The ClpB homolog Hsp78 is required for the efficient degradation of proteins in the mitochondrial matrix

Molecular chaperones perform vital functions in mitochondrial protein import and folding. In yeast mitochondria, two members of the Clp/Hsp100 chaperone family, Hsp78 and Mcx1, have been identified as homologs of the bacterial proteins ClpB and ClpX, respectively. In this report we employed a novel quantitative assay system to assess the role of Hsp78 and Mcx1 in protein degradation within the matrix. Mitochondria were preloaded with large amounts of two purified recombinant reporter proteins exhibiting different folding stabilities. Proteolysis of the imported substrate proteins depended on the mitochondrial level of ATP and was mediated by the matrix protease Pim1/LON. Degradation rates were found to be independent of the folding stability of the reporter proteins. Mitochondria from hsp78Delta cells exhibited a significant defect in the degradation efficiency of both substrates even at low temperature whereas mcx1Delta mitochondria showed wild-type activity. The proteolysis defect in hsp78Delta mitochondria was independent from the aggregation behavior of the substrate proteins. We conclude that Hsp78 is a genuine component of the mitochondrial proteolysis system required for the efficient degradation of substrate proteins in the matrix.