Normal and Friedreich ataxia cells express different isoforms of frataxin with complementary roles in iron-sulfur cluster assembly

Friedreich ataxia (FRDA) is an autosomal recessive degenerative disease caused by insufficient expression of frataxin (FXN), a mitochondrial iron-binding protein required for Fe-S cluster assembly. The development of treatments to increase FXN levels in FRDA requires elucidation of the steps involved in the biogenesis of functional FXN. The FXN mRNA is translated to a precursor polypeptide that is transported to the mitochondrial matrix and processed to at least two forms, FXN^42-210 and FXN^81-210. Previous reports suggested that FXN^42-210 is a transient processing intermediate, whereas FXN^81-210 represents the mature protein. However, we find that both FXN^42-210 and FXN^81-210 are present in control cell lines and tissues at steady-state, and that FXN ^42-210 is consistently more depleted than FXN^81-210 in samples from FRDA patients. Moreover, FXN^42-210 and FXN ^81-210 have strikingly different biochemical properties. A shorter N terminus correlates with monomeric configuration, labile iron binding, and dynamic contacts with components of the Fe-S cluster biosynthetic machinery, i.e. the sulfur donor complex NFS1·ISD11 and the scaffold ISCU. Conversely, a longer N terminus correlates with the ability to oligomerize, store iron, and form stable contacts with NFS1·ISD11 and ISCU. Monomeric FXN^81-210 donates Fe²⁺ for Fe-S cluster assembly on ISCU, whereas oligomeric FXN^42-210 donates either Fe²⁺ or Fe³⁺. These functionally distinct FXN isoforms seem capable to ensure incremental rates of Fe-S cluster synthesis from different mitochondrial iron pools. We suggest that the levels of both isoforms are relevant to FRDA pathophysiology and that the FXN^81-210/FXN^42-210 molar ratio should provide a useful parameter to optimize FXN augmentation and replacement therapies.