Iron-Sulfur Cluster Biosynthesis

123

Friedreich ataxia (FRDA) is an autosomal recessive degenerative disease caused by insufficient expression of frataxin (FXN), a mitochondrial iron-binding protein required for Fe-S cluster assembly. The development of treatments to increase FXN levels in FRDA requires elucidation of the steps involved in the biogenesis of functional FXN. The FXN mRNA is translated to a precursor polypeptide that is transported to the mitochondrial matrix and processed to at least two forms, FXN Friedreich ataxia (FRDA) 2 (OMIM number 229300) is an autosomal recessive disease with an estimated incidence of 1:40,000. Most FRDA patients are apparently healthy at birth and during the first 5-10 years of life; then their gait becomes increasingly unsteady and wide-based and their voluntary movements uncoordinated. Many patients develop hypertrophic cardiomyopathy as well as diabetes, muscle weakness, and skeletal deformities. Although cognitive functions remain largely intact during disease progression, patients develop significant communication difficulties due to dysarthria, which is often compounded by vision and hearing loss. The majority of patients eventually become wheelchair-bound and dependent on others for most daily activities. Cardiac failure is a frequent cause of death at a young age (1).The FRDA locus encodes a mitochondrial protein designated frataxin (FXN), which is expressed at much lower levels in FRDA patients compared with normal individuals (2). In most patients, FXN deficiency results from the presence of an expanded GAA repeat in the first intron of the FRDA gene (2) that causes transcriptional silencing (reviewed in Ref.3). Although FXN is ubiquitously expressed, certain cells (dorsal root ganglia neurons, cardiomyocytes, and pancreatic beta cells) are exquisitely sensitive to frataxin depletion, and the degenerative loss of these particular cells accounts for the major clinical aspects of FRDA (1).Extensive biochemical studies have shown that frataxins across species are conserved iron-binding proteins that can either provide iron for Fe-S cluster assembly and heme synthesis or store iron as a stable mineral (reviewed in Ref. 4). The loss of these properties accounts for impaired iron utilization and increased iron toxicity linked to frataxin deficiency in the mitochondria of such diverse organisms as Saccharomyces cerevisiae, Drosophila, mouse, and humans (5-8). In humans, the mitochondrial alterations caused by FXN deficiency lead to tissue-specific changes in various cellular pathways involved in antioxidant, metabolic, and inflammatory responses, thereby amplifying the pathophysiology of FRDA and promoting disease progression (9 -13).

Section: From the Departments Of Pediatric And Adolescent Medicine And mentioning

confidence: 99%

Normal and Friedreich Ataxia Cells Express Different Isoforms of Frataxin with Complementary Roles in Iron-Sulfur Cluster Assembly

Gakh

Bedekovics

Duncan

et al. 2010

123

“…In human cells data indicate that the interaction is mediated by Isd11 (20). In the case of bacteria, which do not have an Isd11 homolog, the available data indicate that the cysteine desulfurase Nfs1, the ortholog of eukaryotic IscS, is necessary for the interaction (21).…”

mentioning

confidence: 99%

Binding of Yeast Frataxin to the Scaffold for Fe-S Cluster Biogenesis, Isu

Wang

Craig

2008

Friedreich ataxia is caused by reduced activity of frataxin, a conserved iron-binding protein of the mitochondrial matrix, thought to supply iron for formation of Fe-S clusters on the scaffold protein Isu. Frataxin binds Isu in an iron-dependent manner in vitro. However, the biological relevance of this interaction and whether in vivo the interaction between frataxin and Isu is mediated by adaptor proteins is a matter of debate. Here, we report that alterations of conserved, surface-exposed residues of yeast frataxin, which have deleterious effects on cell growth, impair Fe-S cluster biogenesis and interaction with Isu while altering neither iron binding nor oligomerization. Our results support the idea that the surface of the ␤-sheet, adjacent to the acidic, iron binding ridge, is important for interaction of Yfh1 with the Fe-S cluster scaffold and point to a critical role for frataxin in Fe-S cluster biogenesis.Friedreich ataxia (FRDA), 2 a progressive neurodegenerative disease, is caused by a decrease in the level, and in some cases activity, of frataxin, a highly conserved iron-binding protein of the mitochondrial matrix (1-3). The effects of deficiency of frataxin in human cells and Yfh1 in yeast are strikingly similar: decrease in the activity of Fe-S-containing enzymes and increase in intramitochondrial iron levels (4, 5). Not surprisingly considering their 65% amino acid identity, the tertiary structures of frataxin and Yfh1 are very similar, a ␤-sheet packed against two ␣-helices (6 -8). The larger helix and part of the adjacent ␤-strand form an acidic ridge that binds iron. While a Yfh1 monomer can bind 2 ferrous irons (8, 9), it also undergoes iron-dependent oligomerization to form trimers and ultimately oligomers having up to 48 subunits that bind Ͼ2000 iron atoms (10, 11). Alterations in the acidic residues of the ridge can affect oligomerization and thus robust iron binding, alleviating any ability to serve an iron storage function (12, 13).Evidence indicates that Yfh1/frataxin plays an important role in the formation of Fe-S clusters (5, 14) on the highly conserved scaffold Isu prior to transfer to recipient apoproteins (15). Its proposed role as an iron donor is supported by its high affinity, iron-dependent interaction with Isu and its ability to donate iron for cluster formation on Isu in vitro (16). In addition, evidence strongly indicates that Nfs1, an essential cysteine desulfurase, which is required for Fe-S cluster formation on Isu in vivo, functions as the sulfur donor (17, 18). Another essential protein, Isd11, is also required for in vivo Fe-S cluster formation (15,19). It forms a stable complex with Nfs1 and is required for Nfs1 to be stable in vivo.The manner in which the activities of these proteins, including their physical interactions, are coordinated to allow cluster formation on Isu in vivo is a matter of debate. Most relevant to this report, it has been recently argued that the relevant in vivo interaction between frataxin homologs and scaffolds is indirect, despite the fact th...

“…Conversely, Nfs1 and the low levels of Isu1 initially present in ϳ80 -670-kDa fractions shifted to the highest of these fractions (Fig. 3, L versus M and N versus O), possibly reflecting interactions of the Nfs1-Isu1 complex with ironloaded Yfh1 oligomers, as observed for bacterial frataxin in vitro (8). Thus, the broad molecular mass shift exhibited by FIGURE 2.…”

Section: Strainmentioning

confidence: 87%

“…Although one group reported that Yfh1 oligomerization is dispensable in vivo (38), we later showed that mutations affecting Yfh1 assembly are asymptomatic in unstressed cells but become harmful during stress exposure and cause synthetic lethality in cells lacking copper-zinc superoxide dismutase (34). Iron-dependent assembly has also been characterized for the Escherichia coli orthologue, CyaY, and shown to be required for interaction of CyaY with the ISC assembly proteins IscU and IscS in vitro (8,39). Unlike the yeast or E. coli orthologues, human frataxin assembles at physiological iron concentrations during expression in E. coli or in human heart but not in vitro (40,41), indicating that different mechanisms govern frataxin assembly in different species.…”

mentioning

confidence: 99%

“…In either monomeric or oligomeric form, frataxin can serve as a Fe 2ϩ donor for ferrochelatase (2,3), the iron-sulfur cluster (ISC) 2 scaffold protein, IscU/Isu1 (4,5), and the [4Fe-4S] 2ϩ enzyme, aconitase (6). Moreover, frataxin assembles into particles that convert redoxactive iron to a stable mineral, which remains soluble within the protein and is inactive in the generation of free radicals (2,5,7,8). The crystal structure of yeast frataxin trimer has suggested that this form of the protein may be able to perform both iron delivery and iron detoxification (9).…”

mentioning

confidence: 99%

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Assembly of the Iron-binding Protein Frataxin in Saccharomyces cerevisiae Responds to Dynamic Changes in Mitochondrial Iron Influx and Stress Level

Gakh

Smith

Isaya

2008

Defects in frataxin result in Friedreich ataxia, a genetic disease characterized by early onset of neurodegeneration, cardiomyopathy, and diabetes. Frataxin is a conserved mitochondrial protein that controls iron needed for iron-sulfur cluster assembly and heme synthesis and also detoxifies excess iron. Studies in vitro have shown that either monomeric or oligomeric frataxin delivers iron to other proteins, whereas ferritin-like frataxin particles convert redox-active iron to an inert mineral. We have investigated how these different forms of frataxin are regulated in vivo. In Saccharomyces cerevisiae, only monomeric yeast frataxin (Yfh1) was detected in unstressed cells when mitochondrial iron uptake was maintained at a steady, low nanomolar level. Increments in mitochondrial iron uptake induced stepwise assembly of Yfh1 species ranging from trimer to >24-mer, independent of interactions between Yfh1 and its major ironbinding partners, Isu1/Nfs1 or aconitase. The rate-limiting step in Yfh1 assembly was a structural transition that preceded conversion of monomer to trimer. This step was induced, independently or synergistically, by mitochondrial iron increments, overexpression of wild type Yfh1 monomer, mutations that stabilize Yfh1 trimer, or heat stress. Faster assembly kinetics correlated with reduced oxidative damage and higher levels of aconitase activity, respiratory capacity, and cell survival. However, deregulation of Yfh1 assembly resulted in Yfh1 aggregation, aconitase sequestration, and mitochondrial DNA depletion. The data suggest that Yfh1 assembly responds to dynamic changes in mitochondrial iron uptake or stress exposure in a highly controlled fashion and that this may enable frataxin to simultaneously promote respiratory function and stress tolerance.