The two mitochondrial heat shock proteins 70, Ssc1 and Ssq1, compete for the cochaperone Mge1

Two members of the heat shock protein 70 kDa (Hsp70) fan-Lily, Ssc1 and Ssq 1, perform important functions in the mitochondrial matrix. The essential S sc1 is an abundant ATP-binding protein required for both import and folding of mitochondrial proteins. The function of Ssc1 is supported by an interac tion with the preprotein translocase subunit Tim44, the cochaperone Mdj1, a nd the nucleotide exchange factor Mge1. In contrast, only limited informati on is available on Ssq1. So far, a basic characterization of Ssq1 has demon strated its involvement in the maintenance of mitochondrial DNA, the matura tion of the yeast frataxin (Yfh1) after import, and assembly of the mitocho ndrial Fe/S cluster. Here, we analyzed the biochemical properties and the i nteraction partners of Ssq1 in detail. Ssq1 showed typical chaperone proper ties by binding to unfolded substrate proteins in an ATP-regulated manner. Ssq1 was able to form a specific complex with the nucleotide exchange facto r Mge1. In particular, complex formation in organello was enhanced signific antly when Ssc1 was inactivated selectively. However, even under these cond itions, no interaction of Ssq1 with the two other mitochondrial Hsp70-cocha perones, Tim44 and Mdj1, was observed. The Ssq1-Mge1 interaction showed a l ower overall stability but the same characteristic nucleotide-dependence as the Ssc1-Mge1 interaction. A quantitative analysis of the interaction prop erties indicated a competition of Ssq1 with Ssc1 for binding to Mge1. Pertu rbation of Mge1 function or amounts resulted in direct effects on Ssq1 acti vity in intact mitochondria. We conclude that mitochondria represent the un ique case where two Hsp70s compete for the interaction with one nucleotide exchange factor. (C) 2001 Academic Press.