Iron-sulfur (Fe/S) proteins are involved in a wide variety of cellular processes such as enzymatic reactions, respiration, cofactor biosynthesis, ribosome biogenesis, regulation of gene expression, ...and DNA-RNA metabolism. Assembly of Fe/S clusters, small inorganic cofactors, is assisted by complex proteinaceous machineries, which use cysteine as a source of sulfur, combine it with iron to synthesize an Fe/S cluster on scaffold proteins, and finally incorporate the cluster into recipient apoproteins. In eukaryotes, such as yeast and human cells, more than 20 components are known that facilitate the maturation of Fe/S proteins in mitochondria, cytosol, and nucleus. These biogenesis components also perform crucial roles in other cellular pathways, e.g., in the regulation of iron homeostasis or the modification of tRNA. Numerous diseases including several neurodegenerative and hematological disorders have been associated with defects in Fe/S protein biogenesis, underlining the central importance of this process for life.
Iron-sulfur (Fe/S) clusters require a complex set of proteins to become assembled and incorporated into apoproteins in a living cell. Researchers have described three distinct assembly systems in ...eukaryotes that are involved in the maturation of cellular Fe/S proteins. Mitochondria are central for biogenesis. They contain the ISC-the iron-sulfur cluster assembly machinery that was inherited from a similar system of eubacteria in evolution and is involved in biogenesis of all cellular Fe/S proteins. The basic principle of mitochondrial (and bacterial) Fe/S protein maturation is the synthesis of the Fe/S cluster on a scaffold protein before the cluster is transferred to apoproteins. Biogenesis of cytosolic and nuclear Fe/S proteins is facilitated by the cytosolic iron-sulfur protein assembly (CIA) apparatus. This process requires the participation of mitochondria that export a still unknown component via the ISC export machinery, including an ABC transporter.
Iron-sulfur (Fe/S) clusters are essential protein cofactors crucial for many cellular functions including DNA maintenance, protein translation, and energy conversion. De novo Fe/S cluster synthesis ...occurs on the mitochondrial scaffold protein ISCU and requires cysteine desulfurase NFS1, ferredoxin, frataxin, and the small factors ISD11 and ACP (acyl carrier protein). Both the mechanism of Fe/S cluster synthesis and function of ISD11-ACP are poorly understood. Here, we present crystal structures of three different NFS1-ISD11-ACP complexes with and without ISCU, and we use SAXS analyses to define the 3D architecture of the complete mitochondrial Fe/S cluster biosynthetic complex. Our structural and biochemical studies provide mechanistic insights into Fe/S cluster synthesis at the catalytic center defined by the active-site Cys of NFS1 and conserved Cys, Asp, and His residues of ISCU. We assign specific regulatory rather than catalytic roles to ISD11-ACP that link Fe/S cluster synthesis with mitochondrial lipid synthesis and cellular energy status.
The essential process of iron-sulfur (Fe/S) cluster assembly (ISC) in mitochondria occurs in three major phases. First, 2Fe-2S clusters are synthesized on the scaffold protein ISCU2; second, these ...clusters are transferred to the monothiol glutaredoxin GLRX5 by an Hsp70 system followed by insertion into 2Fe-2S apoproteins; third, 4Fe-4S clusters are formed involving the ISC proteins ISCA1–ISCA2–IBA57 followed by target-specific apoprotein insertion. The third phase is poorly characterized biochemically, because previous in vitro assembly reactions involved artificial reductants and lacked at least one of the in vivo-identified ISC components. Here, we reconstituted the maturation of mitochondrial 4Fe-4S aconitase without artificial reductants and verified the 2Fe-2S-containing GLRX5 as cluster donor. The process required all components known from in vivo studies (i.e., ISCA1–ISCA2–IBA57), yet surprisingly also depended on mitochondrial ferredoxin FDX2 and its NADPH-coupled reductase FDXR. Electrons from FDX2 catalyze the reductive 2Fe-2S cluster fusion on ISCA1–ISCA2 in an IBA57-dependent fashion. This previously unidentified electron transfer was occluded during previous in vivo studies due to the earlier FDX2 requirement for 2Fe-2S cluster synthesis on ISCU2. The FDX2 function is specific, because neither FDX1, a mitochondrial ferredoxin involved in steroid production, nor other cellular reducing systems, supported maturation. In contrast to ISC factorassisted 4Fe-4S protein assembly, 2Fe-2S cluster transfer from GLRX5 to 2Fe-2S apoproteins occurred spontaneously within seconds, clearly distinguishing the mechanisms of 2Fe-2S and 4Fe-4S protein maturation. Our study defines the physiologically relevant mechanistic action of late-acting ISC factors in mitochondrial 4Fe-4S cluster synthesis, trafficking, and apoprotein insertion.
Mitochondria play a key role in iron metabolism in that they synthesize heme, assemble iron–sulfur (Fe/S) proteins, and participate in cellular iron regulation. Here, we review the latter two topics ...and their intimate connection. The mitochondrial Fe/S cluster (ISC) assembly machinery consists of 17 proteins that operate in three major steps of the maturation process. First, the cysteine desulfurase complex Nfs1–Isd11 as the sulfur donor cooperates with ferredoxin–ferredoxin reductase acting as an electron transfer chain, and frataxin to synthesize an 2Fe–2S cluster on the scaffold protein Isu1. Second, the cluster is released from Isu1 and transferred toward apoproteins with the help of a dedicated Hsp70 chaperone system and the glutaredoxin Grx5. Finally, various specialized ISC components assist in the generation of 4Fe–4S clusters and cluster insertion into specific target apoproteins. Functional defects of the core ISC assembly machinery are signaled to cytosolic or nuclear iron regulatory systems resulting in increased cellular iron acquisition and mitochondrial iron accumulation. In fungi, regulation is achieved by iron-responsive transcription factors controlling the expression of genes involved in iron uptake and intracellular distribution. They are assisted by cytosolic multidomain glutaredoxins which use a bound Fe/S cluster as iron sensor and additionally perform an essential role in intracellular iron delivery to target metalloproteins. In mammalian cells, the iron regulatory proteins IRP1, an Fe/S protein, and IRP2 act in a post-transcriptional fashion to adjust the cellular needs for iron. Thus, Fe/S protein biogenesis and cellular iron metabolism are tightly linked to coordinate iron supply and utilization. This article is part of a Special Issue entitled: Cell Biology of Metals.
► Iron–sulfur protein biogenesis in mitochondria is a conserved and essential process. ► The process can be sub‐divided into three major biosynthetic steps. ► The process serves as a major regulator for cellular iron homeostasis. ► The process is relevant for neurodegenerative, hematological and metabolic diseases.
Synthesis of iron-sulfur (Fe/S) clusters in living cells requires scaffold proteins for both facile synthesis and subsequent transfer of clusters to target apoproteins. The human mitochondrial ISCU2 ...scaffold protein is part of the core ISC (iron-sulfur cluster assembly) complex that synthesizes a bridging 2Fe-2S cluster on dimeric ISCU2. Initial iron and sulfur loading onto monomeric ISCU2 have been elucidated biochemically, yet subsequent 2Fe-2S cluster formation and dimerization of ISCU2 is mechanistically ill-defined. Our structural, biochemical and cell biological experiments now identify a crucial function of the universally conserved N-terminal Tyr35 of ISCU2 for these late reactions. Mixing two, per se non-functional ISCU2 mutant proteins with oppositely charged Asp35 and Lys35 residues, both bound to different cysteine desulfurase complexes NFS1-ISD11-ACP, restores wild-type ISCU2 maturation demonstrating that ionic forces can replace native Tyr-Tyr interactions during dimerization-induced 2Fe-2S cluster formation. Our studies define the essential mechanistic role of Tyr35 in the reaction cycle of de novo mitochondrial 2Fe-2S cluster synthesis.
Most eukaryotes contain iron-sulfur cluster (ISC) assembly proteins related to Saccharomyces cerevisiae Isa1 and Isa2. We show here that Isa1 but not Isa2 can be functionally replaced by the ...bacterial relatives IscA, SufA, and ErpA. The specific function of these “A-type” ISC proteins within the framework of mitochondrial and bacterial Fe/S protein biogenesis is still unresolved. In a comprehensive in vivo analysis, we show that S. cerevisiae Isa1 and Isa2 form a complex that is required for maturation of mitochondrial 4Fe-4S proteins, including aconitase and homoaconitase. In contrast, Isa1-Isa2 were dispensable for the generation of mitochondrial 2Fe-2S proteins and cytosolic 4Fe-4S proteins. Targeting of bacterial 2Fe-2S and 4Fe-4S ferredoxins to yeast mitochondria further supported this specificity. Isa1 and Isa2 proteins are shown to bind iron in vivo, yet the Isa1-Isa2-bound iron was not needed as a donor for de novo assembly of the 2Fe-2S cluster on the general Fe/S scaffold proteins Isu1-Isu2. Upon depletion of the ISC assembly factor Iba57, which specifically interacts with Isa1 and Isa2, or in the absence of the major mitochondrial 4Fe-4S protein aconitase, iron accumulated on the Isa proteins. These results suggest that the iron bound to the Isa proteins is required for the de novo synthesis of 4Fe-4S clusters in mitochondria and for their insertion into apoproteins in a reaction mediated by Iba57. Taken together, these findings define Isa1, Isa2, and Iba57 as a specialized, late-acting ISC assembly subsystem that is specifically dedicated to the maturation of mitochondrial 4Fe-4S proteins.
Background: The precise molecular function of Isa1 and Isa2 proteins in Fe/S protein biogenesis is unknown.
Results: The Isa1-Isa2 complex binds iron in vivo and acts late in the synthesis of 4Fe-4S clusters.
Conclusion: Mitochondria possess specialized factors for maturation of 4Fe-4S proteins.
Significance: This study defines the role of Isa proteins in mitochondrial Fe/S protein biogenesis.
Assembly of mitochondrial iron-sulfur (Fe/S) proteins is a key process of cells, and defects cause many rare diseases. In the first phase of this pathway, ten Fe/S cluster (ISC) assembly components ...synthesize and insert 2Fe-2S clusters. The second phase is dedicated to the assembly of 4Fe-4S proteins, yet this part is poorly understood. Here, we characterize the BOLA family proteins Bol1 and Bol3 as specific mitochondrial ISC assembly factors that facilitate 4Fe-4S cluster insertion into a subset of mitochondrial proteins such as lipoate synthase and succinate dehydrogenase. Bol1-Bol3 perform largely overlapping functions, yet cannot replace the ISC protein Nfu1 that also participates in this phase of Fe/S protein biogenesis. Bol1 and Bol3 form dimeric complexes with both monothiol glutaredoxin Grx5 and Nfu1. Complex formation differentially influences the stability of the Grx5-Bol-shared Fe/S clusters. Our findings provide the biochemical basis for explaining the pathological phenotypes of patients with mutations in BOLA3.
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.
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![Mitochondrial [4Fe-4S] prot...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0027-8424_1091-6490/small "Mitochondrial [4Fe-4S] protein assembly involves reductive [2Fe-2S] cluster fusion on ISCA1–ISCA2 by electron flow from ferredoxin FDX2")
