Cytochrome c Oxidase from Bakers’ Yeast

Purified cytochrome c oxidase from bakers’ yeast can be resolved into six polypeptide bands by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The apparent molecular weights of these components are I, 42,000; II, 34,500; III, 23,000; IV, 14,000; V, 12,500; and VI, 9,500. (Although Component V actually consists of two distinct polypeptide species, it will be regarded as homogeneous in this study.) In order to study the biosynthesis of these components, yeast cells were labeled with [aH]leucine in the presence of specific inhibitors of mitochondrial and cytoplasmic protein synthesis. Labeled cytochrome c oxidase components were then isolated from crude mitochondrial extracts by immunoprecipitation and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Labeling of the three large Components I, II, and III was insensitive to cycloheximide and sensitive to erythromycin. Labeling of the two largest Components I and II was dependent on the presence of oxygen. The three small Components IV, V, and VI were not labeled in the presence of cycloheximide but became labeled in the presence of erythromycin. These results show that our cytochrome c oxidase preparation contains three polypeptides which are translated on mitochondrial ribosomes and three polypeptides which are translated on cytoplasmic ribosomes. Two of the mitochondrially synthesized polypeptides are only made in the presence of oxygen.