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Friedreich's ataxia is a neurodegenerative disease caused by mutations in the nuclear gene encoding frataxin (FRDA). FRDA is synthesized with an N-terminal signal sequence, which is removed after import into mitochondria. We have shown that FRDA was imported efficiently into isolated mammalian or yeast mitochondria. In both cases, the processing cleavage that removed the N-terminal signal sequence occurred in a single step on import, generating mature products of identical mobility. The processing cleavage could be reconstituted by incubating the FRDA preprotein with rat or yeast matrix processing peptidase (MPP) expressed in Escherichia coli. We used these assays to evaluate the import and processing of an altered form of FRDA containing the disease-causing I154F mutation. No effects on import or maturation of this mutated FRDA were observed. Likewise, no effects were observed on import and maturation of the yeast frataxin homolog (Yfh1p) carrying a homologous I130F mutation. These results argue against the possibility that the I154F mutation interferes with FRDA function via effects on maturation. Other mutations can be screened for effects on FRDA biogenesis as described here, by evaluating import into isolated mitochondria and by testing maturation with purified MPP.

Maturation of Frataxin Within Mammalian and Yeast Mitochondria: One-Step Processing by Matrix Processing Peptidase