Professor Roland Lill

Biosynthesis of iron-sulfur proteins in eukaryotes

During the past decade our research has mainly been focused on the molecular basis of the biosynthesis of iron-sulfur (Fe/S) proteins in eukaryotes. These proteins carry an inorganic Fe/S cofactor which may be used for electron transfer, enzyme catalysis or sensing. It appears that Fe/S protein biogenesis is ancient process with pivotal importance for life. The pathway is tightly linked to several other biochemical processes such as cellular iron regulation, tRNA modification, and nuclear DNA maintenance. Impairment of Fe/S protein biogenesis or of individual Fe/S proteins leads to several neurodegenerative or haematological diseases.

In our work we have identified many of the known factors of Fe/S protein biogenesis, and we have contributed to the understanding of the underlying molecular mechanisms by combining in vivo and in vitro biochemical and cell biological approaches (see Reviews). We found that mitochondria play a crucial role in this pathway being involved in the maturation of all cellular Fe/S proteins, thus becoming indispensable for a living eukaryotic cell. Biogenesis is accomplished by three complex proteinaceous machineries. Mitochondrial Fe/S proteins require the iron-sulfur cluster (ISC) assembly machinery which was inherited by endosymbiosis of bacteria during evolution. Cytosolic and nuclear Fe/S proteins are assembled with the help of the cytosolic iron-sulfur protein assembly (CIA) machinery. This process additionally involves the function of the mitochondrial ISC assembly machinery which contributes an unknown compound exported to the cytosol by the mitochondrial ISC export apparatus. During the past years we have defined some basic principles of biosynthesis. Sulfur is provided by a mitochondrial cysteine desulfurase, and reduced to sulfide by a ferredoxin-dependent electron transfer chain. Fe/S cluster assembly is a two stage reaction. Fe/S clusters are first synthesised on mitochondrial or cytosolic scaffold proteins before they are transferred to and incorporated into apoproteins. The components of all three systems (more than 25 proteins) are highly conserved from yeast to man suggesting similar mechanisms of Fe/S protein assembly. The indispensable function of the ISC assembly machinery within mitochondria is impressively exemplified by the presence of ISC components within mitosomes present in Microsporidia or Giardia, i.e. highly reduced organelles which during evolution have lost all classical functions of mitochondria.

We also have contributed to the tight connection of Fe/S protein biogenesis to cellular iron homeostasis. The efficiency of mitochondrial Fe/S protein assembly serves as a critical sensor for cellular iron homeostasis, both in yeast and mammalian cells, yet via radically different mechanisms. While in yeast mitochondrial Fe/S protein defects lead to the transcriptional activation of the iron regulon (via the transcription factor Aft1 and glutaredoxins), impaired Fe/S protein maturation in mammals causes a post-transcriptional defect in the assembly of the cytosolic Fe/S protein IRP1 (iron regulatory protein 1) which is crucial for the regulation of iron uptake into the cell. Fe/S protein biosynthesis is of medical importance in that more than ten diseases are associated with ISC, CIA or Fe/S protein defects. For instance, depletion of the ISC assembly component frataxin leads to the neurodegenerative disease Friedreich’s ataxia, and a defect in the ISC export protein ABCB7 is associated with X-linked sideroblastic anemia and ataxia (XLSA/A). Fe/S protein defects in the nucleus impairing DNA replication and/or DNA repair link Fe/S protein biogenesis to numerous diseases including various forms of cancer.

Our current work is focussed on the molecular and structural understanding of Fe/S protein assembly. We try to identify and characterise further biogenesis components, and unravel the mechanisms underlying Fe/S protein biosynthesis in mitochondria, the cytosol and nucleus. In particular, we are interested in the contribution of mitochondria to cytosolic/nuclear Fe/S protein biogenesis. We also aim to better understand the molecular basis of the intimate link between Fe/S protein biogenesis and iron homeostasis.