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

Inactivation of the gene for the mitochondrial DnaJ homolog, Mdj1p, in Sacc haromyces cerevisiae results in temperature sensitivity and the loss of res piratory 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 th eir 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 poo r complementation. All DnaJ homologs ensured faithful propagation of the mi tochondrial 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 modulat ion of the substrate binding activity of the cognate Hsp70, we conclude tha t both Mdj1p-linked phenotypes - maintenance of respiratory activity and th e ability to grow at elevated temperature - involve a mitochondrial Hsp70 p artner protein.