Atm1p is an ABC transporter localized in the mitochondrial inner membrane; it functions to export an unknown species into the cytosol and is involved in cellular iron metabolism. Depletion or deletion of Atm1p causes Fe accumulation in mitochondria and a defect in cytosolic Fe/S cluster assembly, but reportedly not a defect in mitochondrial Fe/S cluster assembly. In this study the nature of the accumulated Fe was examined using Mössbauer spectroscopy, EPR, electronic absorption spectroscopy, X-ray absorption spectroscopy, and electron microscopy. The Fe that accumulated in aerobically grown cells was in the form of Fe(III) phosphate nanoparticles similar to that which accumulates in yeast frataxin Yfh1p-deleted or yeast ferredoxin Yah1p-depleted cells. Relative to WT mitochondria, Fe/S cluster and heme levels in Atm1p-depleted mitochondria from aerobic cells were significantly diminished. Atm1p-depletion also caused a build-up of nonheme Fe(II) ions in the mitochondria and an increase in oxidative damage. Atm1p-depleted mitochondria isolated from anaerobically grown cells exhibited WT levels of Fe/S clusters and hemes, and they did not hyper-accumulate Fe. Atm1p-depleted cells lacked Leu1p activity, regardless of whether they were grown aerobically or anaerobically. These results indicate that Atm1p does not participate in mitochondrial Fe/S cluster assembly, and that the species exported by Atm1p is required for cytosolic Fe/S cluster assembly. The Fe/S cluster defect and the Fe-accumulation phenotype, resulting from the depletion of Atm1p in aerobic cells (but not in anaerobic cells), may be secondary effects that are observed only when cells are exposed to oxygen during growth. Reactive oxygen species generated under these conditions might degrade iron-sulfur clusters and lower heme levels in the organelle.

Yah1p, an [Fe 2S 2]-containing ferredoxin located in the matrix of Saccharomyces cerevisiae mitochondria, functions in the synthesis of Fe/S clusters and heme a prosthetic groups. EPR, Mossbauer spectroscopy, and electron microscopy were used to characterize the Fe that accumulates in Yah1p-depleted isolated intact mitochondria. Gal- YAH1 cells were grown in standard rich media (YPD and YPGal) under O 2 or argon atmospheres. Mitochondria were isolated anaerobically, then prepared in the as-isolated redox state, the dithionite-treated state, and the O 2-treated state. The absence of strong EPR signals from Fe/S clusters when Yah1p was depleted confirms that Yah1p is required in Fe/S cluster assembly. Yah1p-depleted mitochondria, grown with O 2 bubbling through the media, accumulated excess Fe (up to 10 mM) that was present as 2-4 nm diameter ferric nanoparticles, similar to those observed in mitochondria from yfh1Delta cells. These particles yielded a broad isotropic EPR signal centered around g = 2, characteristic of superparamagnetic relaxation. Treatment with dithionite caused Fe (3+) ions of the nanoparticles to become reduced and largely exported from the mitochondria. Fe did not accumulate in mitochondria isolated from cells grown under Ar; a significant portion of the Fe in these organelles was in the high-spin Fe (2+) state. This suggests that the O 2 used during growth of Gal- YAH1 cells is responsible, either directly or indirectly, for Fe accumulation and for oxidizing Fe (2+) --> Fe (3+) prior to aggregation. Models are proposed in which the accumulation of ferric nanoparticles is caused either by the absence of a ligand that prevents such precipitation in wild-type mitochondria or by a more oxidizing environment within the mitochondria of Yah1p-depleted cells exposed to O 2. The efficacy of reducing accumulated Fe along with chelating it should be considered as a strategy for its removal in diseases involving such accumulations.

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