Monothiol glutaredoxin-BolA interactions: redox control of Arabidopsis thaliana BolA2 and SufE1

Iron‑sulfur cluster synthesis in plastids by the SUF system: A mechanistic and structural perspective

About 50 proteins expressed in plastids of photosynthetic eukaryotes ligate iron‑sulfur (Fe-S) clusters and ensure vital functions in photosynthesis, sulfur and nitrogen assimilation, but also in the synthesis of pigments, vitamins and hormones. The synthesis of these Fe-S clusters, which are co- or post-translationally incorporated into these proteins, relies on several proteins belonging to the so-called sulfur mobilization (SUF) machinery. An Fe-S cluster is first de novo synthesized on a scaffold protein complex before additional late-acting maturation factors act in the specific transfer, possible conversion and insertion of this cluster into target recipient proteins. In this review, we will summarize what is known about the molecular mechanisms responsible for both the synthesis and transfer steps, focusing in particular on the structural aspects that allow the formation of the required protein complexes.

The cytosolic form of dual localized BolA family protein Bol3 is important for adaptation to iron starvation in Aspergillus fumigatus

Aspergillus fumigatus is the predominant mould pathogen for humans. Adaption to host-imposed iron limitation has previously been demonstrated to be essential for its virulence. [2Fe-2S] clusters are crucial as cofactors of several metabolic pathways and mediate cytosolic/nuclear iron sensing in fungi including A. fumigatus. [2Fe-2S] cluster trafficking has been shown to involve BolA family proteins in both mitochondria and the cytosol/nucleus. Interestingly, both A. fumigatus homologues, termed Bol1 and Bol3, possess mitochondrial targeting sequences, suggesting the lack of cytosolic/nuclear versions. Here, we show by the combination of mutational, proteomic and fluorescence microscopic analyses that expression of the Bol3 encoding gene leads to dual localization of gene products to mitochondria and the cytosol/nucleus via alternative translation initiation downstream of the mitochondrial targeting sequence, which appears to be highly conserved in various Aspergillus species. Lack of either mitochondrial Bol1 or Bol3 was phenotypically inconspicuous while lack of cytosolic/nuclear Bol3 impaired growth during iron limitation but not iron sensing which indicates a particular importance of [2Fe-2S] cluster trafficking during iron limitation. Remarkably, cytosolic/nuclear Bol3 differs from the mitochondrial version only by N-terminal acetylation, a finding that was only possible by mutational hypothesis testing.

D27-like carotenoid isomerases: at the crossroads of strigolactone and abscisic acid biosynthesis

Strigolactones and abscisic acid (ABA) are apocarotenoid-derived plant hormones. Their biosynthesis starts with the conversion of trans-carotenes into cis forms, which serve as direct precursors. Iron-containing DWARF27 isomerases were shown to catalyse or contribute to the trans/cis conversions of these precursor molecules. D27 converts trans-β-carotene into 9-cis-β-carotene, which is the first committed step in strigolactone biosynthesis. Recent studies found that its paralogue, D27-LIKE1, also catalyses this conversion. A crucial step in ABA biosynthesis is the oxidative cleavage of 9-cis-violaxanthin and/or 9-cis-neoxanthin, which are formed from their trans isomers by unknown isomerases. Several lines of evidence point out that D27-like proteins directly or indirectly contribute to 9-cis-violaxanthin conversion, and eventually ABA biosynthesis. Apparently, the diversity of D27-like enzymatic activity is essential for the optimization of cis/trans ratios, and hence act to maintain apocarotenoid precursor pools. In this review, we discuss the functional divergence and redundancy of D27 paralogues and their potential direct contribution to ABA precursor biosynthesis. We provide updates on their gene expression regulation and alleged Fe-S cluster binding feature. Finally, we conclude that the functional divergence of these paralogues is not fully understood and we provide an outlook on potential directions in research.

Cryptochrome PtCPF1 regulates high temperature acclimation of marine diatoms through coordination of iron and phosphorus uptake

Increasing ocean temperatures threaten the productivity and species composition of marine diatoms. High temperature response and regulation are important for the acclimation of marine diatoms to such environments. However, the molecular mechanisms behind their acclimation to high temperature are still largely unknown. In this study, the abundance of PtCPF1 homologs (a member of the cryptochrome-photolyase family in the model diatom Phaeodactylum tricornutum) transcripts in marine phytoplankton is shown to increase with rising temperature based on Tara Oceans datasets. Moreover, the expression of PtCPF1 in P. tricornutum at high temperature (26 °C) was much higher than that at optimum temperature (20 °C). Deletion of PtCPF1 in P. tricornutum disrupted the expression of genes encoding two phytotransferrins (ISIP2A and ISIP1) and two Na+/P co-transporters (PHATRDRAFT_47667 and PHATRDRAFT_40433) at 26 °C. This further impacted the uptake of Fe and P, and eventually caused the arrest of cell division. Gene expression, Fe and P uptake, and cell division were restored by rescue with the native PtCPF1 gene. Furthermore, PtCPF1 interacts with two putative transcription factors (BolA and TF IIA) that potentially regulate the expression of genes encoding phytotransferrins and Na+/P co-transporters. To the best of our knowledge, this is the first study to reveal PtCPF1 as an essential regulator in the acclimation of marine diatoms to high temperature through the coordination of Fe and P uptake. Therefore, these findings help elucidate how marine diatoms acclimate to high temperature.

Regulation of 2,4-diacetylphloroglucinol biosynthesis and biocontrol capacity by the BolA family protein IbaG in Pseudomonas fluorescens 2P24

The monothiol glutaredoxin GrxD plays an essential role in the biosynthesis of the antibiotic 2,4-diacetylphloroglucinol (2,4-DAPG) and the biocontrol capacity of the soil bacterium Pseudomonas fluorescens 2P24. However, the detailed mechanism underlying GrxD-mediated activation of the production of 2,4-DAPG remains unclear. Here, we found that GrxD directly interacted with IbaG, a BolA protein family member. The mutation of ibaG significantly decreased 2,4-DAPG production. Furthermore, expressing ibaG restored the production of 2,4-DAPG in the grxD ibaG double mutant to wild-type levels in the presence of dithiothreitol, suggesting that IbaG was required for GrxD-mediated regulation of 2,4-DAPG production. Transcriptome sequencing analyses revealed that IbaG plays a global role in gene regulation by affecting the expression of numerous genes throughout the genome. We also demonstrated that IbaG is an important regulator of several cellular processes, including swarming motility, biofilm formation, siderophore production, and acid resistance. Altogether, our data suggest that IbaG has an essential role in 2,4-DAPG production, motility, and biofilm formation. We also propose a regulatory mechanism linking GrxD to 2,4-DAPG production via IbaG. IMPORTANCE The production of 2,4-diacetylphloroglucinol (2,4-DAPG) is positively influenced by the monothiol glutaredoxin GrxD in Pseudomonas fluorescens 2P24. However, the regulatory mechanism underlying GrxD-mediated regulation of 2,4-DAPG biosynthesis is mostly uncharacterized. Here, we show the function of the BolA-like protein IbaG in 2,4-DAPG biosynthesis. We also demonstrate that GrxD directly interacts with IbaG and influences the redox state of IbaG. Altogether, this work provides new insights into the role of the highly conserved IbaG protein in regulating 2,4-DAPG synthesis, biofilm formation, and other biocontrol traits of P. fluorescens.

Breakdown of Arabidopsis thaliana thioredoxins and glutaredoxins based on electrostatic similarity-Leads to common and unique interaction partners and functions

The reversible reduction and oxidation of protein thiols was first described as mechanism to control light/dark-dependent metabolic regulation in photosynthetic organisms. Today, it is recognized as an essential mechanism of regulation and signal transduction in all kingdoms of life. Proteins of the thioredoxin (Trx) family, Trxs and glutaredoxins (Grxs) in particular, catalyze thiol-disulfide exchange reactions and are vital players in the operation of thiol switches. Various Trx and Grx isoforms are present in all compartments of the cell. These proteins have a rather broad but at the same time distinct substrate specificity. Understanding the molecular basis of their target specificity is central to the understanding of physiological and pathological redox signaling. Electrostatic complementarity of the redoxins with their target proteins has been proposed as a major reason. Here, we analyzed the electrostatic similarity of all Arabidopsis thaliana Trxs, Grxs, and proteins containing such domains. Clustering of the redoxins based on this comparison suggests overlapping and also distant target specificities and thus functions of the different sub-classes including all Trx isoforms as well as the three classes of Grxs, i.e. CxxC-, CGFS-, and CC-type Grxs. Our analysis also provides a rationale for the tuned substrate specificities of both the ferredoxin- and NADPH-dependent Trx reductases.

Assembly, transfer, and fate of mitochondrial iron-sulfur clusters

Since the discovery of an autonomous iron-sulfur cluster (Fe-S) assembly machinery in mitochondria, significant efforts to examine the nature of this process have been made. The assembly of Fe-S clusters occurs in two distinct steps with the initial synthesis of [2Fe-2S] clusters by a first machinery followed by a subsequent assembly into [4Fe-4S] clusters by a second machinery. Despite this knowledge, we still have only a rudimentary understanding of how Fe-S clusters are transferred and distributed among their respective apoproteins. In particular, demand created by continuous protein turnover and the sacrificial destruction of clusters for synthesis of biotin and lipoic acid reveal possible bottlenecks in the supply chain of Fe-S clusters. Taking available information from other species into consideration, this review explores the mitochondrial assembly machinery of Arabidopsis and provides current knowledge about the respective transfer steps to apoproteins. Furthermore, this review highlights biotin synthase and lipoyl synthase, which both utilize Fe-S clusters as a sulfur source. After extraction of sulfur atoms from these clusters, the remains of the clusters probably fall apart, releasing sulfide as a highly toxic by-product. Immediate refixation through local cysteine biosynthesis is therefore an essential salvage pathway and emphasizes the physiological need for cysteine biosynthesis in plant mitochondria.

Diversity and roles of cysteine desulfurases in photosynthetic organisms

As sulfur is part of many essential protein cofactors such as iron-sulfur clusters, molybdenum cofactors, or lipoic acid, its mobilization from cysteine represents a fundamental process. The abstraction of the sulfur atom from cysteine is catalysed by highly conserved pyridoxal 5'-phosphate-dependent enzymes called cysteine desulfurases. The desulfuration of cysteine leads to the formation of a persulfide group on a conserved catalytic cysteine and the concomitant release of alanine. Sulfur is then transferred from cysteine desulfurases to different targets. Numerous studies have focused on cysteine desulfurases as sulfur-extracting enzymes for iron-sulfur cluster synthesis in mitochondria and chloroplasts but also for molybdenum cofactor sulfuration in the cytosol. Despite this, knowledge about the involvement of cysteine desulfurases in other pathways is quite rudimentary, particularly in photosynthetic organisms. In this review, we summarize current understanding of the different groups of cysteine desulfurases and their characteristics in terms of primary sequence, protein domain architecture, and subcellular localization. In addition, we review the roles of cysteine desulfurases in different fundamental pathways and highlight the gaps in our knowledge to encourage future work on unresolved issues especially in photosynthetic organisms.

New Perspectives on BolA: A Still Mysterious Protein Connecting Morphogenesis, Biofilm Production, Virulence, Iron Metabolism, and Stress Survival

The BolA-like protein family is widespread among prokaryotes and eukaryotes. BolA was originally described in E. coli as a gene induced in the stationary phase and in stress conditions. The BolA overexpression makes cells spherical. It was characterized as a transcription factor modulating cellular processes such as cell permeability, biofilm production, motility, and flagella assembly. BolA is important in the switch between motile and sedentary lifestyles having connections with the signaling molecule c-di-GMP. BolA was considered a virulence factor in pathogens such as Salmonella Typhimurium and Klebsiella pneumoniae and it promotes bacterial survival when facing stresses due to host defenses. In E. coli, the BolA homologue IbaG is associated with resistance to acidic stress, and in Vibrio cholerae, IbaG is important for animal cell colonization. Recently, it was demonstrated that BolA is phosphorylated and this modification is important for the stability/turnover of BolA and its activity as a transcription factor. The results indicate that there is a physical interaction between BolA-like proteins and the CGFS-type Grx proteins during the biogenesis of Fe-S clusters, iron trafficking and storage. We also review recent progress regarding the cellular and molecular mechanisms by which BolA/Grx protein complexes are involved in the regulation of iron homeostasis in eukaryotes and prokaryotes.

Integrative analysis of the role of BOLA2B in human pan-cancer

Objective: BOLA2B is a recently discovered protein-coding gene. Here, pan-cancer analysis was conducted to determine the expression patterns of BOLA2B and its impact on immune response, gene mutation, and possible molecular biological mechanisms in different tumors, together with investigating its potential usefulness for cancer prognosis. Methods: Data on BOLA2B expression and mutations were downloaded from TCGA and GTEx databases. Clinical survival data from TCGA were used to analyze the prognostic value of BOLA2B. TIMER and ESTIMATE algorithms were used to assess correlations between BOLA2B and tumor-infiltrating immune cells, immune cytokines, and immune scores. Results: BOLA2B was found to be highly expressed at both mRNA and protein levels in multiple tumors, where it was associated with worse overall survival (OS), disease-specific survival (DSS), and progression-free interval (PFI) in all cancers apart from ovarian cancer. BOLA2B was also found to be positively correlated with copy number variation (CNV), and mutations in TP53, TTN, and MUC16 were found to influence BOLA2B expression. Post-transcriptional modifications, including m5C, m1A, and m6A, were observed to regulate BOLA2B expression in all cancers. Functional analysis showed that BOLA2B was enriched in pathways associated with iron-sulfur cluster formation, mTOR-mediated autophagy, and cell cycle inhibition. Decreased BOLA2B expression induced the proliferation of breast cancer cells and G2/M cell cycle arrest. Conclusion: BOLA2B was found to be highly expressed in malignant tumors and could be used as a biomarker of poor prognosis in multiple cancers. Further investigation into BOLA2B's role and molecular functions in cancer would provide new insights for cancer diagnosis and treatment.

Crucial role of Arabidopsis glutaredoxin S17 in heat stress response revealed by transcriptome analysis

Heat stress can have detrimental effects on plant growth and development. However, the mechanisms by which the plant is able to perceive changes in ambient temperature, transmit this information, and initiate a temperature-induced response are not fully understood. Previously, we showed that heterologous expression of an Arabidopsis thaliana L. monothiol glutaredoxin AtGRXS17 enhances thermotolerance in various crops, while disruption of AtGRXS17 expression caused hypersensitivity to permissive temperature. In this study, we extend our investigation into the effect of AtGRXS17 and heat stress on plant growth and development. Although atgrxs17 plants were found to exhibit a slight decrease in hypocotyl elongation, shoot meristem development, and root growth compared to wild-type when grown at 22°C, these growth phenotypic differences became more pronounced when growth temperatures were raised to 28°C. Transcriptome analysis revealed significant changes in genome-wide gene expression in atgrxs17 plants compared to wild-type under conditions of heat stress. The expression of genes related to heat stress factors, auxin response, cellular communication, and abiotic stress were altered in atgrxs17 plants in response to heat stress. Overall, our findings indicate that AtGRXS17 plays a critical role in controlling the transcriptional regulation of plant heat stress response pathways.

Metabolic evidence for distinct pyruvate pools inside plant mitochondria

The majority of the pyruvate inside plant mitochondria is either transported into the matrix from the cytosol via the mitochondria pyruvate carrier (MPC) or synthesized in the matrix by alanine aminotransferase (AlaAT) or NAD-malic enzyme (NAD-ME). Pyruvate from these origins could mix into a single pool in the matrix and contribute indistinguishably to respiration via the pyruvate dehydrogenase complex (PDC), or these molecules could maintain a degree of independence in metabolic regulation. Here we demonstrate that feeding isolated mitochondria with uniformly labelled ¹³C-pyruvate and unlabelled malate enables the assessment of pyruvate contribution from different sources to intermediate production in the tricarboxylic acid cycle. Imported pyruvate was the preferred source for citrate production even when the synthesis of NAD-ME-derived pyruvate was optimized. Genetic or pharmacological elimination of MPC activity removed this preference and allowed an equivalent amount of citrate to be generated from the pyruvate produced by NAD-ME. Increasing the mitochondrial pyruvate pool size by exogenous addition affected only metabolites from pyruvate transported by MPC, whereas depleting the pyruvate pool size by transamination to alanine affected only metabolic products derived from NAD-ME. PDC was more membrane-associated than AlaAT and NAD-ME, suggesting that the physical organization of metabolic machinery may influence metabolic rates. Together, these data reveal that the respiratory substrate supply in plants involves distinct pyruvate pools inside the matrix that can be flexibly mixed on the basis of the rate of pyruvate transport from the cytosol. These pools are independently regulated and contribute differentially to organic acid export from plant mitochondria.

Full-Length Transcriptional Analysis of the Same Soybean Genotype With Compatible and Incompatible Reactions to Heterodera glycines Reveals Nematode Infection Activating Plant Defense Response

Full-length transcriptome sequencing with long reads is a powerful tool to analyze transcriptional and post-transcriptional events; however, it has not been applied on soybean (Glycine max). Here, a comparative full-length transcriptome analysis was performed on soybean genotype 09-138 infected with soybean cyst nematode (SCN, Heterodera glycines) race 4 (SCN4, incompatible reaction) and race 5 (SCN5, compatible reaction) using Oxford Nanopore Technology. Each of 9 full-length samples collected 8 days post inoculation with/without nematodes generated an average of 6.1 GB of clean data and a total of 65,038 transcript sequences. After redundant transcripts were removed, 1,117 novel genes and 41,096 novel transcripts were identified. By analyzing the sequence structure of the novel transcripts, a total of 28,759 complete open reading frame (ORF) sequences, 5,337 transcription factors, 288 long non-coding RNAs, and 40,090 novel transcripts with function annotation were predicted. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses of differentially expressed genes (DEGs) revealed that growth hormone, auxin-activated signaling pathway and multidimensional cell growth, and phenylpropanoid biosynthesis pathway were enriched by infection with both nematode races. More DEGs associated with stress response elements, plant-hormone signaling transduction pathway, and plant-pathogen interaction pathway with more upregulation were found in the incompatible reaction with SCN4 infection, and more DEGs with more upregulation involved in cell wall modification and carbohydrate bioprocess were detected in the compatible reaction with SCN5 infection when compared with each other. Among them, overlapping DEGs with a quantitative difference was triggered. The combination of protein-protein interaction with DEGs for the first time indicated that nematode infection activated the interactions between transcription factor WRKY and VQ (valine-glutamine motif) to contribute to soybean defense. The knowledge of the SCN-soybean interaction mechanism as a model will present more understanding of other plant-nematode interactions.

The C-terminus of a diatom plant-like cryptochrome influences the FAD redox state and binding of interaction partners

A plant-like cryptochrome of diatom microalgae, CryP, acts as a photoreceptor involved in transcriptional regulation. It contains FAD and 5,10-methenyltetrahydrofolate as chromophores. Here, we demonstrate that the unstructured C-terminal extension (CTE) of CryP has an influence on the redox state of the flavin. In CryP lacking the CTE, the flavin is in the oxidized state (FADox), whereas it is a neutral radical (FADH•) in the full-length protein. When the CTE of CryP is coupled to another diatom cryptochrome that naturally binds FADox, this chimera also binds FADH•. In full-length CryP, FADH• is the most stable redox state and oxidation to FADox is extremely slow, whereas reduction to FADH2 is reversible in the dark in approximately 1 h. We also identified novel interaction partners of this algal CRY and characterized two of them in depth regarding their binding activities. BolA, a putative transcription factor, binds to monomeric and to dimeric CryP via the CTE, independent of the redox state of the flavin. In contrast, an unknown protein, ID42612, which occurs solely in heterokont algae, binds only to CryP dimers. This binding is independent of the CTE and shows slight differences in strength depending on the flavin's redox state.

Functional characterization of monothiol and dithiol glutaredoxins from Leptospira interrogans

Thiol redox proteins and low molecular mass thiols have essential functions in maintaining cellular redox balance in almost all living organisms. In the pathogenic bacterium Leptospira interrogans, several redox components have been described, namely, typical 2-Cys peroxiredoxin, a functional thioredoxin system, glutathione synthesis pathway, and methionine sulfoxide reductases. However, until now, information about proteins linked to GSH metabolism has not been reported in this pathogen. Glutaredoxins (Grxs) are GSH-dependent oxidoreductases that regulate and maintain the cellular redox state together with thioredoxins. This work deals with recombinant production at a high purity level, biochemical characterization, and detailed kinetic and structural study of the two Grxs (Lin1CGrx and Lin2CGrx) identified in L. interrogans serovar Copenhageni strain Fiocruz L1-130. Both recombinant LinGrxs exhibited the classical in vitro GSH-dependent 2-hydroxyethyl disulfide and dehydroascorbate reductase activity. Strikingly, we found that Lin2CGrx could serve as a substrate of methionine sulfoxide reductases A1 and B from L. interrogans. Distinctively, only recombinant Lin1CGrx contained a [2Fe2S] cluster confirming a homodimeric structure. The functionality of both LinGrxs was assessed by yeast complementation in null grx mutants, and both isoforms were able to rescue the mutant phenotype. Finally, our data suggest that protein glutathionylation as a post-translational modification process is present in L. interrogans. As a whole, our results support the occurrence of two new redox actors linked to GSH metabolism and iron homeostasis in L. interrogans.

Maturation and Assembly of Iron-Sulfur Cluster-Containing Subunits in the Mitochondrial Complex I From Plants

In plants, the mitochondrial complex I is the protein complex encompassing the largest number of iron-sulfur (Fe-S) clusters. The whole, membrane-embedded, holo-complex is assembled stepwise from assembly intermediates. The Q and N modules are combined to form a peripheral arm in the matrix, whereas the so-called membrane arm is formed after merging a carbonic anhydrase (CA) module with so-called Pp (proximal) and the Pd (distal) domains. A ferredoxin bridge connects both arms. The eight Fe-S clusters present in the peripheral arm for electron transfer reactions are synthesized via a dedicated protein machinery referred to as the iron-sulfur cluster (ISC) machinery. The de novo assembly occurs on ISCU scaffold proteins from iron, sulfur and electron delivery proteins. In a second step, the preformed Fe-S clusters are transferred, eventually converted and inserted in recipient apo-proteins. Diverse molecular actors, including a chaperone-cochaperone system, assembly factors among which proteins with LYR motifs, and Fe-S cluster carrier/transfer proteins, have been identified as contributors to the second step. This mini-review highlights the recent progresses in our understanding of how specificity is achieved during the delivery of preformed Fe-S clusters to complex I subunits.

Oxygen and reactive oxygen species-dependent regulation of plant growth and development

Oxygen and reactive oxygen species (ROS) have been co-opted during evolution into the regulation of plant growth, development, and differentiation. ROS and oxidative signals arising from metabolism or phytohormone-mediated processes control almost every aspect of plant development from seed and bud dormancy, liberation of meristematic cells from the quiescent state, root and shoot growth, and architecture, to flowering and seed production. Moreover, the phytochrome and phytohormone-dependent transmissions of ROS waves are central to the systemic whole plant signaling pathways that integrate root and shoot growth. The sensing of oxygen availability through the PROTEOLYSIS 6 (PRT6) N-degron pathway functions alongside ROS production and signaling but how these pathways interact in developing organs remains poorly understood. Considerable progress has been made in our understanding of the nature of hydrogen peroxide sensors and the role of thiol-dependent signaling networks in the transmission of ROS signals. Reduction/oxidation (redox) changes in the glutathione (GSH) pool, glutaredoxins (GRXs), and thioredoxins (TRXs) are important in the control of growth mediated by phytohormone pathways. Although, it is clear that the redox states of proteins involved in plant growth and development are controlled by the NAD(P)H thioredoxin reductase (NTR)/TRX and reduced GSH/GRX systems of the cytosol, chloroplasts, mitochondria, and nucleus, we have only scratched the surface of this multilayered control and how redox-regulated processes interact with other cell signaling systems.

Iron-sulfur proteins in plant mitochondria: roles and maturation

Iron-sulfur (Fe-S) clusters are prosthetic groups ensuring electron transfer reactions, activating substrates for catalytic reactions, providing sulfur atoms for the biosynthesis of vitamins or other cofactors, or having protein-stabilizing effects. Hence, metalloproteins containing these cofactors are essential for numerous and diverse metabolic pathways and cellular processes occurring in the cytoplasm. Mitochondria are organelles where the Fe-S cluster demand is high, notably because the activity of the respiratory chain complexes I, II, and III relies on the correct assembly and functioning of Fe-S proteins. Several other proteins or complexes present in the matrix require Fe-S clusters as well, or depend either on Fe-S proteins such as ferredoxins or on cofactors such as lipoic acid or biotin whose synthesis relies on Fe-S proteins. In this review, we have listed and discussed the Fe-S-dependent enzymes or pathways in plant mitochondria including some potentially novel Fe-S proteins identified based on in silico analysis or on recent evidence obtained in non-plant organisms. We also provide information about recent developments concerning the molecular mechanisms involved in Fe-S cluster synthesis and trafficking steps of these cofactors from maturation factors to client apoproteins.

[4Fe-4S] cluster trafficking mediated by Arabidopsis mitochondrial ISCA and NFU proteins

Numerous iron-sulfur (Fe-S) proteins with diverse functions are present in the matrix and respiratory chain complexes of mitochondria. Although [4Fe-4S] clusters are the most common type of Fe-S cluster in mitochondria, the molecular mechanism of [4Fe-4S] cluster assembly and insertion into target proteins by the mitochondrial iron-sulfur cluster (ISC) maturation system is not well-understood. Here we report a detailed characterization of two late-acting Fe-S cluster-carrier proteins from Arabidopsis thaliana, NFU4 and NFU5. Yeast two-hybrid and bimolecular fluorescence complementation studies demonstrated interaction of both the NFU4 and NFU5 proteins with the ISCA class of Fe-S carrier proteins. Recombinant NFU4 and NFU5 were purified as apo-proteins after expression in Escherichia coliIn vitro Fe-S cluster reconstitution led to the insertion of one [4Fe-4S]2+ cluster per homodimer as determined by UV-visible absorption/CD, resonance Raman and EPR spectroscopy, and analytical studies. Cluster transfer reactions, monitored by UV-visible absorption and CD spectroscopy, showed that a [4Fe-4S]2+ cluster-bound ISCA1a/2 heterodimer is effective in transferring [4Fe-4S]2+ clusters to both NFU4 and NFU5 with negligible back reaction. In addition, [4Fe-4S]2+ cluster-bound ISCA1a/2, NFU4, and NFU5 were all found to be effective [4Fe-4S]2+ cluster donors for maturation of the mitochondrial apo-aconitase 2 as assessed by enzyme activity measurements. The results demonstrate rapid, unidirectional, and quantitative [4Fe-4S]2+ cluster transfer from ISCA1a/2 to NFU4 or NFU5 that further delineates their respective positions in the plant ISC machinery and their contributions to the maturation of client [4Fe-4S] cluster-containing proteins.

The Arabidopsis Mitochondrial Glutaredoxin GRXS15 Provides [2Fe-2S] Clusters for ISCA-Mediated [4Fe-4S] Cluster Maturation

Iron-sulfur (Fe-S) proteins are crucial for many cellular functions, particularly those involving electron transfer and metabolic reactions. An essential monothiol glutaredoxin GRXS15 plays a key role in the maturation of plant mitochondrial Fe-S proteins. However, its specific molecular function is not clear, and may be different from that of the better characterized yeast and human orthologs, based on known properties. Hence, we report here a detailed characterization of the interactions between Arabidopsis thaliana GRXS15 and ISCA proteins using both in vivo and in vitro approaches. Yeast two-hybrid and bimolecular fluorescence complementation experiments demonstrated that GRXS15 interacts with each of the three plant mitochondrial ISCA1a/1b/2 proteins. UV-visible absorption/CD and resonance Raman spectroscopy demonstrated that coexpression of ISCA1a and ISCA2 resulted in samples with one [2Fe-2S]²⁺ cluster per ISCA1a/2 heterodimer, but cluster reconstitution using as-purified [2Fe-2S]-ISCA1a/2 resulted in a [4Fe-4S]²⁺ cluster-bound ISCA1a/2 heterodimer. Cluster transfer reactions monitored by UV-visible absorption and CD spectroscopy demonstrated that [2Fe-2S]-GRXS15 mediates [2Fe-2S]²⁺ cluster assembly on mitochondrial ferredoxin and [4Fe-4S]²⁺ cluster assembly on the ISCA1a/2 heterodimer in the presence of excess glutathione. This suggests that ISCA1a/2 is an assembler of [4Fe-4S]²⁺ clusters, via two-electron reductive coupling of two [2Fe-2S]²⁺ clusters. Overall, the results provide new insights into the roles of GRXS15 and ISCA1a/2 in effecting [2Fe-2S]²⁺ to [4Fe-4S]²⁺ cluster conversions for the maturation of client [4Fe-4S] cluster-containing proteins in plants.

Leveraging computational genomics to understand the molecular basis of metal homeostasis

Genome-based data is helping to reveal the diverse strategies plants and algae use to maintain metal homeostasis. In addition to acquisition, distribution and storage of metals, acclimating to feast or famine can involve a wealth of genes that we are just now starting to understand. The fast-paced acquisition of genome-based data, however, is far outpacing our ability to experimentally characterize protein function. Computational genomic approaches are needed to fill the gap between what is known and unknown. To avoid misconstruing bioinformatically derived data, which is the root cause of the inaccurate functional annotations that plague databases, functional inferences from diverse sources and contextualization of that evidence with a robust understanding of protein family evolution is needed. Phylogenomic- and comparative-genomic-based studies can aid in the interpretation of experimental data or provide a spark for the discovery of a new function. These analyses not only lead to novel insight into a target protein's function but can generate thought-provoking insights across protein families.

Sinorhizobium meliloti YrbA binds divalent metal cations using two conserved histidines

Sinorhizobium meliloti is a nitrogen-fixing bacterium forming symbiotic nodules with the legume Medicago truncatula. S. meliloti possesses two BolA-like proteins (BolA and YrbA), the function of which is unknown. In organisms where BolA proteins and monothiol glutaredoxins (Grxs) are present, they contribute to the regulation of iron homeostasis by bridging a [2Fe–2S] cluster into heterodimers. A role in the maturation of iron–sulfur (Fe–S) proteins is also attributed to both proteins. In the present study, we have performed a structure–function analysis of SmYrbA showing that it coordinates diverse divalent metal ions (Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺) using His³² and His⁶⁷ residues, that are also used for Fe–S cluster binding in BolA–Grx heterodimers. It also possesses the capacity to form heterodimers with the sole monothiol glutaredoxin (SmGrx2) present in this species. Using cellular approaches analyzing the metal tolerance of S. meliloti mutant strains inactivated in the yrbA and/or bolA genes, we provide evidence for a connection of YrbA with the regulation of iron homeostasis. The mild defects in M. truncatula nodulation reported for the yrbA bolA mutant as compared with the stronger defects in nodule development previously observed for a grx2 mutant suggest functions independent of SmGrx2. These results help in clarifying the physiological role of BolA-type proteins in bacteria.

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.

Iron-sulfur cluster biogenesis, trafficking, and signaling: Roles for CGFS glutaredoxins and BolA proteins

The synthesis and trafficking of iron-sulfur (Fe-S) clusters in both prokaryotes and eukaryotes requires coordination within an expanding network of proteins that function in the cytosol, nucleus, mitochondria, and chloroplasts in order to assemble and deliver these ancient and essential cofactors to a wide variety of Fe-S-dependent enzymes and proteins. This review focuses on the evolving roles of two ubiquitous classes of proteins that operate in this network: CGFS glutaredoxins and BolA proteins. Monothiol or CGFS glutaredoxins possess a Cys-Gly-Phe-Ser active site that coordinates an Fe-S cluster in a homodimeric complex. CGFS glutaredoxins also form [2Fe-2S]-bridged heterocomplexes with BolA proteins, which possess an invariant His and an additional His or Cys residue that serve as cluster ligands. Here we focus on recent discoveries in bacteria, fungi, humans, and plants that highlight the shared and distinct roles of CGFS glutaredoxins and BolA proteins in Fe-S cluster biogenesis, Fe-S cluster storage and trafficking, and Fe-S cluster signaling to transcriptional factors that control iron metabolism--.

Infection of human lymphomononuclear cells by SARS-CoV-2

Although SARS-CoV-2 severe infection is associated with a hyperinflammatory state, lymphopenia is an immunological hallmark, and correlates with poor prognosis in COVID-19. However, it remains unknown if circulating human lymphocytes and monocytes are susceptible to SARS-CoV-2 infection. In this study, SARS-CoV-2 infection of human peripheral blood mononuclear cells (PBMCs) was investigated both in vitro and in vivo . We found that in vitro infection of whole PBMCs from healthy donors was productive of virus progeny. Results revealed that monocytes, as well as B and T lymphocytes, are susceptible to SARS-CoV-2 active infection and viral replication was indicated by detection of double-stranded RNA. Moreover, flow cytometry and immunofluorescence analysis revealed that SARS-CoV-2 was frequently detected in monocytes and B lymphocytes from COVID-19 patients, and less frequently in CD4 ⁺ T lymphocytes. The rates of SARS-CoV-2-infected monocytes in PBMCs from COVID-19 patients increased over time from symptom onset. Additionally, SARS-CoV-2-positive monocytes and B and CD4+T lymphocytes were detected by immunohistochemistry in post mortem lung tissue. SARS-CoV-2 infection of blood circulating leukocytes in COVID-19 patients may have important implications for disease pathogenesis, immune dysfunction, and virus spread within the host.

Identification of client iron-sulfur proteins of the chloroplastic NFU2 transfer protein in Arabidopsis thaliana

Iron-sulfur (Fe-S) proteins have critical functions in plastids, notably participating in photosynthetic electron transfer, sulfur and nitrogen assimilation, chlorophyll metabolism, and vitamin or amino acid biosynthesis. Their maturation relies on the so-called SUF (sulfur mobilization) assembly machinery. Fe-S clusters are synthesized de novo on a scaffold protein complex and then delivered to client proteins via several transfer proteins. However, the maturation pathways of most client proteins and their specificities for transfer proteins are mostly unknown. In order to decipher the proteins interacting with the Fe-S cluster transfer protein NFU2, one of the three plastidial representatives found in Arabidopsis thaliana, we performed a quantitative proteomic analysis of shoots, roots, and seedlings of nfu2 plants, combined with NFU2 co-immunoprecipitation and binary yeast two-hybrid experiments. We identified 14 new targets, among which nine were validated in planta using a binary bimolecular fluorescence complementation assay. These analyses also revealed a possible role for NFU2 in the plant response to desiccation. Altogether, this study better delineates the maturation pathways of many chloroplast Fe-S proteins, considerably extending the number of NFU2 clients. It also helps to clarify the respective roles of the three NFU paralogs NFU1, NFU2, and NFU3.

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.

Regulation of Iron Homeostasis and Use in Chloroplasts

Iron (Fe) is essential for life because of its role in protein cofactors. Photosynthesis, in particular photosynthetic electron transport, has a very high demand for Fe cofactors. Fe is commonly limiting in the environment, and therefore photosynthetic organisms must acclimate to Fe availability and avoid stress associated with Fe deficiency. In plants, adjustment of metabolism, of Fe utilization, and gene expression, is especially important in the chloroplasts during Fe limitation. In this review, we discuss Fe use, Fe transport, and mechanisms of acclimation to Fe limitation in photosynthetic lineages with a focus on the photosynthetic electron transport chain. We compare Fe homeostasis in Cyanobacteria, the evolutionary ancestors of chloroplasts, with Fe homeostasis in green algae and in land plants in order to provide a deeper understanding of how chloroplasts and photosynthesis may cope with Fe limitation.

NMR-Metabolomics Shows That BolA Is an Important Modulator of Salmonella Typhimurium Metabolic Processes under Virulence Conditions

BolA is a ubiquitous global transcription factor. Despite its clear role in the induction of important stress-resistant physiological changes and its recent implication in the virulence of Salmonella, further research is required to shed light on the pathways modulated by BolA. In this study, we resorted to untargeted ¹H-NMR metabolomics to understand the impact of BolA on the metabolic profile of Salmonella Typhimurium, under virulence conditions. Three strains of S. Typhimurium SL1344 were studied: An SL1344 strain transformed with an empty plasmid (control), a bolA knockout mutant (ΔbolA), and a strain overexpressing bolA (bolA⁺). These strains were grown in a minimal virulence-inducing medium and cells were collected at the end of the exponential and stationary phases. The extracts were analyzed by NMR, and multivariate and univariate statistical analysis were performed to identify significant alterations. Principal component analysis (PCA) and partial least squares discriminant analysis (PLS-DA) of ¹H-NMR data allowed the discrimination between the metabolic profiles of these strains, revealing increased levels of acetate, valine, alanine, NAD⁺, succinate, coenzyme A, glutathione, and putrescine in bolA⁺. These results indicate that BolA regulates pathways related to stress resistance and virulence, being an important modulator of the metabolic processes needed for S. Typhimurium infection.

Iron-sulfur protein NFU2 is required for branched-chain amino acid synthesis in Arabidopsis roots

Numerous proteins require a metallic co-factor for their function. In plastids, the maturation of iron-sulfur (Fe-S) proteins necessitates a complex assembly machinery. In this study, we focused on Arabidopsis thaliana NFU1, NFU2, and NFU3, which participate in the final steps of the maturation process. According to the strong photosynthetic defects observed in high chlorophyll fluorescence 101 (hcf101), nfu2, and nfu3 plants, we determined that NFU2 and NFU3, but not NFU1, act immediately upstream of HCF101 for the maturation of [Fe4S4]-containing photosystem I subunits. An additional function of NFU2 in the maturation of the [Fe2S2] cluster of a dihydroxyacid dehydratase was obvious from the accumulation of precursors of the branched-chain amino acid synthesis pathway in roots of nfu2 plants and from the rescue of the primary root growth defect by supplying branched-chain amino acids. The absence of NFU3 in roots precluded any compensation. Overall, unlike their eukaryotic and prokaryotic counterparts, which are specific to [Fe4S4] proteins, NFU2 and NFU3 contribute to the maturation of both [Fe2S2] and [Fe4S4] proteins, either as a relay in conjunction with other proteins such as HCF101 or by directly delivering Fe-S clusters to client proteins. Considering the low number of Fe-S cluster transfer proteins relative to final acceptors, additional targets probably await identification.

Is There a Role for Glutaredoxins and BOLAs in the Perception of the Cellular Iron Status in Plants?

Glutaredoxins (GRXs) have at least three major identified functions. In apoforms, they exhibit oxidoreductase activity controlling notably protein glutathionylation/deglutathionylation. In holoforms, i.e., iron-sulfur (Fe-S) cluster-bridging forms, they act as maturation factors for the biogenesis of Fe-S proteins or as regulators of iron homeostasis contributing directly or indirectly to the sensing of cellular iron status and/or distribution. The latter functions seem intimately connected with the capacity of specific GRXs to form [2Fe-2S] cluster-bridging homodimeric or heterodimeric complexes with BOLA proteins. In yeast species, both proteins modulate the localization and/or activity of transcription factors regulating genes coding for proteins involved in iron uptake and intracellular sequestration in response notably to iron deficiency. Whereas vertebrate GRX and BOLA isoforms may display similar functions, the involved partner proteins are different. We perform here a critical evaluation of the results supporting the implication of both protein families in similar signaling pathways in plants and provide ideas and experimental strategies to delineate further their functions.

The thioredoxin-mediated recycling of Arabidopsis thaliana GRXS16 relies on a conserved C-terminal cysteine

BACKGROUND

Glutaredoxins (GRXs) are oxidoreductases involved in diverse cellular processes through their capacity to reduce glutathionylated proteins and/or to coordinate iron‑sulfur (Fe-S) clusters. Among class II GRXs, the plant-specific GRXS16 is a bimodular protein formed by an N-terminal endonuclease domain fused to a GRX domain containing a ¹⁵⁸CGFS signature.

METHODS

The biochemical properties (redox activity, sensitivity to oxidation, pK_a of cysteine residues, midpoint redox potential) of Arabidopsis thaliana GRXS16 were investigated by coupling oxidative treatments to alkylation shift assays, activity measurements and mass spectrometry analyses.

RESULTS

Activity measurements using redox-sensitive GFP2 (roGFP2) as target protein did not reveal any significant glutathione-dependent reductase activity of A. thaliana GRXS16 whereas it was able to catalyze the oxidation of roGFP2 in the presence of glutathione disulfide. Accordingly, Arabidopsis GRXS16 reacted efficiently with oxidized forms of glutathione, leading to the formation of an intramolecular disulfide between Cys¹⁵⁸ and the semi-conserved Cys²¹⁵, which has a midpoint redox potential of - 298 mV at pH 7.0 and is reduced by plastidial thioredoxins (TRXs) but not GSH. By promoting the formation of this disulfide, Cys²¹⁵ modulates GRXS16 oxidoreductase activity.

CONCLUSION

The reduction of AtGRXS16, which is mandatory for its oxidoreductase activity and the binding of Fe-S clusters, depends on light through the plastidial FTR/TRX system. Hence, disulfide formation may constitute a redox switch mechanism controlling GRXS16 function in response to day/night transition or oxidizing conditions.

GENERAL SIGNIFICANCE

From the in vitro data obtained with roGFP2, one can postulate that GRXS16 would mediate protein glutathionylation/oxidation in plastids but not their deglutathionylation.

Conserved functions of Arabidopsis mitochondrial late-acting maturation factors in the trafficking of iron‑sulfur clusters

Numerous proteins require iron‑sulfur (Fe-S) clusters as cofactors for their function. Their biogenesis is a multi-step process occurring in the cytosol and mitochondria of all eukaryotes and additionally in plastids of photosynthetic eukaryotes. A basic model of Fe-S protein maturation in mitochondria has been obtained based on studies achieved in mammals and yeast, yet some molecular details, especially of the late steps, still require investigation. In particular, the late-acting biogenesis factors in plant mitochondria are poorly understood. In this study, we expressed the factors belonging to NFU, BOLA, SUFA/ISCA and IBA57 families in the respective yeast mutant strains. Expression of the Arabidopsis mitochondrial orthologs was usually sufficient to rescue the growth defects observed on specific media and/or to restore the abundance or activity of the defective Fe-S or lipoic acid-dependent enzymes. These data demonstrate that the plant mitochondrial counterparts, including duplicated isoforms, likely retained their ancestral functions. In contrast, the SUFA1 and IBA57.2 plastidial isoforms cannot rescue the lysine and glutamate auxotrophies of the respective isa1-isa2Δ and iba57Δ strains or of the isa1-isa2-iba57Δ triple mutant when expressed in combination. This suggests a specialization of the yeast mitochondrial and plant plastidial factors in these late steps of Fe-S protein biogenesis, possibly reflecting substrate-specific interactions in these different compartments.

Roles and maturation of iron-sulfur proteins in plastids

One reason why iron is an essential element for most organisms is its presence in prosthetic groups such as hemes or iron–sulfur (Fe–S) clusters, which are notably required for electron transfer reactions. As an organelle with an intense metabolism in plants, chloroplast relies on many Fe–S proteins. This includes those present in the electron transfer chain which will be, in fact, essential for most other metabolic processes occurring in chloroplasts, e.g., carbon fixation, nitrogen and sulfur assimilation, pigment, amino acid, and vitamin biosynthetic pathways to cite only a few examples. The maturation of these Fe–S proteins requires a complex and specific machinery named SUF (sulfur mobilisation). The assembly process can be split in two major steps, (1) the de novo assembly on scaffold proteins which requires ATP, iron and sulfur atoms, electrons, and thus the concerted action of several proteins forming early acting assembly complexes, and (2) the transfer of the preformed Fe–S cluster to client proteins using a set of late-acting maturation factors. Similar machineries, having in common these basic principles, are present in the cytosol and in mitochondria. This review focuses on the currently known molecular details concerning the assembly and roles of Fe–S proteins in plastids.

Assembly and Transfer of Iron-Sulfur Clusters in the Plastid

Iron-Sulfur (Fe-S) clusters and proteins are essential to many growth and developmental processes. In plants, they exist in the plastids, mitochondria, cytosol, and nucleus. Six types of Fe-S clusters are found in the plastid: classic 2Fe-2S, NEET-type 2Fe-2S, Rieske-type 2Fe-2S, 3Fe-4S, 4Fe-4S, and siroheme 4Fe-4S. Classic, NEET-type, and Rieske-type 2Fe-2S clusters have the same 2Fe-2S core; similarly, common and siroheme 4Fe-4S clusters have the same 4Fe-4S core. Plastidial Fe-S clusters are assembled by the sulfur mobilization (SUF) pathway, which contains cysteine desulfurase (EC 2.8.1.7), sulfur transferase (EC 2.8.1.3), Fe-S scaffold complex, and Fe-S carrier proteins. The plastidial cysteine desulfurase-sulfur transferase-Fe-S-scaffold complex system is responsible for de novo assembly of all plastidial Fe-S clusters. However, different types of Fe-S clusters are transferred to recipient proteins via respective Fe-S carrier proteins. This review focuses on recent discoveries on the molecular functions of different assembly and transfer factors involved in the plastidial SUF pathway. It also discusses potential points for regulation of the SUF pathway, relationships among the plastidial, mitochondrial, and cytosolic Fe-S assembly and transfer pathways, as well as several open questions about the carrier proteins for Rieske-type 2Fe-2S, NEET-type 2Fe-2S, and 3F-4S clusters.

Thiol Based Redox Signaling in Plant Nucleus

Reactive oxygen species (ROS) are well-described by-products of cellular metabolic activities, acting as signaling molecules and regulating the redox state of proteins. Solvent exposed thiol residues like cysteines are particularly sensitive to oxidation and their redox state affects structural and biochemical capacities of many proteins. While thiol redox regulation has been largely studied in several cell compartments like in the plant chloroplast, little is known about redox sensitive proteins in the nucleus. Recent works have revealed that proteins with oxidizable thiols are important for the regulation of many nuclear functions, including gene expression, transcription, epigenetics, and chromatin remodeling. Moreover, thiol reducing molecules like glutathione and specific isoforms of thiols reductases, thioredoxins and glutaredoxins were found in different nuclear subcompartments, further supporting that thiol-dependent systems are active in the nucleus. This mini-review aims to discuss recent progress in plant thiol redox field, taking examples of redox regulated nuclear proteins and focusing on major thiol redox systems acting in the nucleus.

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.

Erv1 of Arabidopsis thaliana can directly oxidize mitochondrial intermembrane space proteins in the absence of redox-active Mia40

Background

Many proteins of the mitochondrial intermembrane space (IMS) contain structural disulfide bonds formed by the mitochondrial disulfide relay. In fungi and animals, the sulfhydryl oxidase Erv1 ‘generates’ disulfide bonds that are passed on to the oxidoreductase Mia40, which oxidizes substrate proteins. A different structural organization of plant Erv1 proteins compared to that of animal and fungal orthologs was proposed to explain its inability to complement the corresponding yeast mutant.

Results

Herein, we have revisited the biochemical and functional properties of Arabidopsis thaliana Erv1 by both in vitro reconstituted activity assays and complementation of erv1 and mia40 yeast mutants. These mutants were viable, however, they showed severe defects in the biogenesis of IMS proteins. The plant Erv1 was unable to oxidize yeast Mia40 and rather even blocked its activity. Nevertheless, it was able to mediate the import and folding of mitochondrial proteins.

Conclusions

We observed that plant Erv1, unlike its homologs in fungi and animals, can promote protein import and oxidative protein folding in the IMS independently of the oxidoreductase Mia40. In accordance to the absence of Mia40 in many protists, our study suggests that the mitochondrial disulfide relay evolved in a stepwise reaction from an Erv1-only system to which Mia40 was added in order to improve substrate specificity.

Graphical Abstract

Arabidopsis Glutaredoxin S17 Contributes to Vegetative Growth, Mineral Accumulation, and Redox Balance during Iron Deficiency

Iron (Fe) is an essential mineral nutrient and a metal cofactor required for many proteins and enzymes involved in the processes of DNA synthesis, respiration, and photosynthesis. Iron limitation can have detrimental effects on plant growth and development. Such effects are mediated, at least in part, through the generation of reactive oxygen species (ROS). Thus, plants have evolved a complex regulatory network to respond to conditions of iron limitations. However, the mechanisms that couple iron deficiency and oxidative stress responses are not fully understood. Here, we report the discovery that an Arabidopsis thaliana monothiol glutaredoxin S17 (AtGRXS17) plays a critical role in the plants ability to respond to iron deficiency stress and maintain redox homeostasis. In a yeast expression assay, AtGRXS17 was able to suppress the iron accumulation in yeast ScGrx3/ScGrx4 mutant cells. Genetic analysis indicated that plants with reduced AtGRXS17 expression were hypersensitive to iron deficiency and showed increased iron concentrations in mature seeds. Disruption of AtGRXS17 caused plant sensitivity to exogenous oxidants and increased ROS production under iron deficiency. Addition of reduced glutathione rescued the growth and alleviates the sensitivity of atgrxs17 mutants to iron deficiency. These findings suggest AtGRXS17 helps integrate redox homeostasis and iron deficiency responses.

Structural insights into the molecular function of human [2Fe-2S] BOLA1-GRX5 and [2Fe-2S] BOLA3-GRX5 complexes

Members of the monothiol glutaredoxin family and members of the BolA-like protein family have recently emerged as specific interacting partners involved in iron-sulfur protein maturation and redox regulation pathways. It is known that human mitochondrial BOLA1 and BOLA3 form [2Fe-2S] cluster-bridged dimeric heterocomplexes with the monothiol glutaredoxin GRX5. The structure and cluster coordination of the two [2Fe-2S] heterocomplexes as well as their molecular function are, however, not defined yet. Experimentally-driven structural models of the two [2Fe-2S] cluster-bridged dimeric heterocomplexes, the relative stability of the two complexes and the redox properties of the [2Fe-2S] cluster bound to these complexes are here presented on the basis of UV/vis, CD, EPR and NMR spectroscopies and computational protein-protein docking. While the BOLA1-GRX5 complex coordinates a reduced, Rieske-type [2Fe-2S]¹⁺ cluster, an oxidized, ferredoxin-like [2Fe-2S]²⁺ cluster is present in the BOLA3-GRX5 complex. The [2Fe-2S] BOLA1-GRX5 complex is preferentially formed over the [2Fe-2S] BOLA3-GRX5 complex, as a result of a higher cluster binding affinity. All these observed differences provide the first indications discriminating the molecular function of the two [2Fe-2S] heterocomplexes.

Arabidopsis Glutaredoxin S17 Contributes to Vegetative Growth, Mineral Accumulation, and Redox Balance during Iron Deficiency

Iron (Fe) is an essential mineral nutrient and a metal cofactor required for many proteins and enzymes involved in the processes of DNA synthesis, respiration, and photosynthesis. Iron limitation can have detrimental effects on plant growth and development. Such effects are mediated, at least in part, through the generation of reactive oxygen species (ROS). Thus, plants have evolved a complex regulatory network to respond to conditions of iron limitations. However, the mechanisms that couple iron deficiency and oxidative stress responses are not fully understood. Here, we report the discovery that an Arabidopsis thaliana monothiol glutaredoxin S17 (AtGRXS17) plays a critical role in the plants ability to respond to iron deficiency stress and maintain redox homeostasis. In a yeast expression assay, AtGRXS17 was able to suppress the iron accumulation in yeast ScGrx3/ScGrx4 mutant cells. Genetic analysis indicated that plants with reduced AtGRXS17 expression were hypersensitive to iron deficiency and showed increased iron concentrations in mature seeds. Disruption of AtGRXS17 caused plant sensitivity to exogenous oxidants and increased ROS production under iron deficiency. Addition of reduced glutathione rescued the growth and alleviates the sensitivity of atgrxs17 mutants to iron deficiency. These findings suggest AtGRXS17 helps integrate redox homeostasis and iron deficiency responses.

In Vitro Alkylation Methods for Assessing the Protein Redox State

Cysteines are important residues for protein structure, function, and regulation. Owing to their modified reactivity, some cysteines can undergo very diverse redox posttranslational modifications, including the reversible formation of disulfide bonds, a widespread protein regulatory process as well exemplified in plant chloroplasts for Calvin-Benson cycle enzymes. Both core- and peripheral-photorespiratory enzymes possess conserved cysteines, some of which have been identified as being subject to oxidative modifications. This is not surprising considering their presence in subcellular compartments where the production of reactive species can be important. However, in most cases, the types of modifications and their biochemical effect on protein activity have not been validated, meaning that the possible impact of these modifications in a complex physiological context, such as photorespiration, remains obscure.We here describe a detailed set of protocols for alkylation methods that have been used so far to (1) study the protein cysteine redox state either in vitro by submitting purified recombinant proteins to reducing/oxidation treatments or in vivo by western blots on protein extracts from plants subject to environmental constraints, and its dependency on the two major reducing systems in the cell, i.e., the thioredoxin and glutathione/glutaredoxin systems, and (2) determine two key redox parameters, i.e., the cysteine pK _a and the redox midpoint potential.

The Escherichia coli BolA Protein IbaG Forms a Histidine-Ligated [2Fe-2S]-Bridged Complex with Grx4

Two ubiquitous protein families have emerged as key players in iron metabolism, the CGFS-type monothiol glutaredoxins (Grxs) and the BolA proteins. Monothiol Grxs and BolA proteins form heterocomplexes that have been implicated in Fe-S cluster assembly and trafficking. The Escherichia coli genome encodes members of both of these proteins families, namely, the monothiol glutaredoxin Grx4 and two BolA family proteins, BolA and IbaG. Previous work has demonstrated that E. coli Grx4 and BolA interact as both apo and [2Fe-2S]-bridged heterodimers that are spectroscopically distinct from [2Fe-2S]-bridged Grx4 homodimers. However, the physical and functional interactions between Grx4 and IbaG are uncharacterized. Here we show that co-expression of Grx4 with IbaG yields a [2Fe-2S]-bridged Grx4-IbaG heterodimer. In vitro interaction studies indicate that IbaG binds the [2Fe-2S] Grx4 homodimer to form apo Grx4-IbaG heterodimer as well as the [2Fe-2S] Grx4-IbaG heterodimer, altering the cluster stability and coordination environment. Additionally, spectroscopic and mutagenesis studies provide evidence that IbaG ligates the Fe-S cluster via the conserved histidine that is present in all BolA proteins and by a second conserved histidine that is present in the H/C loop of two of the four classes of BolA proteins. These results suggest that IbaG may function in Fe-S cluster assembly and trafficking in E. coli as demonstrated for other BolA homologues that interact with monothiol Grxs.

The Arabidopsis Iron-Sulfur Protein GRXS17 is a Target of the Ubiquitin E3 Ligases RGLG3 and RGLG4

The stability of signaling proteins in eukaryotes is often controlled by post-translational modifiers. For polyubiquitination, specificity is assured by E3 ubiquitin ligases. Although plant genomes encode hundreds of E3 ligases, only few targets are known, even in the model Arabidopsis thaliana. Here, we identified the monothiol glutaredoxin GRXS17 as a substrate of the Arabidopsis E3 ubiquitin ligases RING DOMAIN LIGASE 3 (RGLG3) and RGLG4 using a substrate trapping approach involving tandem affinity purification of RING-dead versions. Simultaneously, we used a ubiquitin-conjugating enzym (UBC) panel screen to pinpoint UBC30 as a cognate E2 UBC capable of interacting with RGLG3 and RGLG4 and mediating auto-ubiquitination of RGLG3 and ubiquitination of GRXS17 in vitro. Accordingly, GRXS17 is ubiquitinated and degraded in an RGLG3- and RGLG4-dependent manner in planta. The truncated hemoglobin GLB3 also interacted with RGLG3 and RGLG4 but appeared to obstruct RGLG3 ubiquitination activity rather than being its substrate. Our results suggest that the RGLG family is intimately linked to the essential element iron.

New nodes and edges in the glucosinolate molecular network revealed by proteomics and metabolomics of Arabidopsis myb28/29 and cyp79B2/B3 glucosinolate mutants

Glucosinolates present in Brassicales are important for human health and plant defense against insects and pathogens. Here we investigate the proteomes and metabolomes of Arabidopsis myb28/29 and cyp79B2/B3 mutants deficient in aliphatic glucosinolates and indolic glucosinolates, respectively. Quantitative proteomics of the myb28/29 and cyp79B2/B3 mutants led to the identification of 2785 proteins, of which 142 proteins showed significant changes in the two mutants compared to wild type (WT). By mapping the differential proteins using STRING, we detected 59 new edges in the glucosinolate metabolic network. These connections can be classified as primary with direct roles in glucosinolate metabolism, secondary related to plant stress responses, and tertiary involved in other biological processes. Gene Ontology analysis of the differential proteins showed high level of enrichment in the nodes belonging to metabolic process including glucosinolate biosynthesis and response to stimulus. Using metabolomics, we quantified 292 metabolites covering a broad spectrum of metabolic pathways, and 89 exhibited differential accumulation patterns between the mutants and WT. The changing metabolites (e.g., γ-glutamyl amino acids, auxins and glucosinolate hydrolysis products) complement our proteomics findings. This study contributes toward engineering and breeding of glucosinolate profiles in plants in efforts to improve human health, crop quality and productivity.

BIOLOGICAL SIGNIFICANCE

Glucosinolates in Brassicales constitute an important group of natural metabolites important for plant defense and human health. Its biosynthetic pathways and transcriptional regulation have been well-studied. Using Arabidopsis mutants of important genes in glucosinolate biosynthesis, quantitative proteomics and metabolomics led to identification of many proteins and metabolites that are potentially related to glucosinolate metabolism. This study provides a comprehensive insight into the molecular networks of glucosinolate metabolism, and will facilitate efforts toward engineering and breeding of glucosinolate profiles for enhanced crop defense, and nutritional value.

The Poplar Rust-Induced Secreted Protein (RISP) Inhibits the Growth of the Leaf Rust Pathogen Melampsora larici-populina and Triggers Cell Culture Alkalinisation

Plant cells secrete a wide range of proteins in extracellular spaces in response to pathogen attack. The poplar rust-induced secreted protein (RISP) is a small cationic protein of unknown function that was identified as the most induced gene in poplar leaves during immune responses to the leaf rust pathogen Melampsora larici-populina, an obligate biotrophic parasite. Here, we combined in planta and in vitro molecular biology approaches to tackle the function of RISP. Using a RISP-mCherry fusion transiently expressed in Nicotiana benthamiana leaves, we demonstrated that RISP is secreted into the apoplast. A recombinant RISP specifically binds to M. larici-populina urediniospores and inhibits their germination. It also arrests the growth of the fungus in vitro and on poplar leaves. Interestingly, RISP also triggers poplar cell culture alkalinisation and is cleaved at the C-terminus by a plant-encoded mechanism. Altogether our results indicate that RISP is an antifungal protein that has the ability to trigger cellular responses.

Regulation of BolA abundance mediates morphogenesis in Fremyella diplosiphon

Filamentous cyanobacterium Fremyella diplosiphon is known to alter its pigmentation and morphology during complementary chromatic acclimation (CCA) to efficiently harvest available radiant energy for photosynthesis. F. diplosiphon cells are rectangular and filaments are longer under green light (GL), whereas smaller, spherical cells and short filaments are prevalent under red light (RL). Light regulation of bolA morphogene expression is correlated with photoregulation of cellular morphology in F. diplosiphon. Here, we investigate a role for quantitative regulation of cellular BolA protein levels in morphology determination. Overexpression of bolA in WT was associated with induction of RL-characteristic spherical morphology even when cultures were grown under GL. Overexpression of bolA in a ΔrcaE background, which lacks cyanobacteriochrome photosensor RcaE and accumulates lower levels of BolA than WT, partially reverted the cellular morphology of the strain to a WT-like state. Overexpression of BolA in WT and ΔrcaE backgrounds was associated with decreased cellular reactive oxygen species (ROS) levels and an increase in filament length under both GL and RL. Morphological defects and high ROS levels commonly observed in ΔrcaE could, thus, be in part due to low accumulation of BolA. Together, these findings support an emerging model for RcaE-dependent photoregulation of BolA in controlling the cellular morphology of F. diplosiphon during CCA.

The mitochondrial monothiol glutaredoxin S15 is essential for iron-sulfur protein maturation in Arabidopsis thaliana

The iron-sulfur cluster (ISC) is an ancient and essential cofactor of many proteins involved in electron transfer and metabolic reactions. In Arabidopsis, three pathways exist for the maturation of iron-sulfur proteins in the cytosol, plastids, and mitochondria. We functionally characterized the role of mitochondrial glutaredoxin S15 (GRXS15) in biogenesis of ISC containing aconitase through a combination of genetic, physiological, and biochemical approaches. Two Arabidopsis T-DNA insertion mutants were identified as null mutants with early embryonic lethal phenotypes that could be rescued by GRXS15. Furthermore, we showed that recombinant GRXS15 is able to coordinate and transfer an ISC and that this coordination depends on reduced glutathione (GSH). We found the Arabidopsis GRXS15 able to complement growth defects based on disturbed ISC protein assembly of a yeast Δgrx5 mutant. Modeling of GRXS15 onto the crystal structures of related nonplant proteins highlighted amino acid residues that after mutation diminished GSH and subsequently ISC coordination, as well as the ability to rescue the yeast mutant. When used for plant complementation, one of these mutant variants, GRXS15K83/A, led to severe developmental delay and a pronounced decrease in aconitase activity by approximately 65%. These results indicate that mitochondrial GRXS15 is an essential protein in Arabidopsis, required for full activity of iron-sulfur proteins.