Loss of the mitochondrial Hsp70 functions causes aggregation of mitochondria in yeast cells

Ssc1p, a member of the Hsp70 family in the mitochondrial matrix of budding yeast, mediates protein import into mitochondria and prevents irreversible aggregation of proteins in the mitochondrial matrix during folding/ assembl y or at elevated temperature. Here, we show that functional inactivation of the mitochondrial Hsp70 system causes aggregation of mitochondria. When te mperature-sensitive mitochondrial Hsp70 mutant cells were incubated at rest rictive temperature, a tubular network of mitochondria was collapsed to for m aggregates. Inhibition of protein synthesis in the cytosol did not suppre ss the mitochondrial aggregation and functional impairment of Tim23, a subu nit of mitochondrial protein translocator in the inner membrane, did not ca use mitochondrial aggregation. Therefore defects of the Hsp70 function in p rotein import into mitochondria or resulting accumulation of precursor form s of mitochondrial proteins outside the mitochondria are not the causal rea son for the aberrant mitochondrial morphology. By contrast, deletion of Mdj 1p, a functional partner for mitochondrial Hsp70 in prevention of irreversi ble protein aggregation in the matrix, but not in protein import into mitoc hondria, caused aggregation of mitochondria, which was enhanced at elevated temperature (37 degreesC). The aggregation of mitochondria at 37 degreesC was reversed when the temperature was lowered to 23 degreesC unless protein synthesis was blocked. On the basis of these results, we propose that the mitochondrial matrix contains a protein that is responsible for the mainten ance of mitochondrial morphology and requires mitochondrial Hsp70 for its f unction.