The in vivo mitochondrial two-step maturation of human frataxin

Abstract

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 (m(56)-FXN). However, two independent reports have challenged these studies describing two different forms encoded by amino acids 78-210 (m(78)-FXN) and 81-210 (m(81)-FXN). Here, we provide evidence that mature human frataxin corresponds to m(81)-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 m(56)-FXN and m(78)-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 m(78)-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.

Dual role of the mitochondrial protein frataxin in astrocytic tumors

“…The transgenic lines U87-FXN and U87-Neo showed patterns, which were identical for all markers analyzed, proving these cell lines to be derivatives of U87MG cells, which had been originally obtained from the ATCC. Three of these markers, amelogenin (X), vWA (alleles 15,17) and THO1 (allele 9.3) are published for the parental line U87MG in the ATCC database (http://www.ATCC.org). OLN-93 was originally derived from a primary rat brain glial culture and resembles primary oligodendrocytes.…”

Frataxin Depletion in Yeast Triggers Up-regulation of Iron Transport Systems before Affecting Iron-Sulfur Enzyme Activities

“…In human cells, the presence of an extramitochondrial form of frataxin has been reported that could be involved in IRP1 regulation (42). It should be noted, however, that the existence of such an extramitochondrial form of frataxin has been questioned by other authors (43). To analyze the existence of such an extramitochondrial form of Yfh1, yeast cells carrying an HA-tagged Yfh1 (strain BQS204) were fractionated and analyzed by Western blot for the presence of Yfh1-HA in the cytosolic and mitochondrial fractions (Fig.…”

The alteration of the C‐terminal region of human frataxin distorts its structural dynamics and function

“…In this organelle, the protein is first processed to the intermediate , and then to the mature form, FXN 81-210 [1,2]. Mature FXN plays an essential role in iron homeostasis; it may act as an iron chaperone delivering iron to enzyme partners during heme and Fe-S cluster biosynthesis [3][4][5].…”