Friedreich's ataxia (FDRA) is a rare neurodegenerative and cardiac disease affecting about 1 birth in 50,000. FDRA is due to a mutation of the gene encoding frataxin (FXN), a protein located in the mitochondria, the "energy center" of the cell. Frataxin mutation is responsible for a lack of synthesis of "iron-sulfur clusters", assemblies of iron and sulfide ions essential to the activity of the proteins to which they are linked to. The iron-sulfur proteins have essential biological functions: energy production, protein synthesis, maintenance of the integrity of the genome... The mechanism of Fe-S clusters assembly is very poorly understood. In eukaryotes, two machineries operates for Fe-S clusters biosynthesis: the ISC machinery located in the mitochondria and the CIA machinery located in the cytosol. The ISC machinery encompasses several proteins (Figure 1): the scaffold protein ISCU on which Fe-S clusters are assembled; NFS1, a pyridoxal phosphate-dependent cysteine desulfurase wich provides sulfur in the form of a cysteine bound persulfide (Cys-SSH); the [2Fe-2S] ferredoxin FDX2 associated with its cognate reductase FDXR which delivers electrons; and three proteins including frataxin, both of elusive functions. Scientists imagine that the assembly process is based on a reduction of persulfide to sulphide ions, potentially catalyzed by FDX2, and that would be coordinated with the insertion of iron in ISCU to prevent futile cycles of sulphide ion production in the absence of iron, and promote rapid bonding of sulphide ions to iron to prevent their diffusion, potentially toxic. However, this hypothesis has not been formally proven. Frataxin has been suggested to act as a chaperone bringing iron to ISCU. It is still a matter of debate.

One of the limitations to the study of Fe-S clusters assembly is to obtain an in vitro functional ISC machinery. ISC must be reconstituted with no less than seven purified proteins. The heterogeneity of the results collected so far suggests that these conditions have not yet been obtained. In the previous reported reconstructions, Fe-S clusters are formed from free sulfide ions generated via an iron-uncoupled process. In an article published in Nature Communications, researchers from SBIGEM (CEA-Joliot / I2BC), in collaboration with French^[1], German^[2] and Spanish^[3] teams, show that one of the biases with previous reconstructions is the systematic presence of a zinc ion in the assembly site of ISCU. Zinc hinders iron binding but fosters reduction of the persulfide of NFS1 by L-cysteine which leads to release of free sulfide. Fe-S clusters are then formed in ISCU by the association of free sulfide and iron ions but this process cannot be considered as physiologically relevant: the synthesis being slow, inefficient and not confined to ISCU.

By exchanging zinc with iron, researchers managed to generate an iron-loaded ISCU protein allowing Fe-S clusters synthesis in a manner that appears to reproduce the physiological process. The assembly mechanism comprises a minimum of four steps (Figure 2) initiated by the insertion of iron which triggers the transfer of the persulfide of NFS1 to ISCU. This persulfide is subsequently reduced into sulfide by FDX2. Data indicate that a key feature of the Fe-ISCU based reaction is the iron-dependency of both persulfide transfer and reduction, which most likely ensure that sulfure transfer and sulfide production are coordinated with iron availability in ISCU, thereby preventing futile persulfide transfer cycles and allowing instantaneous binding of the nascent sulfide to the nearby iron. Eventually, a center [2Fe2S] will form, probably by ISCU dimerization. Frataxine is not necessary for the insertion of iron in ISCU but accelerates the transfer of NFS1 persulfide to ISCU.

These results establish the first model of the assembly process of Fe-S clusters highlighting a confined sulfide production in ISCU catalyzed by FDX2 and coordinated with the presence of iron in the assembly site. In addition, data show that frataxin operates in Fe-S cluster biogenesis as a kinetic activator of persulfide transfer. This work should help to better understand the pathophysiology of Friedreich's ataxia and perhaps pave the way for the development of new therapeutic approaches.