Frataxin is a novel mitochondrial iron-storage protein
Frataxin is a highly conserved mitochondrial protein that plays a critical role in iron homeostasis. A deficiency of frataxin is the primary cause of Friedreich ataxia (FRDA) (http://www.ncbi.nlm.nih.gov/Omim), an autosomal recessive degenerative disease. Abnormal cellular iron distribution and increased oxidative damage are associated with frataxin defects in yeast as well as FRDA but the underlying pathogenic mechanism is unknown. We have shown that ferrous iron stimulates self-assembly of yeast frataxin (Yfh1p) into regular multimers capable of storing as many as 3,000 atoms of iron in soluble form (Adamec et al. 2000).
Atomic Force Microscopy of assembled yeast frataxin
Yfh1p binds Fe2+ and keeps it in reduced and readily available form for a limited time in aqueous solution at neutral pH. During this time, Yfh1p can protect other biomolecules from iron-induced oxidative damage and provide Fe2+ ions for heme synthesis in the presence of ferrochelatase and protoporphyrin IX. The Fe2+ ions bound to Yfh1p are progressively oxidized and stored in soluble form within the assembled protein (Adamec et al., submitted).
Human frataxin can rescue iron homeostasis and OXPHOS activity when expressed in yeast cells lacking Yfh1p, indicating that what we have learned about Yfh1p is applicable to the human protein as well (Cavadini et al. 2000a).
We have recently investigated the function of human frataxin in E. coli and S. cerevisiae. When expressed in E. coli, the mature form of human frataxin assembles into a stable homopolymer that can bind at least 10 atoms of iron per molecule of frataxin. The iron loaded protein can be detected on native PAGE by either protein or iron staining, which demonstrates a stable association between frataxin and iron. As analyzed by size-exclusion chromatography and EM, the assembled protein consists of globular particles of ~1 MDa and ordered rod-shaped polymers of these particles that accumulate small electron-dense iron cores, shown in the figure below.
When the human frataxin precursor is expressed in S. cerevisiae, the mitochondrially-generated mature form is separated by gel filtration into monomer and a high-MW form of >600 kDa. Low- and high-MW forms of frataxin are also detected in mouse heart indicating that frataxin can assemble under native conditions. In radiolabeled yeast cells, human frataxin is recovered by immunoprecipitation with ~5 atoms of 55Fe bound per molecule (Cavadini et al., in press).
Our working hypothesis is that the ability to control Fe2+oxidation enables frataxin to promote iron usage within the mitochondrion or iron export to the cytoplasm, preventing participation of Fe2+ ions in ROS reactions and mitochondrial accumulation of insoluble ferric iron oxides (Patel & Isaya, 2001)(see model below).
(A) Proposed function of frataxin
(B) Consequences of frataxin deficiency