title: Normal and Friedreich ataxia cells express different isoforms of frataxin with complementary roles in iron-sulfur cluster assembly.
abstract: 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 Fe(2+) for Fe-S cluster assembly on ISCU, whereas oligomeric FXN(42-210) donates either Fe(2+) or Fe(3+). 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.
authors.affiliation: Department of Pediatric & Adolescent Medicine, Mayo Clinic, Rochester, Minnesota 55905, USA.
authors.forename: Oleksandr
authors.initials: O
authors.lastname: Gakh
authors.forename: Tibor
authors.initials: T
authors.lastname: Bedekovics
authors.forename: Samantha F
authors.initials: SF
authors.lastname: Duncan
authors.forename: Douglas Y
authors.initials: DY
authors.lastname: Smith
authors.forename: Donald S
authors.initials: DS
authors.lastname: Berkholz
authors.forename: Grazia
authors.initials: G
authors.lastname: Isaya
mesh_terms: D000293:Adolescent; D000328:Adult; D013437:Carbon-Sulfur Lyases / Q000235:genetics / Q000378:metabolism; D002461:Cell Line, Transformed; D002648:Child; D005260:Female; D005621:Friedreich Ataxia / Q000235:genetics / Q000378:metabolism*; D005786:Gene Expression Regulation; D006801:Humans; D007501:Iron / Q000378:metabolism; D033862:Iron-Binding Proteins / Q000096:biosynthesis* / Q000235:genetics; D035925:Iron-Regulatory Proteins / Q000235:genetics / Q000378:metabolism; D007506:Iron-Sulfur Proteins / Q000235:genetics / Q000378:metabolism; D008297:Male; D008928:Mitochondria / Q000235:genetics / Q000378:metabolism*; D024101:Mitochondrial Proteins / Q000096:biosynthesis* / Q000235:genetics; D014176:Protein Biosynthesis / Q000235:genetics; D020033:Protein Isoforms / Q000096:biosynthesis / Q000235:genetics; D011498:Protein Precursors / Q000096:biosynthesis* / Q000235:genetics; D017434:Protein Structure, Tertiary; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D000096923:Frataxin
chemical_list: D013437:Carbon-Sulfur Lyases; D007501:Iron; D033862:Iron-Binding Proteins; D035925:Iron-Regulatory Proteins; D007506:Iron-Sulfur Proteins; D024101:Mitochondrial Proteins; D020033:Protein Isoforms; D011498:Protein Precursors; D012333:RNA, Messenger; D000096923:Frataxin; C506160:ISCU protein, human; C544445:LYRM4 protein, human; C513492:NFS1 protein, human