Evidence that yeast frataxin is not an iron storage protein in vivo

Yeast cells deficient in the yeast frataxin homolog (Yfh1p) accumulate iron in their mitochondria. Whether this iron is toxic, however, remains unclear. We showed that large excesses of iron in the growth medium did not inhibit growth and did not decrease cell viability. Increasing the ratio of mitochondrial iron-to-Yfh1p by decreasing the steady-state level of Yfh1p to less than 100 molecules per cell had very few deleterious effects on cell physiology, even though the mitochondrial iron concentration greatly exceeded the iron-binding capacity of Whip in these conditions. Mossbauer spectroscopy and FPLC analyses of whole mitochondria or of isolated mitochondrial matrices showed that the chemical and biochemical forms of the accumulated iron in mitochondria of mutant yeast strains (Delta yfh1, Delta ggc1 and Delta ssq1) displayed a nearly identical distribution. This was also the case for Delta ggc1 cells, in which Yfhip was overproduced. In these mitochondria, most of the iron was insoluble, and the ratio of soluble-to-insoluble iron did not change when the amount of Yfh1p was increased up to 4500 molecules per cell. Our results do not privilege the hypothesis of Yfh1 p being an iron storage protein in vivo. (C) 2010 Elsevier BM. All rights reserved.