Proteolysis of mitochondrial-coded and nuclear-coded proteins found in yeast mitochondria.

The rate of degradation of radioactive labeled mitochondrial proteins synthesized both in vitro and in vivo by isolated yeast mitochondria and growing yeast cells respectively, has been studied. It was found that the in vitro-synthesized mitochondrial proteins are rapidly degraded by an energy-dependent proteolytic system. Under the same experimental conditions the in vivo-synthesized mitochondrial proteins are slowly degraded to a limited extent by a protease which is slightly inhibited by ATP. During this period, the mitochondria are coupled and metabolically active. It is proposed that mitochondria possess an energy-dependent proteolytic system that recognizes as substrates either "abnormal" proteins or unassembled protein subunits encoded in the mitochondrial genome. An apparently different system, which is independent of energy, seems to be responsible for the slow and limited degradation of "normal" mitochondrial proteins.