A novel mutation selectively decreases complex I and cytochrome c oxidase subunits in Chinese hamster mitochondria

Chinese hamster subunits of mitochondrial respiratory Complex I (NADH-ubiquinone reductase), Complex IV (cytochrome c oxidase), and Complex V (oligomycin-sensitive ATPase) were identified by immunoprecipitation and/or Western immunoblotting with antibody to the corresponding beef heart complexes. In the Chinese hamster lung cell mutant Gal 32, cytochrome c oxidase activity and its mitochondrially synthesized subunits (I, II, and III) are substantially decreased, but a cytoplasmically synthesized subunit (IV) is present at wild type levels. Complex I activity and five of its subunits are greatly diminished in Gal 32; several of the affected Complex I subunits correspond in mobility to mitochondrial translation products. In contrast, ATPase activity and its mitochondrially and cytoplasmically synthesized subunits are not greatly modified in the mutant. Our data suggest that the ATPase complex contains two rather than one mitochondrially synthesized peptides. The simultaneous correction of this pleiotropic phenotype in a spontaneous revertant of Gal 32 selected for its ability to grow on galactose suggests that the Gal 32 phenotype is a consequence of a single mutation. Therefore, it is concluded that Complex I may contain a previously unrecognized mitochondrially synthesized subunit(s), and that the lowered activity of both Complex I and cytochrome c oxidase in the mutant is due to decreased levels of their mitochondrially encoded subunits.