The Nfs1 interacting protein Isd11 has an essential role in Fe/S cluster biogenesis in mitochondria

Abstract

Formation of iron/sulfur (Fe/S) clusters, protein translocation and protein folding are essential processes in the mitochondria of Saccharomyces cerevisiae. In a systematic approach to characterize essential proteins involved in these processes, we identified a novel essential protein of the mitochondrial matrix, which is highly conserved from yeast to human and which we termed Isd11. Depletion of Isd11 caused a strong reduction in the levels of the Fe/S proteins aconitase and the Rieske protein, and a massive decrease in the enzymatic activities of aconitase and succinate dehydrogenase. Incorporation of iron into the Fe/S protein Leu1 and formation of the Fe/S cluster containing holoform of the mitochondrial ferredoxin Yah1 were inhibited in the absence of Isd11. This strongly suggests that Isd11 is required for the assembly of Fe/S proteins. We show that Isd11 forms a stable complex with Nfs1, the cysteine desulfurase of the mitochondrial machinery for Fe/S cluster assembly. In the absence of Isd11, Nfs1 is prone to aggregation. We propose that Isd11 acts together with Nfs1 in an early step in the biogenesis of Fe/S proteins.

The Fe/S Cluster Assembly Protein Isd11 Is Essential for tRNA Thiolation in Trypanosoma brucei

“…In this work we demonstrate that Isd11 is essential for Fe/S cluster assembly of both mitochondrial and cytosolic proteins of T. brucei, like in other eukaryotes (6,7,9,11,13). We also show for the first time that Isd11 is required for thiolation of both cytosolic and mitochondrial tRNAs and that Isd11 is not solely an Nfs chaperone for Fe/S cluster assembly, whereas Mtu1 is essential for mitochondrial tRNA thiolation only.…”

“…Isd11, a eukaryotic invention in the Fe/S cluster assembly pathway, is present in all sequenced eukaryotic genomes as a single-copy gene (6,7,12). Interestingly, the model flagellate T. brucei represents an exception, because a Blast search identified three putative homologs of Isd11 in its genome.…”

“…Isd11 Forms a Stable Complex with Nfs and IscU-In yeast, microsporidian, and human cells, Isd11 was shown to be associated with the Nfs protein (6,7,9,11,13). To explore whether similar interactions of Isd11 occur in trypanosomes, a TAPtagged version of this protein was generated and expressed under control of a tetracycline-inducible promoter.…”

“…Somewhat unexpectedly, a third component of this Fe/S cluster assembly complex, a small protein named Isd11, was identified as a stable and essential binding partner of the eukaryotic Nfs protein (6 -9). Based on in vitro experiments, Nfs was originally thought to catalyze sulfur transfer by itself (10), but it was recently shown to require the assistance of Isd11 for the formation of Fe/S clusters on the IscU scaffold (6,7). Co-expression in Escherichia coli of Nfs and Isd11 from the microsporidian Trachipleistophora hominis confirmed that it is indeed a complex of both tightly bound proteins that represents the functional cysteine desulfurase (11).…”

“…Isd11 belongs to the LYR family (9) and in yeast stabilizes mitochondrial Nfs1, which is in its absence prone to aggregation. Although in yeast Isd11 seems to be confined to the mitochondrion (6,7), it is equally abundant in the mitochondrion and nucleus of human cells, where its depletion resulted in inactivation of Fe/S cluster-containing cytosolic and mitochondrial aconitases. Furthermore, its down-regulation in human cells disrupts iron homeostasis (9).…”