Severe combined deficiency of the 2-oxoacid dehydrogenases, associated with a defect in lipoate synthesis and accompanied by defects in complexes I, II, and III of the mitochondrial respiratory chain, is a rare autosomal recessive syndrome with no obvious causative gene defect. A candidate locus for this syndrome was mapped to chromosomal region 2p14 by microcell-mediated chromosome transfer in two unrelated families. Unexpectedly, analysis of genes in this area identified mutations in two different genes, both of which are involved in [Fe-S] cluster biogenesis. A homozygous missense mutation, c.545G>A, near the splice donor of exon 6 in NFU1 predicting a p.Arg182Gln substitution was found in one of the families. The mutation results in abnormal mRNA splicing of exon 6, and no mature protein could be detected in fibroblast mitochondria. A single base-pair duplication c.123dupA was identified in BOLA3 in the second family, causing a frame shift that produces a premature stop codon (p.Glu42Argfs(∗)13). Transduction of fibroblast lines with retroviral vectors expressing the mitochondrial, but not the cytosolic isoform of NFU1 and with isoform 1, but not isoform 2 of BOLA3 restored both respiratory chain function and oxoacid dehydrogenase complexes. NFU1 was previously proposed to be an alternative scaffold to ISCU for the biogenesis of [Fe-S] centers in mitochondria, and the function of BOLA3 was previously unknown. Our results demonstrate that both play essential roles in the production of [Fe-S] centers for the normal maturation of lipoate-containing 2-oxoacid dehydrogenases, and for the assembly of the respiratory chain complexes.

Iron-sulfur (Fe-S) clusters are required for the functions of mitochondrial aconitase, mammalian iron regulatory protein 1, and many other proteins in multiple subcellular compartments. Recent studies in Saccharomyces cerevisiae indicated that Fe-S cluster biogenesis also has an important role in mitochondrial iron homeostasis. Here we report the functional analysis of the mitochondrial and cytosolic isoforms of the human Fe-S cluster scaffold protein, ISCU. Suppression of human ISCU by RNAi not only inactivated mitochondrial and cytosolic aconitases in a compartment-specific manner but also inappropriately activated the iron regulatory proteins and disrupted intracellular iron homeostasis. Furthermore, endogenous ISCU levels were suppressed by iron deprivation. These results provide evidence for a coordinated response to iron deficiency that includes activation of iron uptake, redistribution of intracellular iron, and decreased utilization of iron in Fe-S proteins.

Iron-sulphur clusters are important cofactors for proteins that are involved in many cellular processes, including electron transport, enzymatic catalysis and regulation. The enzymes that catalyse the formation of iron-sulphur clusters are widely conserved from bacteria to humans. Recent studies in model systems and humans reveal that iron-sulphur proteins have important roles in mitochondrial iron homeostasis and in the pathogenesis of the human disease Friedreich ataxia.

Iron sulfur (Fe-S) clusters, preassembled on the ISCU scaffold, are transferred to target proteins or to intermediate scaffolds by a dedicated chaperone-cochaperone system. However, the molecular mechanisms that underlie substrate discrimination and guide delivery of nascent clusters to specific subsets of Fe-S recipients are poorly understood. Here, we identified interacting partners of the cochaperone HSC20 and discovered that LYR motifs are molecular signatures of specific recipient Fe-S proteins or accessory factors that assist Fe-S cluster delivery. In succinate dehydrogenase B, two LYR motifs engage the ISCU-HSC20-HSPA9 complex to aid incorporation of three Fe-S clusters within the final structure of complex II. Moreover, we show that members of the LYR motif family which assist assembly of complexes II or III, SDHAF1 and LYRM7, respectively, are HSC20 binding partners. Our studies unveil a network of interactions between HSC20 and LYR motif-containing proteins that are key to the assembly and function of complexes I, II, and III.

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Iron–sulfur (Fe–S) clusters are cofactors found in many proteins that have important redox, catalytic or regulatory functions. In mammalian cells, almost all known Fe–S proteins are found in the mitochondria, but at least one is found in the cytosol. Here we report cloning of the human homologs to IscU and NifU, iron‐binding proteins that play a critical role in Fe–S cluster assembly in bacteria. In human cells, alternative splicing of a common pre‐mRNA results in synthesis of two proteins that differ at the N‐terminus and localize either to the cytosol (IscU1) or to the mitochondria (IscU2). Biochemical analyses demonstrate that IscU proteins specifically associate with IscS, a cysteine desulfurase that is proposed to sequester inorganic sulfur for Fe–S cluster assembly. Protein complexes containing IscU and IscS can be found in the mitochondria as well as in the cytosol, implying that Fe–S cluster assembly takes place in multiple subcellular compartments in mammalian cells. The possible roles of the IscU proteins in mammalian cells and the potential implications of compartmentalization of Fe–S cluster assembly are discussed.

The LYR family consists of proteins of diverse functions that contain the conserved tripeptide 'LYR' near the N-terminus, and it includes Isd11, which was previously observed to have an important role in iron-sulfur (Fe-S) cluster biogenesis in Saccharomyces cerevisiae. Here, we have cloned and characterized human ISD11 and shown that human ISD11 forms a stable complex in vivo with the human cysteine desulfurase (ISCS), which generates the inorganic sulfur needed for Fe-S protein biogenesis. Similar to ISCS, we have found that ISD11 localizes to the mitochondrial compartment, as expected, but also to the nucleus of mammalian cells. Using RNA-interference techniques, we have shown that suppression of human ISD11 inactivated mitochondrial and cytosolic aconitases. In addition, ISD11 suppression activated iron-responsive element-binding activity of iron regulatory protein 1, increased protein levels of iron regulatory protein 2, and resulted in abnormal punctate ferric iron accumulations in cells. These results indicate that ISD11 is important in the biogenesis of Fe-S clusters in mammalian cells, and its loss disrupts normal mitochondrial and cytosolic iron homeostasis.

Iron and citrate are essential for the metabolism of most organisms, and regulation of iron and citrate biology at both the cellular and systemic levels is critical for normal physiology and survival. Mitochondrial and cytosolic aconitases catalyze the interconversion of citrate and isocitrate, and aconitase activities are affected by iron levels, oxidative stress and by the status of the Fe-S cluster biogenesis apparatus. Assembly and disassembly of Fe-S clusters is a key process not only in regulating the enzymatic activity of mitochondrial aconitase in the citric acid cycle, but also in controlling the iron sensing and RNA binding activities of cytosolic aconitase (also known as iron regulatory protein IRP1). This review discusses the central role of aconitases in intermediary metabolism and explores how iron homeostasis and Fe-S cluster biogenesis regulate the Fe-S cluster switch and modulate intracellular citrate flux.