Evolutionary conservation and in vitro reconstitution of microsporidian iron-sulfur cluster biosynthesis.

Microsporidians are obligate intracellular parasites that have minimized their genome content and sub-cellular structures by reductive evolution. Here, we demonstrate that cristae-deficient mitochondria (mitosomes) of Trachipleistophora hominis are the functional site of iron-sulfur cluster (ISC) assembly, which we suggest is the essential task of these organelles. Cell fractionation, fluorescence imaging and immunoelectron microscopy demonstrate that mitosomes contain a complete pathway for [2Fe-2S] cluster biosynthesis that we biochemically reconstituted using purified mitosomal ISC proteins. The T. hominis cytosolic iron-sulfur protein assembly (CIA) pathway includes the essential Cfd1-Nbp35 scaffold complex that assembles a [4Fe-4S] cluster as shown by spectroscopic methods in vitro. Phylogenetic analyses reveal that the ISC and CIA pathways are predominantly bacterial, but their cytosolic and nuclear target Fe/S proteins are mainly archaeal. This mixed evolutionary history of Fe/S-related proteins and pathways, and their strong conservation among highly reduced parasites, provides compelling evidence for the ancient chimeric ancestry of eukaryotes.

The role of mitochondria in cellular iron-sulfur protein biogenesis and iron metabolism.

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.

Cytosolic monothiol glutaredoxins function in intracellular iron sensing and trafficking via their bound iron-sulfur cluster.

Iron is an essential nutrient for cells. It is unknown how iron, after its import into the cytosol, is specifically delivered to iron-dependent processes in various cellular compartments. Here, we identify an essential function of the conserved cytosolic monothiol glutaredoxins Grx3 and Grx4 in intracellular iron trafficking and sensing. Depletion of Grx3/4 specifically impaired all iron-requiring reactions in the cytosol, mitochondria, and nucleus, including the synthesis of Fe/S clusters, heme, and di-iron centers. These defects were caused by impairment of iron insertion into proteins and iron transfer to mitochondria, indicating that intracellular iron is not bioavailable, despite highly elevated cytosolic levels. The crucial task of Grx3/4 is mediated by a bridging, glutathione-containing Fe/S center that functions both as an iron sensor and in intracellular iron delivery. Collectively, our study uncovers an important role of monothiol glutaredoxins in cellular iron metabolism, with a surprising connection to cellular redox and sulfur metabolisms.

Localization and functionality of microsporidian iron-sulphur cluster assembly proteins.

Microsporidia are highly specialized obligate intracellular parasites of other eukaryotes (including humans) that show extreme reduction at the molecular, cellular and biochemical level. Although microsporidia have long been considered as early branching eukaryotes that lack mitochondria, they have recently been shown to contain a tiny mitochondrial remnant called a mitosome. The function of the mitosome is unknown, because microsporidians lack the genes for canonical mitochondrial functions, such as aerobic respiration and haem biosynthesis. However, microsporidial genomes encode several components of the mitochondrial iron-sulphur (Fe-S) cluster assembly machinery. Here we provide experimental insights into the metabolic function and localization of these proteins. We cloned, functionally characterized and localized homologues of several central mitochondrial Fe-S cluster assembly components for the microsporidians Encephalitozoon cuniculi and Trachipleistophora hominis. Several microsporidial proteins can functionally replace their yeast counterparts in Fe-S protein biogenesis. In E. cuniculi, the iron (frataxin) and sulphur (cysteine desulphurase, Nfs1) donors and the scaffold protein (Isu1) co-localize with mitochondrial Hsp70 to the mitosome, consistent with it being the functional site for Fe-S cluster biosynthesis. In T. hominis, mitochondrial Hsp70 and the essential sulphur donor (Nfs1) are still in the mitosome, but surprisingly the main pools of Isu1 and frataxin are cytosolic, creating a conundrum of how these key components of Fe-S cluster biosynthesis coordinate their function. Together, our studies identify the essential biosynthetic process of Fe-S protein assembly as a key function of microsporidian mitosomes.

The effects of mitochondrial iron homeostasis on cofactor specificity of superoxide dismutase 2.

Many metalloproteins have the capacity to bind diverse metals, but in living cells connect only with their cognate metal cofactor. In eukaryotes, this metal specificity can be achieved through metal-specific metallochaperone proteins. Herein, we describe a mechanism whereby Saccharomyces cerevisiae manganese superoxide dismutase (SOD2) preferentially binds manganese over iron based on the differential bioavailability of these ions within mitochondria. The bulk of mitochondrial iron is normally unavailable to SOD2, but when mitochondrial iron homeostasis is disrupted, for example, by mutations in S. cerevisiae mtm1, ssq1 and grx5, iron accumulates in a reactive form that potently competes with manganese for binding to SOD2, inactivating the enzyme. Studies in mtm1 mutants indicate that iron inactivation of SOD2 involves the Mrs3p/Mrs4p mitochondrial carriers and iron-binding frataxin (Yfh1p). A small pool of SOD2-reactive iron also exists under normal iron homeostasis conditions and binds SOD2 when mitochondrial manganese is low. The ability to control this reactive pool of iron is critical to maintaining SOD2 activity and has important potential implications for oxidative stress in disorders of iron overload.