Functional specificity of the mitochondrial DnaJ protein, Mdj1p, in Saccharomyces cerevisiae
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Inactivation of the gene for the mitochondrial DnaJ homolog, Mdj1p, in Saccharomyces cerevisiae results in temperature sensitivity and the loss of respiratory activity; the latter phenotype has been attributed to the loss of mitochondrial DNA. To investigate the functional specificity of Mdj1p, non-mitochondrial DnaJ proteins were targeted to mitochondria and tested for their ability to substitute for Mdj1p. The tested DnaJ proteins were able to complement the two Mdj1p-linked phenotypes, i.e., respiratory activity and growth at 37 °C, to different extents, ranging from full to very poor complementation. All DnaJ homologs ensured faithful propagation of the mitochondrial genome. N-terminal fragments of Mdj1p and Escherichia coli DnaJ comprising the well-characterized J domain partially substituted for Mdj1p. As the only hitherto known function of the N-terminal fragment is modulation of the substrate binding activity of the cognate Hsp70, we conclude that both Mdj1p-linked phenotypes – maintenance of respiratory activity and the ability to grow at elevated temperature – involve a mitochondrial Hsp70 partner protein.
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