Lutz T, Westermann B, Neupert W, Herrmann J M
Institut für Physiologische, Chemie der Universität München, Goethestrasse 33, München, 80336, Germany.
J Mol Biol. 2001 Mar 30;307(3):815-25. doi: 10.1006/jmbi.2001.4527.
Mitochondria of the yeast Saccharomyces cerevisiae contain three different Hsp70 chaperones, Ssc1, Ecm10 and Ssq1. Ssc1 is an essential protein that mediates the import of nuclear-encoded proteins into the organelle and their subsequent folding. The nucleotide state of Ssc1 is thereby regulated by the nucleotide exchange factor Mge1. Here, we show that Mge1 interacts with Ssq1 in an ATP-dependent manner, suggesting that Mge1 also regulates Ssq1 function. In contrast to Ssc1, Ssq1 does not associate with the Tim44 subunit of the protein translocating complex, indicating a different function of both chaperones. Mutants in Ssq1 were reported to have low levels of iron sulfur (FeS) cluster-containing enzymes. Employing an assay that allowed us to monitor the conversion of the apoform of mitochondrial ferredoxin into its FeS-containing holoform, Ssq1 was demonstrated to be required for the FeS cluster assembly in mitochondria. The mitochondrial DnaJ homolog Jac1 is crucial for this process, whereas Mdj1 function is dispensable. Furthermore, the presence of frataxin is necessary for FeS cluster assembly into ferredoxin suggesting a role for frataxin at the level of the formation of holo-ferredoxin.
酿酒酵母的线粒体含有三种不同的热休克蛋白70（Hsp70）伴侣蛋白，即Ssc1、Ecm10和Ssq1。Ssc1是一种必需蛋白，它介导核编码蛋白导入该细胞器并使其随后折叠。Ssc1的核苷酸状态由此受到核苷酸交换因子Mge1的调节。在此，我们表明Mge1以ATP依赖的方式与Ssq1相互作用，这表明Mge1也调节Ssq1的功能。与Ssc1不同，Ssq1不与蛋白质转运复合物的Tim44亚基结合，这表明这两种伴侣蛋白具有不同的功能。据报道，Ssq1突变体中含铁硫（FeS）簇的酶水平较低。采用一种能让我们监测线粒体铁氧化还原蛋白的脱辅基形式转化为其含FeS的全辅基形式的检测方法，结果表明Ssq1是线粒体中FeS簇组装所必需的。线粒体DnaJ同源物Jac1对这一过程至关重要，而Mdj1的功能则是可有可无的。此外，铁调素的存在对于FeS簇组装到铁氧化还原蛋白中是必需的，这表明铁调素在全辅基铁氧化还原蛋白形成水平上发挥作用。
