Divalent metal ion-dependent mitochondrial degradation of unassembled subunits 2 and 3 of cytochrome c oxidase.

Intracellular protein degradation plays an important role in maintaining the stoichiometry of the different subunits of an oligomeric enzyme. In a Saccharomyces cerevisiae mutant defective in cytochrome c oxidase subunit 4 encoded in nuclear DNA, mitochondrial-encoded subunits 2 and 3 cannot assemble normally [Dowhan et al. (1985) EMBO J. 4, 179-184]. In this study, we show that those unassembled forms of subunits 2 and 3 in this strain are eliminated rapidly by degradation. Reduction of the intracellular ATP level by inhibiting the glycolytic pathway, or inhibition of the entry of ATP into mitochondria by bongkrekic acid, both of which are expected to reduce the intramitochondrial ATP level in respiratory-deficient cells such as WD1, significantly suppressed the degradation, suggesting that the degradation requires intramitochondrial ATP. The degradation was also inhibited by o-phenanthroline, a membrane-permeable metal chelator, and this inhibitory effect was suppressed by addition of an excess amount of Co2+, Mn2+, or Zn2+, but not by Ca2+ or Mg2+, suggesting a novel metal-dependence of the degradation of unassembled Cox II and Cox III which has not been reported previously for mitochondrial metabolic protein degradation systems. A potential advantage of using this strain for identifying the factor(s) involved by a genetical approach is discussed.