Numerous cytosolic and nuclear proteins involved in metabolism, DNA maintenance, protein translation, or iron homeostasis depend on iron-sulfur (Fe/S) cofactors, yet their assembly is poorly defined. ...Here, we identify and characterize human CIA2A (FAM96A), CIA2B (FAM96B), and CIA1 (CIAO1) as components of the cytosolic Fe/S protein assembly (CIA) machinery. CIA1 associates with either CIA2A or CIA2B and the CIA-targeting factor MMS19. The CIA2B-CIA1-MMS19 complex binds to and facilitates assembly of most cytosolic-nuclear Fe/S proteins. In contrast, CIA2A specifically matures iron regulatory protein 1 (IRP1), which is critical for cellular iron homeostasis. Surprisingly, a second layer of iron regulation involves the stabilization of IRP2 by CIA2A binding or upon depletion of CIA2B or MMS19, even though IRP2 lacks an Fe/S cluster. In summary, CIA2B-CIA1-MMS19 and CIA2A-CIA1 assist different branches of Fe/S protein assembly and intimately link this process to cellular iron regulation via IRP1 Fe/S cluster maturation and IRP2 stabilization.
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•CIA1, CIA2A, and CIA2B function in cytosolic-nuclear iron-sulfur protein maturation•The CIA1-CIA2B-MMS19 targeting complex assists maturation of dedicated apoproteins•CIA2A is specific for iron-sulfur cluster assembly of IRP1 and stabilization of IRP2•CIA2A and CIA2B integrate iron-sulfur protein assembly and cellular iron regulation
Not long after the Big Bang, iron began to play a central role in the Universe and soon became mired in the tangle of biochemistry that is the prima essentia of life. Since life's addiction to iron ...transcends the oxygenation of the Earth's atmosphere, living things must be protected from the potentially dangerous mix of iron and oxygen. The human being possesses grams of this potentially toxic transition metal, which is shuttling through his oxygen-rich humor. Since long before the birth of modern medicine, the blood—vibrant red from a massive abundance of hemoglobin iron—has been a focus for health experts.
We describe the current understanding of iron metabolism, highlight the many important discoveries that accreted this knowledge, and describe the perils of dysfunctional iron handling.
Isaac Newton famously penned, “If I have seen further than others, it is by standing upon the shoulders of giants”. We hope that this review will inspire future scientists to develop intellectual pursuits by understanding the research and ideas from many remarkable thinkers of the past.
The history of iron research is a long, rich story with early beginnings, and is far from being finished. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.
► Iron has been a requirement for nearly all life from the very first organisms. ► We document the history of iron metabolism in health and disease. ► Unique and specific systems for handling iron evolved to control its catalytic nature. ► The plasma protein transferrin plays a key role in iron metabolism. ► Disturbed iron homeostasis can result in catastrophic consequences for humans.
Heme is a cofactor that is essential to almost all forms of life. The production of heme is a balancing act between the generation of the requisite levels of the end-product and protection of the ...cell and/or organism against any toxic substrates, intermediates and, in this case, end-product. In this review, we provide an overview of our understanding of the formation and regulation of this metallocofactor and discuss new research on the cell biology of heme homeostasis, with a focus on putative transmembrane transporters now proposed to be important regulators of heme distribution. The main text is complemented by a discussion dedicated to the intricate chemistry and biochemistry of heme, which is often overlooked when new pathways of heme transport are conceived.
Heme is essential for life, but is toxic when in excess. Two well-characterized mechanisms keep cellular heme at appropriate levels: it is synthesized in quantities that satisfy the requirements for the formation of heme proteins, while the levels of ‘toxic heme’ are maintained at a minimum.
Heme toxicity can be prevented by heme oxygenase 1 (HO-1), the enzyme that catalyzes heme degradation.
A new concept of heme ‘detoxification’, based on the hypothesis that toxic heme can be extruded from cells, is emerging. In particular, it was proposed that a putative ‘heme exporter’ on erythroid precursors is required to protect them from heme toxicity.
HO-1, recently shown to be expressed in erythroid progenitors, can degrade ‘free heme’. This, together with the fact that heme is not detectable in normal plasma, indicates that these and other cells (by extension) may not need to export heme.
In erythroid cells, more than 90% of transferrin-derived iron enters mitochondria where ferrochelatase inserts Fe2+ into protoporphyrin IX. However, the path of iron from endosomes to mitochondrial ...ferrochelatase remains elusive. The prevailing opinion is that, after its export from endosomes, the redox-active metal spreads into the cytosol and mysteriously finds its way into mitochondria through passive diffusion. In contrast, this study supports the hypothesis that the highly efficient transport of iron toward ferrochelatase in erythroid cells requires a direct interaction between transferrin-endosomes and mitochondria (the “kiss-and-run” hypothesis). Using a novel method (flow sub-cytometry), we analyze lysates of reticulocytes after labeling these organelles with different fluorophores. We have identified a double-labeled population definitively representing endosomes interacting with mitochondria, as demonstrated by confocal microscopy. Moreover, we conclude that this endosome-mitochondrion association is reversible, since a “chase” with unlabeled holotransferrin causes a time-dependent decrease in the size of the double-labeled population. Importantly, the dissociation of endosomes from mitochondria does not occur in the absence of holotransferrin. Additionally, mutated recombinant holotransferrin, that cannot release iron, significantly decreases the uptake of 59Fe by reticulocytes and diminishes 59Fe incorporation into heme. This suggests that endosomes, which are unable to provide iron to mitochondria, cause a “traffic jam” leading to decreased endocytosis of holotransferrin. Altogether, our results suggest that a molecular mechanism exists to coordinate the iron status of endosomal transferrin with its trafficking. Besides its contribution to the field of iron metabolism, this study provides evidence for a new intracellular trafficking pathway of organelles.
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•Iron is directly delivered from endosomes to mitochondria in erythroid cells.•Intra-endosomal transferrin saturation governs interorganellar association.•Endosomal mitochondrial interface regulates the cellular iron uptake.•The study's findings are important for the understanding of sideroblastic anemia.
Mammalian adrenodoxin (ferredoxin 1; Fdx1) is essential for the synthesis of various steroid hormones in adrenal glands. As a member of the 2Fe-2S cluster-containing ferredoxin family, Fdx1 reduces ...mitochondrial cytochrome P450 enzymes, which then catalyze; e.g., the conversion of cholesterol to pregnenolone, aldosterone, and cortisol. The high protein sequence similarity between Fdx1 and its yeast adrenodoxin homologue (Yah1) suggested that Fdx1, like Yah1, may be involved in the biosynthesis of heme A and Fe/S clusters, two versatile and essential protein cofactors. Our study, employing RNAi technology to deplete human Fdx1, did not confirm this expectation. Instead, we identified a Fdx1-related mitochondrial protein, designated ferredoxin 2 (Fdx2) and found it to be essential for heme A and Fe/S protein biosynthesis. Unlike Fdx1, Fdx2 was unable to efficiently reduce mitochondrial cytochromes P450 and convert steroids, indicating that the two ferredoxini isoforms are highly specific for their substrates in distinct biochemical pathways. Moreover, Fdx2 deficiency had a severe impact, via impaired Fe/S protein biogenesis, on cellular iron homeostasis, leading to increased cellular iron uptake and iron accumulation in mitochondria. We conclude that mammals depend on two distinct mitochondrial ferredoxins for the specific production of either steroid hormones or heme A and Fe/S proteins.
Members of the bacterial and mitochondrial iron-sulfur cluster (ISC) assembly machinery include the so-called A-type ISC proteins, which support the assembly of a subset of Fe/S apoproteins. The ...human genome encodes two A-type proteins, termed ISCA1 and ISCA2, which are related to Saccharomyces cerevisiae Isa1 and Isa2, respectively. An additional protein, Iba57, physically interacts with Isa1 and Isa2 in yeast. To test the cellular role of human ISCA1, ISCA2, and IBA57, HeLa cells were depleted for any of these proteins by RNA interference technology. Depleted cells contained massively swollen and enlarged mitochondria that were virtually devoid of cristae membranes, demonstrating the importance of these proteins for mitochondrial biogenesis. The activities of mitochondrial 4Fe-4S proteins, including aconitase, respiratory complex I, and lipoic acid synthase, were diminished following depletion of the three proteins. In contrast, the mitochondrial 2Fe-2S enzyme ferrochelatase and cellular heme content were unaffected. We further provide evidence against a localization and direct Fe/S protein maturation function of ISCA1 and ISCA2 in the cytosol. Taken together, our data suggest that ISCA1, ISCA2, and IBA57 are specifically involved in the maturation of mitochondrial 4Fe-4S proteins functioning late in the ISC assembly pathway.
The mitochondrion is well known for its key role in energy transduction. However, it is less well appreciated that it is also a focal point of iron metabolism. Iron is needed not only for heme and ...iron sulfur cluster (ISC)-containing proteins involved in electron transport and oxidative phosphorylation, but also for a wide variety of cytoplasmic and nuclear functions, including DNA synthesis. The mitochondrial pathways involved in the generation of both heme and ISCs have been characterized to some extent. However, little is known concerning the regulation of iron uptake by the mitochondrion and how this is coordinated with iron metabolism in the cytosol and other organelles (e.g., lysosomes). In this article, we discuss the burgeoning field of mitochondrial iron metabolism and trafficking that has recently been stimulated by the discovery of proteins involved in mitochondrial iron storage (mitochondrial ferritin) and transport (mitoferrin-1 and -2). In addition, recent work examining mitochondrial diseases (e.g., Friedreich's ataxia) has established that communication exists between iron metabolism in the mitochondrion and the cytosol. This finding has revealed the ability of the mitochondrion to modulate whole-cell iron-processing to satisfy its own requirements for the crucial processes of heme and ISC synthesis. Knowledge of mitochondrial iron-processing pathways and the interaction between organelles and the cytosol could revolutionize the investigation of iron metabolism.
Iron is a transition metal whose physicochemical properties make it the focus of vital biologic processes in virtually all living organisms. Among numerous roles, iron is essential for oxygen ...transport, cellular respiration, and DNA synthesis. Paradoxically, the same characteristics that biochemistry exploits make iron a potentially lethal substance. In the presence of oxygen, ferrous iron (Fe2+) will catalyze the production of toxic hydroxyl radicals from hydrogen peroxide. In addition, Fe3+ is virtually insoluble at physiologic pH. To protect tissues from deleterious effects of Fe, mammalian physiology has evolved specialized mechanisms for extracellular, intercellular, and intracellular iron handling. Here we show that developing erythroid cells, which are taking up vast amounts of Fe, deliver the metal directly from transferrin-containing endosomes to mitochondria (the site of heme biosynthesis), bypassing the oxygen-rich cytosol. Besides describing a new means of intracellular transport, our finding is important for developing therapies for patients with iron loading disorders.
Vertebrate heme synthesis requires three substrates: succinyl-CoA, which regenerates in the tricarboxylic acid cycle, iron and glycine. For each heme molecule synthesized, one atom of iron and eight ...molecules of glycine are needed. Inadequate delivery of iron to immature erythroid cells leads to a decreased production of heme, but virtually nothing is known about the consequence of an insufficient supply of extracellular glycine on the process of hemoglobinization. To address this issue, we exploited mice in which the gene encoding glycine transporter 1 (GlyT1) was disrupted. Primary erythroid cells isolated from fetal livers of GlyT1 knockout (GlyT1
) and GlyT1-haplodeficient (GlyT1
) embryos had decreased cellular uptake of 2
Cglycine and heme synthesis as revealed by a considerable decrease in 2-
Cglycine and
Fe incorporation into heme. Since GlyT1
mice die during the first postnatal day, we analyzed blood parameters of newborn pups and found that GlyT1
animals develop hypochromic microcytic anemia. Our finding that Glyt1-deficiency causes decreased heme synthesis in erythroblasts is unexpected, since glycine is a non-essential amino acid. It also suggests that GlyT1 represents a limiting step in heme and, consequently, hemoglobin production.
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