ATP-DEPENDENT PROTEOLYSIS IN MITOCHONDRIA - M-AAA PROTEASE AND PIM1 PROTEASE EXERT OVERLAPPING SUBSTRATE SPECIFICITIES AND COOPERATE WITH THE MTHSP70 SYSTEM

To analyze protein degradation in mitochondria and the role of molecul ar chaperone proteins in this process, bovine apocytochrome P450scc wa s employed as a model protein. When imported into isolated yeast mitoc hondria, P450scc was mislocalized to the matrix and rapidly degraded. This proteolytic breakdown was mediated by the ATP-dependent PIM1 prot ease, a Lon-like protease in the mitochondrial matrix, in cooperation with the mtHsp70 system. In addition, a derivative of P450scc was stud ied to which a heterologous transmembrane region was fused at the amin o terminus. This protein became anchored to the inner membrane upon im port and was degraded by the membrane-embedded, ATP-dependent m-AAA pr otease, Again, degradation depended on the mtHsp70 system; it was inhi bited at nonpermissive temperature in mitochondria carrying temperatur e-sensitive mutant forms of Ssc1p, Mdj1p, or Mge1p. These results demo nstrate overlapping substrate specificities of PIM1 and the m-AAA prot ease, and they assign a central role to the mtHsp70 system during the degradation of misfolded polypeptides by both proteases.