Structure of the human frataxin-bound iron-sulfur cluster assembly complex provides insight into its activation mechanism

Abstract

Iron-sulfur clusters (ISC) are essential in all life forms and carry out many crucial cellular functions.The core machinery for de novo ISC biosynthesis, located in the mitochondria matrix, is a fiveprotein complex containing the cysteine desulfurase NFS1 that is activated by frataxin (FXN), scaffold protein ISCU, accessory protein ISD11, and acyl-carrier protein ACP. Deficiency in FXN leads to the loss-of-function neurodegenerative disorder Friedreich's ataxia (FRDA). Recently crystal structures depicting the inactive 3-and 4-way sub-complexes of the ISC biosynthesis machinery, lacking the key activator FXN, have been determined. Here, the 3.2 Å resolution cryo-electron microscopy structure of the FXN-bound active human complex, containing two copies of the NFS1-ISD11-ACP-ISCU-FXN hetero-pentamer, delineates for the first time in any organism the interactions of FXN with the component proteins. FXN binds at the interface of two NFS1 and one ISCU subunits, modifying the local environment of a bound zinc ion that would otherwise inhibit NFS1 activity in complexes without FXN. Our structure sheds light on how FXN facilitates ISC production through unlocking the zinc inhibition and stabilizing key loop conformations of NFS1 and ISCU at the protein-protein interfaces, and offers an explanation of how FRDA clinical mutations affect complex formation and FXN activation.

Iron-Sulfur Cluster Complex Assembly in the Mitochondria of Arabidopsis thaliana

“…Moreover, the critical amino acid residues for the interaction have been mapped [ 47 ]. Recently, it was reported the structure of a heterodecamer multiprotein complex that would be key for the biosynthesis of mitochondrial Fe-S clusters in humans containing two copies of each of NFS1, frataxin (FXN), ISCU, ISD11, and ACP [ 48 ].…”

“…Also, FXN Asp124 form a salt-bridge with NFS1 Arg289, and carbonyl backbone groups from FXN residues Glu121 and Tyr123 could interact via hydrogen-bonds with NFS1 Arg119 and Arg272. It was also reported that FXN directly interacts with a second NFS1 subunit near the catalytic site via hydrophobic interaction between FXN Trp155 and NFS1 Leu386, and a hydrogen-bond between FXN Asn146 and NFS1 Ala384 [ 48 ].…”

“…The analysis of the predicted model and the identification of key amino acids present in AtFH, AtNFS1, and AtISD11 showed that some structural characteristics are conserved with respect to the human ISC complex, however, some differences were found. It was reported that HsFH can interact with ISD11 [ 42 ], and also with ISCU at the interface that this protein forms with Nfs1 [ 48 ]. We were unable to find any interaction between AtFH and AtISD11 or AtFH and AtISU1 (not shown), suggesting that AtISU1 would have weak binding or would not bind to the proposed plant ISC complex.…”

“…The amino acid residues involved in such interaction were mapped and results showed that some residues present in the helix regions of the ISD11 protein would have an essential role in the interaction [ 47 ]. Moreover, it has recently been shown in human mitochondria that the complex involved in the synthesis of Fe-S clusters is a heterodecamer composed of five proteins: NFS1, frataxin, the scaffold protein ISCU, ISD11, and an acyl-carrier protein (ACP) [ 48 ].…”

Molecular and Cellular Substrates for the Friedreich Ataxia. Significance of Contactin Expression and of Antioxidant Administration

“…The neural and the cardiovascular phenotypes share a common pathogenic mechanism, based on the evidence that FA is a mitochondrial disorder, affecting at the same time the nervous and the hearth tissue [ 6 , 12 , 20 , 21 , 22 , 23 ] as a consequence of the mutation of the gene encoding the mitochondrial protein Frataxin [ 24 ]. This protein plays a relevant role in modulating oxidative phosphorylation, consistent with the contribution of the mutation of the underlying gene to the disorder pathogenesis, in turn implying mitochondrial iron and sulfur accumulation within the nervous and the cardiac tissues, which results in their degeneration [ 7 , 12 , 23 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ].…”