Functional specificity of the mitochondrial DnaJ protein, Mdj1p, in Saccharomyces cerevisiae.

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 degrees 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 Mdjlp 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.