Integrity of mitochondria in a mammalian cell mutant defective in mitochondrial protein synthesis.

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Abstract
A defect in mitochondrial protein synthesis was previously identified in the respiration-deficient Chinese hamster lung fibroblast mutant V79-G7. The characterization of this mutant is extended. Mutant mitochondria apparently synthesize all mitochondrially encoded peptides, but in significantly reduced amounts. The difference is seen when isolated mitochondria are tested for in vitro protein synthesis. To distinguish between a defect in the translational machinery and a defect in the transcription of mitochondrial DNA, the synthesis of the 16S and 12S mitochondrial rRNA species was studied; they were made in normal amounts in G7 mitochondria. The rRNA species appear to be assembled into subunits whose sedimentation behavior is virtually indistinguishable from that of the wild-type subunits. The consequences of the defect in mitochondrial protein synthesis on mutant cells and their mitochondria-utilizing techniques of EM, 2-dimensional gel electrophoresis and immunochemical analysis were examined. G7 mitochondria have a characteristic ultrastructure distinguished by predominantly tubular cristae, but the overall biochemical composition of mitochondrial membrane and matrix fractions appears essentially unaltered except for the absence of a few characteristic peptides. The absence of 2 mitochondrially encoded subunits of cytochrome c oxidase was identified on 2-dimensional gels and a drastic reduction of both cytoplasmically and mitochondrially synthesized subunits of this enzyme in immunoprecipitates of G7 mitochondria was shown.