Evidence that cytochrome b is the antimycin-binding component of the yeast mitochondrial cytochrome bc1 complex

The binding of antimycin was studied in several mutant strains of yeast that have specific defects in cytochrome b. The strains have mutations in a part of the mitochondrial DNA that contains the structural gene for the apoprotein of cytochome b. Two of the mutants lack this protein and have no spectral cytochrome b. These mutants also lack the strong antimycin-binding site that is present in wild-type yeast mitochondria in the ratio of one site per two cytochrome b molecules. A third mutant which contains normal levels of spectral cytochrome b, but shows an altered absorption maximum for cytochrome b at 77 °K, was found to bind normal amounts of antimycin. However, the fluorescence of antimycin bound to mitochondria of this strain was found to be less efficiently quenched than in the case of the wild-type strain. In another mutant which contains only 20% of the normal spectral level of cytochrome b, the number of antimycin-binding sites was proportionately less. In an antimycin-resistant mutant, the binding of antimycin was too weak to be detected. The simultaneous modification of the structure of cytochrome b and the alteration of the antimycin-binding site in these mutants suggests that the antimycin-binding site is located on the apoprotein of cytochrome b.