Role of protein-glutathione contacts in defining glutaredoxin-3 [2Fe–2S] cluster chirality, ligand exchange and transfer chemistry

Cysteine import via the high-affinity GSH transporter Hgt1 rescues GSH auxotrophy in yeast

Glutathione (GSH) is an abundant thiol-containing tripeptide that functions in redox homeostasis, protein folding, and iron (Fe) metabolism. In Saccharomyces cerevisiae, GSH depletion leads to increased sensitivity to oxidants and other toxic compounds, disruption of iron-sulfur (Fe-S) cluster biogenesis, and eventually cell death. GSH pools are supplied by intracellular biosynthesis and GSH import from the extracellular environment. Consequently, in GSH-depleted growth media, deletion of the gene encoding the first enzyme in the GSH biosynthetic pathway (GSH1) is lethal in yeast. At the other extreme, GSH overaccumulation via overexpression of the high-affinity GSH transporter Hgt1 is also toxic to cells, leading to reductive stress. Here, we engineered a yeast strain that combines gsh1 deletion with HGT1 overexpression to study the cellular effects of oscillating between GSH-deplete and -replete conditions. Surprisingly, we find that constitutive expression of HGT1 in gsh1Δ cells rescues the GSH auxotrophy of this strain. We also show that addition of cysteine or cysteine derivatives to the growth media is required for this rescue. GSH limitation in yeast causes intracellular Fe overload because of disruption of an Fe-S cluster-dependent pathway that regulates the activity of the low Fe-sensing transcription factors Aft1 and Aft2. Analysis of Fe regulation and other Fe-S cluster-dependent pathways reveals that HGT1 overexpression partially alleviates the Fe starvation-like response of gsh1Δ cells. Taken together, these results suggest that HGT1 overexpression facilitates import of cysteine or cysteine derivatives that allow limited Fe-S cluster biogenesis to sustain cell growth in the absence of GSH.

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.

Regulations of mitoNEET by the key redox homeostasis molecule glutathione

Human mitoNEET (mNT) and CISD2 are two NEET proteins characterized by an atypical [2Fe-2S] cluster coordination involving three cysteines and one histidine. They act as redox switches with an active state linked to the oxidation of their cluster. In the present study, we show that reduced glutathione but also free thiol-containing molecules such as β-mercaptoethanol can induce a loss of the mNT cluster under aerobic conditions, while CISD2 cluster appears more resistant. This disassembly occurs through a radical-based mechanism as previously observed with the bacterial SoxR. Interestingly, adding cysteine prevents glutathione-induced cluster loss. At low pH, glutathione can bind mNT in the vicinity of the cluster. These results suggest a potential new regulation mechanism of mNT activity by glutathione, an essential actor of the intracellular redox state.

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

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

Biochemical impact of a disease-causing Ile67Asn substitution on BOLA3 protein

Iron-sulfur (Fe-S) cluster biosynthesis involves the action of a variety of functionally distinct proteins, most of which are evolutionarily conserved. Mutations in these Fe-S scaffold and trafficking proteins can cause diseases such as multiple mitochondrial dysfunctions syndrome (MMDS), sideroblastic anemia, and mitochondrial encephalopathy. Herein, we investigate the effect of Ile67Asn substitution in the BOLA3 protein that results in the MMDS2 phenotype. Although the exact functional role of BOLA3 in Fe-S cluster biosynthesis is not known, the [2Fe-2S]-bridged complex of BOLA3 with GLRX5, another Fe-S protein, has been proposed as a viable intermediary cluster carrier to downstream targets. Our investigations reveal that the Ile67Asn substitution impairs the ability of BOLA3 to bind its physiological partner GLRX5, resulting in a failure to form the [2Fe-2S]-bridged complex. Although no drastic structural change in BOLA3 arises from the substitution, as evidenced by wild-type and mutant BOLA3 1H-15N HSQC and ion mobility native mass spectrometry experiments, this substitution appears to influence cluster reconstitution on downstream proteins leading to the disease phenotype. By contrast, substituted derivatives of the holo homodimeric form of BOLA3 are formed and remain active toward cluster exchange.

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

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

Cluster exchange reactivity of [2Fe-2S]-bridged heterodimeric BOLA1-GLRX5

Mitochondrial BOLA1 is known to form a [2Fe-2S] cluster-bridged heterodimeric complex with mitochondrial monothiol glutaredoxin GLRX5; however, the function of this heterodimeric complex is unclear. Some reports suggest redundant roles for BOLA1 and a related protein, BOLA3, with both involved in the maturation of [4Fe-4S] clusters in a subset of mitochondrial proteins. However, a later report on the structure of BOLA1-GLRX5 heterodimeric complex demonstrated a buried cluster environment and predicted a redox role instead of the cluster trafficking role suggested for the BOLA3-GLRX5 heterodimeric complex. Herein, we describe a detailed kinetic study of relative cluster exchange reactivity involving heterodimeric complex of BOLA1 with GLRX5. By the use of CD spectroscopy, it is demonstrated that [2Fe-2S]-bridged BOLA1-GLRX5 can be readily formed by cluster uptake from donors such as ISCU or [2Fe-2S](GS)₄ complex, but not from ISCA1 or ISCA2. Rapid holo-formation following delivery from [2Fe-2S](GS)₄ supports possible physiological relevance in the cellular labile iron pool. Holo [2Fe-2S] BOLA1-GLRX5 heterodimeric complex is incapable of donating cluster to apo protein acceptors, providing experimental support for a nontrafficking role. Finally, we report the formation and reactivity of the holo [2Fe-2S]-bridged BOLA1 homodimer (lacking a partner GLRX). While the holo-heterodimer is thermodynamically more stable, by contrast the holo BOLA1 homodimer does demonstrate facile cluster exchange reactivity.

Kendomycin Cytotoxicity against Bacterial, Fungal, and Mammalian Cells Is Due to Cation Chelation

Kendomycin is a small-molecule natural product that has gained significant attention due to reported cytotoxicity against pathogenic bacteria and fungi as well as a number of cancer cell lines. Despite significant biomedical interest and attempts to reveal its mechanism of action, the cellular target of kendomycin remains disputed. Herein it is shown that kendomycin induces cellular responses indicative of cation stress comparable to the effects of established iron chelators. Furthermore, addition of excess iron and copper attenuated kendomycin cytotoxicity in bacteria, yeast, and mammalian cells. Finally, NMR analysis demonstrated a direct interaction with cations, corroborating a close link between the observed kendomycin polypharmacology across different species and modulation of iron and/or copper levels.

Characterization of [2Fe-2S]-Cluster-Bridged Protein Complexes and Reaction Intermediates by use of Native Mass Spectrometric Methods

Many iron-sulfur proteins involved in cluster trafficking form [2Fe-2S]-cluster-bridged complexes that are often challenging to characterize because of the inherent instability of the cluster at the interface. Herein, we illustrate the use of fast, online buffer exchange coupled to a native mass spectrometry (OBE nMS) method to characterize [2Fe-2S]-cluster-bridged proteins and their transient cluster-transfer intermediates. The use of this mechanistic and protein-characterization tool is demonstrated with holo glutaredoxin 5 (GLRX5) homodimer and holo GLRX5:BolA-like protein 3 (BOLA3) heterodimer. Using the OBE nMS method, cluster-transfer reactions between the holo-dimers and apo-ferredoxin (FDX2) are monitored, and intermediate [2Fe-2S] species, such as (FDX2:GLRX5:[2Fe-2S]:GSH) and (FDX2:BOLA3:GLRX5:[2Fe-2S]:GSH) are detected. The OBE nMS method is a robust technique for characterizing iron-sulfur-cluster-bridged protein complexes and transient iron-sulfur-cluster transfer intermediates.

Reconstitution, characterization, and [2Fe-2S] cluster exchange reactivity of a holo human BOLA3 homodimer

A new class of mitochondrial disease has been identified and characterized as Multiple Mitochondrial Dysfunctions Syndrome (MMDS). Four different forms of the disease have each been attributed to point mutations in proteins involved in iron-sulfur (Fe-S) biosynthesis; in particular, MMDS2 has been associated with the protein BOLA3. To date, this protein has been characterized in vitro concerning its ability to form heterodimeric complexes with two putative Fe-S cluster-binding partners: GLRX5 and NFU. However, BOLA3 has yet to be characterized in its own discrete holo form. Herein we describe procedures to isolate and characterize the human holo BOLA3 protein in terms of Fe-S cluster binding and trafficking and demonstrate that human BOLA3 can form a functional homodimer capable of engaging in Fe-S cluster transfer.

Cluster exchange reactivity of [2Fe-2S] cluster-bridged complexes of BOLA3 with monothiol glutaredoxins

The [2Fe-2S] cluster-bridged complex of BOLA3 with GLRX5 has been implicated in cluster trafficking, but cluster exchange involving this heterocomplex has not been reported. Herein we describe an investigation of the cluster exchange reactivity of holo BOLA3-GLRX complexes using two different monothiol glutaredoxins, H.s. GLRX5 and S.c. Grx3, which share significant identity. We observe that a 1 : 1 mixture of apo BOLA3 and glutaredoxin protein is able to accept a cluster from donors such as ISCU and a [2Fe-2S](GS)4 complex, with preferential formation of the cluster-bridged heterodimer over the plausible holo homodimeric glutaredoxin. Holo BOLA3-GLRX5 transfers clusters to apo acceptors at rates comparable to other Fe-S cluster trafficking proteins. Isothermal titration calorimetry experiments with apo proteins demonstrated a strong binding of BOLA3 with both GLRX5 and Grx3, while binding with an alternative mitochondrial partner, NFU1, was weak. Cluster exchange and calorimetry experiments resulted in a very similar behavior for yeast Grx3 (cytosolic) and human GLRX5 (mitochondrial), indicating conservation across the monothiol glutaredoxin family for interactions with BOLA3 and supporting a functional role for the BOLA3-GLRX5 heterocomplex relative to the previously proposed BOLA3-NFU1 interaction. The results also demonstrate rapid formation of the heterocomplexed holo cluster via delivery from a glutathione-complexed cluster, again indicative of the physiological relevance of the [2Fe-2S](GS)4 complex in the cellular labile iron pool.

Iron-sulphur cluster biogenesis via the SUF pathway

Iron-sulphur (Fe-S) clusters are versatile cofactors, which are essential for key metabolic processes in cells, such as respiration and photosynthesis, and which may have also played a crucial role in establishing life on Earth. They can be found in almost all living organisms, from unicellular prokaryotes and archaea to multicellular animals and plants, and exist in diverse forms. This review focuses on the most ancient Fe-S cluster assembly system, the sulphur utilization factor (SUF) mechanism, which is crucial in bacteria for cell survival under stress conditions such as oxidation and iron starvation, and which is also present in the chloroplasts of green microalgae and plants, where it is responsible for plastidial Fe-S protein maturation. We explain the SUF Fe-S cluster assembly process, the proteins involved, their regulation and provide evolutionary insights. We specifically focus on examples from Fe-S cluster synthesis in the model organisms Escherichia coli and Arabidopsis thaliana and discuss in an in vivo context the assembly of the [FeFe]-hydrogenase H-cluster from Chlamydomonas reinhardtii.

Investigation of glutathione-derived electrostatic and hydrogen-bonding interactions and their role in defining Grx5 [2Fe-2S] cluster optical spectra and transfer chemistry

Human glutaredoxin 5 (Grx5) is one of the core components of the Isc (iron-sulfur cluster) assembly and trafficking machinery, and serves as an intermediary cluster carrier, putatively delivering cluster from the Isu scaffold protein to target proteins. The tripeptide glutathione is intimately involved in this role, providing cysteinyl coordination to the iron center of the Grx5-bound [2Fe-2S] cluster. Grx5 has a well-defined glutathione-binding pocket with protein amino acid residues providing many ionic and hydrogen binding contacts to the bound glutathione. In this report, we investigated the importance of these interactions in cluster chirality and exchange reactivity by systematically perturbing the crucial contacts by use of natural and non-natural amino acid substitutions to disrupt the binding contacts from both the protein and glutathione. Native Grx5 could be reconstituted with all of the glutathione analogs used, as well as other thiol ligands, such as DTT or L-cysteine, by in vitro chemical reconstitution, and the holo proteins were found to transfer [2Fe-2S] cluster to apo ferredoxin 1 at comparable rates. However, the circular dichroism spectra of these derivatives displayed prominent differences that reflect perturbations in local cluster chirality. These studies provided a detailed molecular understanding of glutathione-protein interactions in holo Grx5 that define both cluster spectroscopy and exchange chemistry.