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Review
. 2021 Jun 2;22(11):6006.
doi: 10.3390/ijms22116006.

Molecular Details of the Frataxin-Scaffold Interaction during Mitochondrial Fe-S Cluster Assembly

Courtney J Campbell  1 , Ashley E Pall  1 , Akshata R Naik  1 , Lindsey N Thompson  1 , Timothy L Stemmler  1
Affiliations
Review

Molecular Details of the Frataxin-Scaffold Interaction during Mitochondrial Fe-S Cluster Assembly

Courtney J Campbell et al. Int J Mol Sci. .

Abstract

Iron-sulfur clusters are essential to almost every life form and utilized for their unique structural and redox-targeted activities within cells during many cellular pathways. Although there are three different Fe-S cluster assembly pathways in prokaryotes (the NIF, SUF and ISC pathways) and two in eukaryotes (CIA and ISC pathways), the iron-sulfur cluster (ISC) pathway serves as the central mechanism for providing 2Fe-2S clusters, directly and indirectly, throughout the entire cell in eukaryotes. Proteins central to the eukaryotic ISC cluster assembly complex include the cysteine desulfurase, a cysteine desulfurase accessory protein, the acyl carrier protein, the scaffold protein and frataxin (in humans, NFS1, ISD11, ACP, ISCU and FXN, respectively). Recent molecular details of this complex (labeled NIAUF from the first letter from each ISC protein outlined earlier), which exists as a dimeric pentamer, have provided real structural insight into how these partner proteins arrange themselves around the cysteine desulfurase, the core dimer of the (NIAUF)2 complex. In this review, we focus on both frataxin and the scaffold within the human, fly and yeast model systems to provide a better understanding of the biophysical characteristics of each protein alone and within the FXN/ISCU complex as it exists within the larger NIAUF construct. These details support a complex dynamic interaction between the FXN and ISCU proteins when both are part of the NIAUF complex and this provides additional insight into the coordinated mechanism of Fe-S cluster assembly.

Keywords: Fe-S cluster biosynthesis; ISC machinery; frataxin.

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Conflict of interest statement

There are no conflicts of interest.

Figures

Figure 1
Figure 1
The mitochondrial iron–sulfur cluster (ISC) assembly machinery. Schematic of the de novo mitochondrial Fe–S cluster biosynthetic pathway. Iron (Fe+2) is imported via mitoferrin (MFRN1/2) while cysteine desulfurase (NFS1) provides sulfur, from L-cysteine, in the form of a persulfide (-SSH). ISD11 and ACP stabilize NFS1. The 2Fe–2S cluster is formed on the scaffold protein (ISCU) and ferredoxin (FDX2) provides the electron required for this process. FXN promotes NFS1 activity and Fe loading of ISCU. HSC20, HSPA9 and GLRX5 receive the 2Fe–2S cluster from the ISC complex and promote downstream delivery. The conversion of 2Fe–2S to 4Fe–4S is still uncharacterized but involves a complex of ISA1, ISA2 and IBA57 proteins. The 2Fe–2S cluster is exported out of the mitochondria as an unknown sulfur-containing moiety (X-S) via the ATP binding cassette (ABCB7).
Figure 2
Figure 2
Crystal structure of the human NIAUF complex. (A) The dimeric pentamer of the ISC assembly complex has NFS1 as the central linker that forms the dimeric interface. (B) ISC assembly complex rotated 90 degrees. NFS1 is represented in green, ISD11 is represented in orange, Acp (bacterial) is represented in blue, ISCU is represented in yellow and FXN is represented in red. PDB ID: 6NZU.
Figure 3
Figure 3
Molecular details of frataxin protein orthologs. (A) Sequence homology between human, fly, and yeast frataxin orthologs. Brown colored letters represent iron-binding residues, green indicates residues interacting with cysteine desulfurase, yellow indicates residues involved in ISCU partnering, and purple indicates residues implicated in stability. Likewise, roman numerals are used to represent residues involved in multiple interactions. Roman numeral I represents iron-binding residues, III indicates residues implicated in protein stability, IV indicates residues involved in ISCU partnering. (B) Crystal structure of human FXN (PDB ID: 1EKG). (C) Crystal structure of fly frataxin modeled using PyMol. (D) Crystal structure of yeast frataxin (PDB ID:2GA5).
Figure 4
Figure 4
Key residues of biophysical relevance on the human FXN structure. Crystal structure of human FXN with positions of residues for: (A) Fe binding (brown), (B) cysteine desulfurase binding (green), (C) ISCU binding (Yellow) and (D) stability (purple).
Figure 5
Figure 5
Molecular details of ISCU protein orthologs. (A) Sequence homology between human, fly, and yeast scaffold orthologs. Magenta colored letters indicate active site residues, green indicates residues interacting with cysteine desulfurase, red indicates residues involved in frataxin partnering, and purple indicates residues implicated in stability. Roman numerals are used to represent residues involved in multiple interactions. Roman numeral I represents iron-binding residues, II is active site residues, III indicates residues implicated in protein stability and IV indicates resides involved in frataxin partnering. (B) Crystal structure of human ISCU (PDB ID: 6NZU). (C) Crystal structure of human ISCU (PDB ID: 6NZU) with active site residues highlighted in magenta.
Figure 6
Figure 6
Key residues of biophysical relevance on the human ISCU structure. Crystal structure of human ISCU with positions of residues for: (A) Fe binding (brown), (B) cysteine desulfurase binding (green), (C) FXN binding (red) and (D) stability (purple).
Figure 7
Figure 7
Structure of the human FXN–ISCU interface from the NIAUF crystal structure. (A) Residues with direct interaction on FXN (red) and ISCU (yellow). (B) Residues at the ISCU zinc-loaded active site with the cysteine loop and catalytic residue C381 of NFS1 (green) depicted behind the protein partners. (C) Residues at the FXN–ISCU interface whose association are supported by biochemical data. Adapted from PDB ID:6NZU.
Figure 8
Figure 8
The structural orientation of ISCU residue H137 in the Zn-NIAU crystal structure compared to Zn-NIAUF. (A) The Zn-loaded NIAU crystal structure (PDB: 5WLW) with the imidazole side chain of human ISCU (yellow) residue H137 pointing towards the ISCU active site. (B) The Zn-loaded NIAUF structure with the imidazole side chain of human ISCU residue H137 projecting away from the ISCU active site to orient parallel with the aromatic plane of human FXN (red) residue W155. Adapted from PDB ID:6NZU.
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