The role of mitochondria in cellular iron-sulfur protein biogenesis: mechanisms, connected processes, and diseases

Abstract

Iron-sulfur (Fe/S) clusters belong to the most ancient protein cofactors in life, and fulfill functions in electron transport, enzyme catalysis, homeostatic regulation, and sulfur activation. The synthesis of Fe/S clusters and their insertion into apoproteins requires almost 30 proteins in the mitochondria and cytosol of eukaryotic cells. This review summarizes our current biochemical knowledge of mitochondrial Fe/S protein maturation. Because this pathway is essential for various extramitochondrial processes, we then explain how mitochondria contribute to the mechanism of cytosolic and nuclear Fe/S protein biogenesis, and to other connected processes including nuclear DNA replication and repair, telomere maintenance, and transcription. We next describe how the efficiency of mitochondria to assemble Fe/S proteins is used to regulate cellular iron homeostasis. Finally, we briefly summarize a number of mitochondrial "Fe/S diseases" in which various biogenesis components are functionally impaired owing to genetic mutations. The thorough understanding of the diverse biochemical disease phenotypes helps with testing the current working model for the molecular mechanism of Fe/S protein biogenesis and its connected processes.

Figure 1.

The biogenesis of cellular Fe/S proteins in eukaryotes and the links to cellular iron homeostasis, protein translation, and nuclear genome integrity. Eukaryotic Fe/S proteins are located in mitochondria, cytosol, and nucleus where they perform diverse functions in cellular metabolism and regulation. The mitochondrial Fe/S cluster (ISC) assembly machinery matures all organellar Fe/S proteins, and additionally contributes to the biogenesis of cytosolic and nuclear Fe/S proteins by producing an unknown sulfur-containing compound (X-S) that is exported to the cytosol and used by the cytosolic Fe/S protein assembly (CIA) machinery. Hence, mitochondria are directly responsible for the essential functions (e.g., of nuclear Fe/S proteins involved in DNA metabolism and genome maintenance). Additionally, the ISC assembly machinery exerts a regulatory role on cellular iron homeostasis (see text for details). Red and yellow circles indicate iron and sulfur ions, respectively.

Figure 2.

The three key steps of mitochondrial Fe/S protein assembly. In the first step (1), a [2Fe-2S] cluster is assembled on the Isu1 scaffold protein, which tightly interacts with the cysteine desulfurase complex Nfs1-Isd11 serving as the sulfur donor. The initially formed persulfide (-SSH) on Nfs1 by conversion of cysteine to alanine is possibly next transferred to Isu1. Fe/S cluster assembly on Isu1 then requires Yfh1 (frataxin) and the electron transfer chain consisting of the [2Fe-2S] ferredoxin Yah1 and ferredoxin reductase Arh1, which receives electrons from NAD(P)H. The mitochondrial carrier proteins Mrs3-Mrs4 supply ferrous iron. In the second step (2), the Fe/S cluster is released from Isu1, which involves the specialized Hsp70 chaperone system comprised of Ssq1, Jac1, and Mge1. This may lead to Fe/S cluster transfer to the monothiol glutaredoxin Grx5, which binds the metallo-cofactor in a glutathione (GSH)-dependent fashion. All of these ISC components form the core ISC assembly machinery, and are required for biogenesis of all cellular Fe/S proteins including those assembled by the CIA machinery (see Fig. 3) in the cytosol and nucleus (blue arrows). Finally, the third step (3) involves dedicated ISC-targeting factors, which transfer the cluster to specific apoproteins and assemble it into the polypeptide chains. Biogenesis of [4Fe-4S] clusters is facilitated by Isa1-Isa2 and Iba57, which may bind folate. Nfu1 and Aim1 are essential for the specific maturation of respiratory complexes I and II, and for lipoate synthase. Complex I assembly further needs the P-loop ATPase Ind1, which, similar to Nfu1, transiently coordinates a [4Fe-4S] cluster.

Figure 3.

A model for assembly of cytosolic and nuclear Fe/S proteins. The CIA machinery comprises eight known proteins. In a first step, a bridging [4Fe-4S] cluster is assembled on the Cfd1-Nbp35 scaffold complex. This reaction requires a sulfur source (X-S) generated by the mitochondrial ISC assembly machinery and exported by the mitochondrial ABC transporter Atm1 (human ABCB7). Generation of the functionally essential amino-terminal Fe/S cluster of Nbp35 (bottom) depends on the flavoprotein Tah18 and the Fe/S protein Dre2, which serve as an NADPH-dependent electron transfer chain. In the second step, the bridging Fe/S cluster is released from Cfd1-Nbp35, a reaction mediated by Nar1 and the CIA-targeting complex Cia1-Cia2-Mms19. The latter two proteins interact with target (apo)proteins and assure specific Fe/S cluster insertion. Biogenesis further requires, at an unknown step, the cytosolic multidomain monothiol glutaredoxins Grx3-Grx4 (human PICOT), which bind a glutathione-coordinated, bridging [2Fe-2S] cluster (not shown). These proteins also play a role in intracellular iron trafficking.

Figure 4.

Diseases associated with the assembly of mitochondrial and cytosolic Fe/S proteins. The model is a simplified version of Fig. 2 showing the human ISC protein names. The boxes highlight ISC proteins, which are mutated in two different kinds of Fe/S diseases. Red boxes indicate Fe/S diseases that are associated with a mitochondrial iron accumulation, whereas mutations in ISC proteins indicated by green boxes do not affect the iron metabolism. In addition, the model depicts a mitochondrial iron import disorder associated with a mutation in the inner membrane carrier mitoferrin 1 (gray box). XLSA/A, X-linked sideroblastic anemia and cerebellar ataxia; MMDS, multiple mitochondrial dysfunction syndrome.