Human Molecular Genetics Advance Access originally published online on August 25, 2008
Human Molecular Genetics 2008 17(22):3521-3531; doi:10.1093/hmg/ddn244
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
The in vivo mitochondrial two-step maturation of human frataxin
1 IGBMC (Institut de Génétique et de Biologie Moléculaire et Cellulaire), 1 rue Laurent Fries, BP 10142, Illkirch F-67400, France, 2 INSERM, U596, Illkirch F-67400, France, 3 CNRS, UMR7104, Illkirch F-67400, France, 4 Université Louis Pasteur, Strasbourg F-67000, France 5 Collège de France, Chaire de génétique humaine, Illkirch F-67400, France
* To whom correspondence should be addressed. Tel: +33 388653264; Fax: +33 388653246; Email: hpuccio{at}igbmc.fr
Received July 28, 2008; Accepted August 12, 2008
Deficiency in the nuclear-encoded mitochondrial protein frataxin causes Friedreich ataxia (FRDA), a progressive neurodegenerative disorder associating spinocerebellar ataxia and cardiomyopathy. Although the exact function of frataxin is still a matter of debate, it is widely accepted that frataxin is a mitochondrial iron chaperone involved in iron–sulfur cluster and heme biosynthesis. Frataxin is synthesized as a precursor polypeptide, directed to the mitochondrial matrix where it is proteolytically cleaved by the mitochondrial processing peptidase to the mature form via a processing intermediate. The mature form was initially reported to be encoded by amino acids 56–210 (m56-FXN). However, two independent reports have challenged these studies describing two different forms encoded by amino acids 78–210 (m78-FXN) and 81–210 (m81-FXN). Here, we provide evidence that mature human frataxin corresponds to m81-FXN, and can rescue the lethal phenotype of fibroblasts completely deleted for frataxin. Furthermore, our data demonstrate that the migration profile of frataxin depends on the experimental conditions, a behavior which most likely contributed to the confusion concerning the endogenous mature frataxin. Interestingly, we show that m56-FXN and m78-FXN can be generated when the normal maturation process of frataxin is impaired, although the physiological relevance is not clear. Furthermore, we determine that the d-FXN form, previously reported to be a degradation product, corresponds to m78-FXN. Finally, we demonstrate that all frataxin isoforms are generated and localized within the mitochondria. The clear identification of the N-terminus of mature FXN is an important step for designing therapeutic approaches for FRDA based on frataxin replacement.