomes. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

Iron binding activity is essential for the function of IscA in iron-sulphur cluster biogenesis

Iron-sulphur cluster biogenesis requires coordinated delivery of iron and sulphur to scaffold proteins, followed by transfer of the assembled clusters from scaffold proteins to target proteins. This complex process is accomplished by a group of dedicated iron-sulphur cluster assembly proteins that are conserved from bacteria to humans. While sulphur in iron-sulphur clusters is provided by L-cysteine via cysteine desulfurase, the iron donor(s) for iron-sulphur cluster assembly remains largely elusive. Here we report that among the primary iron-sulphur cluster assembly proteins, IscA has a unique and strong binding activity for mononuclear iron in vitro and in vivo. Furthermore, the ferric iron centre tightly bound in IscA can be readily extruded by l-cysteine, followed by reduction to ferrous iron for iron-sulphur cluster biogenesis. Substitution of the highly conserved residue tyrosine 40 with phenylalanine (Y40F) in IscA results in a mutant protein that has a diminished iron binding affinity but retains the iron-sulphur cluster binding activity. Genetic complementation studies show that the IscA Y40F mutant is inactive in vivo, suggesting that the iron binding activity is essential for the function of IscA in iron-sulphur cluster biogenesis.

Iron/sulfur proteins biogenesis in prokaryotes: formation, regulation and diversity

Iron/sulfur centers are key cofactors of proteins intervening in multiple conserved cellular processes, such as gene expression, DNA repair, RNA modification, central metabolism and respiration. Mechanisms allowing Fe/S centers to be assembled, and inserted into polypeptides have attracted much attention in the last decade, both in eukaryotes and prokaryotes. Basic principles and recent advances in our understanding of the prokaryotic Fe/S biogenesis ISC and SUF systems are reviewed in the present communication. Most studies covered stem from investigations in Escherichia coli and Azotobacter vinelandii. Remarkable insights were brought about by complementary structural, spectroscopic, biochemical and genetic studies. Highlights of the recent years include scaffold mediated assembly of Fe/S cluster, A-type carriers mediated delivery of clusters and regulatory control of Fe/S homeostasis via a set of interconnected genetic regulatory circuits. Also, the importance of Fe/S biosynthesis systems in mediating soft metal toxicity was documented. A brief account of the Fe/S biosynthesis systems diversity as present in current databases is given here. Moreover, Fe/S biosynthesis factors have themselves been the object of molecular tailoring during evolution and some examples are discussed here. An effort was made to provide, based on the E. coli system, a general classification associating a given domain with a given function such as to help next search and annotation of genomes. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

Monothiol glutaredoxins and A-type proteins: partners in Fe-S cluster trafficking

Monothiol glutaredoxins (Grxs) are proposed to function in Fe-S cluster storage and delivery, based on their ability to exist as apo monomeric forms and dimeric forms containing a subunit-bridging [Fe(2)S(2)](2+) cluster, and to accept [Fe(2)S(2)](2+) clusters from primary scaffold proteins. In addition yeast cytosolic monothiol Grxs interact with Fra2 (Fe repressor of activation-2), to form a heterodimeric complex with a bound [Fe(2)S(2)](2+) cluster that plays a key role in iron sensing and regulation of iron homeostasis. In this work, we report on in vitro UV-visible CD studies of cluster transfer between homodimeric monothiol Grxs and members of the ubiquitous A-type class of Fe-S cluster carrier proteins ((Nif)IscA and SufA). The results reveal rapid, unidirectional, intact and quantitative cluster transfer from the [Fe(2)S(2)](2+) cluster-bound forms of A. thaliana GrxS14, S. cerevisiae Grx3, and A. vinelandii Grx-nif homodimers to A. vinelandii(Nif)IscA and from A. thaliana GrxS14 to A. thaliana SufA1. Coupled with in vivo evidence for interaction between monothiol Grxs and A-type Fe-S cluster carrier proteins, the results indicate that these two classes of proteins work together in cellular Fe-S cluster trafficking. However, cluster transfer is reversed in the presence of Fra2, since the [Fe(2)S(2)](2+) cluster-bound heterodimeric Grx3-Fra2 complex can be formed by intact [Fe(2)S(2)](2+) cluster transfer from (Nif)IscA. The significance of these results for Fe-S cluster biogenesis or repair and the cellular regulation of the Fe-S cluster status are discussed.

The iron-sulfur cluster assembly machineries in plants: current knowledge and open questions

Many metabolic pathways and cellular processes occurring in most sub-cellular compartments depend on the functioning of iron-sulfur (Fe-S) proteins, whose cofactors are assembled through dedicated protein machineries. Recent advances have been made in the knowledge of the functions of individual components through a combination of genetic, biochemical and structural approaches, primarily in prokaryotes and non-plant eukaryotes. Whereas most of the components of these machineries are conserved between kingdoms, their complexity is likely increased in plants owing to the presence of additional assembly proteins and to the existence of expanded families for several assembly proteins. This review focuses on the new actors discovered in the past few years, such as glutaredoxin, BOLA and NEET proteins as well as MIP18, MMS19, TAH18, DRE2 for the cytosolic machinery, which are integrated into a model for the plant Fe-S cluster biogenesis systems. It also discusses a few issues currently subjected to an intense debate such as the role of the mitochondrial frataxin and of glutaredoxins, the functional separation between scaffold, carrier and iron-delivery proteins and the crosstalk existing between different organelles.

Iron-sulphur clusters, their biosynthesis, and biological functions in protozoan parasites

Fe-S clusters are ensembles of sulphide-linked di-, tri-, and tetra-iron centres of a variety of metalloproteins that play important roles in reduction and oxidation of mitochondrial electron transport, energy metabolism, regulation of gene expression, cell survival, nitrogen fixation, and numerous other metabolic pathways. The Fe-S clusters are assembled by one of four distinct systems: NIF, SUF, ISC, and CIA machineries. The ISC machinery is a house-keeping system conserved widely from prokaryotes to higher eukaryotes, while the other systems are present in a limited range of organisms and play supplementary roles under certain conditions such as stress. Fe-S cluster-containing proteins and the components required for Fe-S cluster biosynthesis are modulated under stress conditions, drug resistance, and developmental stages. It is also known that a defect in Fe-S proteins and Fe-S cluster biogenesis leads to many genetic disorders in humans, which indicates the importance of the systems. In this review, we describe the biological and physiological significance of Fe-S cluster-containing proteins and their biosynthesis in parasitic protozoa including Plasmodium, Trypanosoma, Leishmania, Giardia, Trichomonas, Entamoeba, Cryptosporidium, Blastocystis, and microsporidia. We also discuss the roles of Fe-S cluster biosynthesis in proliferation, differentiation, and stress response in protozoan parasites. The heterogeneity of the systems and the compartmentalization of Fe-S cluster biogenesis in the protozoan parasites likely reflect divergent evolution under highly diverse environmental niches, and influence their parasitic lifestyle and pathogenesis. Finally, both Fe-S cluster-containing proteins and their biosynthetic machinery in protozoan parasites are remarkably different from those in their mammalian hosts. Thus, they represent a rational target for the development of novel chemotherapeutic and prophylactic agents against protozoan infections.