The mitochondrial proteins Ssq1 and Jac1 are required for the assembly of iron sulfur clusters in mitochondria

Mitochondria of the yeast Saccharomyces cerevisiae contain three different Hsp70 chaperones, Ssc1, Ecm10 and Ssq1. Ssc1 is an essential protein that m ediates the import of nuclear-encoded proteins into the organelle and their subsequent folding. The nucleotide state of Ssc1 is thereby regulated by t he nucleotide exchange factor Mge1. Here, we show that Mge1 interacts with Ssq1 in an ATP-dependent manner, suggesting that Mge1 also regulates Ssq1 f unction. in contrast to Ssc1, Ssq1 does not associate with the Tim44 subuni t of the protein translocating complex, indicating a different function of both chaperones. Mutants in Ssql were reported to have low levels of iron s ulfur (FeS) cluster-containing enzymes. Employing an assay that allowed us to monitor the conversion of the apoform of mitochondrial ferredoxin into i ts FeS-containing holoform, Ssq1 was demonstrated to be required for the Fe S cluster assembly in mitochondria. The mitochondrial DnaJ homolog Tad is c rucial for this process, whereas Mdj1 function is dispensable. Furthermore, the presence of frataxin is necessary for FeS cluster assembly into ferred oxin suggesting a role for frataxin at the level of the formation of holo-f erredoxin. (C) 2001 Academic Press.