Research in our group concentrates on the molecular mechanisms of the biogenesis of iron-sulfur (Fe/S) proteins in eukaryotes. These proteins contain Fe/S clusters as inorganic co-factors, and they participate in electron transfer reactions, enzyme catalysis and the regulation of various cellular processes. Important cellular processes such as respiration, citric acid cycle, numerous metabolic reactions, DNA synthesis and repair, ribosome biogenesis, tRNA modification and iron regulation require assistance of these ancient protein cofactors. Assembly of the Fe/S clusters and their insertion into apoproteins in a living cell is a complex process involving more than 25 proteins. The mitochondrial ISC assembly machinery (see Figure) was inherited from bacteria and is involved in biogenesis of all cellular Fe/S proteins, i.e. proteins in mitochondria, cytosol and nucleus. Maturation of extra-mitochondrial proteins additionally needs components of the mitochondrial ISC export machinery and proteins of the cytosolic CIA machinery. According to a model mitochondria export a factor (X in Figure) which is essential for Fe/S protein biogenesis in the cytosol by the CIA machinery.

            The process of iron-sulfur biogenesis is essential for the viability of the eukaryotic cell. Surprisingly, the indispensable character of Fe/S protein biogenesis can be explained by crucial functions performed by extra-mitochondrial Fe/S proteins. One of these essential cytosolic Fe/S proteins (Rli1) performs an central role in the biogenesis and function of cytosolic ribosomes. Apparently, two central processes of a eukaryotic cell, namely Fe/S protein biogenesis in mitochondria and protein translation in the cytosol, are intimately linked via this protein. Recently mitosomes were discovered in Microsporidia, Giardia and Entamoeba. These organelles are evolutionary reduced mitochondria having lost most known functions of classical mitochondria. These remnant organelles appear to have maintained Fe/S protein biogenesis as the only (known) function again demonstrating the importance of the ISC machinery.

            Another topic of our research is the intracellular compartmentalization and regulation of iron. In particular, we are interested in the components and mechanisms involved in iron transport and regulation in mitochondria. Mitochondrial import depends on reduced (ferrous) iron, a membrane potential, and involves the mitochondrial carrier proteins Mrs3/Mrs4 (see Figure). Additional iron transporters remain to be identified. Defects in mitochondrial ISC components result in an increased cellular iron uptake and in iron accumulation in mitochondria. The molecular basis underlying the (dys)regulation of cellular iron compartmentation as a result of Fe/S protein defects is unclear to date.

            Our studies are performed in the model organism baker’s yeast (Saccharomyces cerevisiae) and in human cell culture. Mutations in ISC biogenesis components are causative of human disease, e.g., the iron storage diseases Friedreich’s ataxia (FRDA) and X-chromosome-linked sideroblastic anemia and cerebellar ataxia (XLSA/A). Defects in (putative) Fe/S proteins involved in DNA repair are connected to multiple diseases such as skin cancer and Fanconi anemia.

  We are grateful for generous financial support by

DFG, SFB 593, Behring-Röntgen-Stiftung, FCI, MPG

Figure: Working model for the maturation of cellular iron-sulfur proteins.

The biogenesis of iron-sulfur proteins in mitochondria requires the ISC assembly machinery. This machinery is comprised of the cysteine desulfurase complex Nfs1-Isd11 releasing sulfur from cysteine, and frataxin (Yfh1) that presumably serves as an iron donor. The two elements are combined in an unknown chemical mechanism on the Isu proteins which serve as a scaffold for de novo Fe/S cluster synthesis. This step further requires an electron transfer chain consisting of NADH, ferredoxin reductase (Arh1) and ferredoxin (Yah1). The Isu-bound Fe/S cluster is labile and is transferred from the Isu proteins to target apoproteins by functional involvement of the Hsp70- and DnaJ-like chaperones Ssq1 and Jac1, respectively, and the glutaredoxin Grx5. Specialized ISC assembly proteins catalyze formation of aconitase-like and radical SAM proteins (Isa1-Isa2-Iba57) and complex I (Ind1).

The maturation of extra-mitochondrial iron-sulfur proteins requires the participation of both the ISC assembly and export machineries. The latter comprises the ABC transporter Atm1 of the inner membrane which exports an unknown compound (X) to the cytosol, the sulfhydryl oxidase Erv1 of the intermembrane space, and the tripeptide glutathione (GSH). Since Nfs1 is required inside mitochondria for extra-mitochondrial Fe/S protein biogenesis, at least sulfur has to be exported from mitochondria. Maturation of the Fe/S apoproteins in the cytosol is accomplished by a number of recently identified proteins of the CIA machinery. Cfd1-Nbp35 form a complex and serve as scaffolds for Fe/S cluster formation, whereas Nar1, Cia1 and Cia2 perform functions in Fe/S cluster transfer to apoproteins. the function of the CIA protein Dre2 is still unknown.