Analysis of cytochrome-b amino acid residues forming the contact face with the iron-sulfur subunit of ubiquinol:cytochrome-c reductase in Saccharomyces cerevisiae

Four mutations in the mitochondrial cytochrome b of Saccharomyces cerevisiae have been characterized with respect to catalytic properties, inhibitor resistance and subunit interaction. The respiratory-deficient mutant [Gl37E]cytochrome b and the pseudo-wild-type revertant [G137E, N256L]cytochrome b were described previously [di Rago, J.-P., Netter, P. & Sionimski, P. P. (1990) J. Biol. Chem. 265, 3332-3339; di Rago, J.-P., Netter, P. & Slonimski, P. P. (1990) J. Biol. Chem. 265, 15750-15757]. Two new mutants [N256K]cytochrome b and [N256I]cytochrome b were isolated by dissociation of the second-site suppressor from the original target mutation. The mutants [Gl37e]cytochrome b and [G137E, N256K]cytochrome b exhibited a high resistance against methoxyacrylate inhibitors, whereas the suppressors [N256K]cytochrome b and [N256I]cytochrome b showed only a slight resistance. Remarkably, all mutants exhibited stigmatellin cross-resistance. The electron-transfer activity from the substrate nonylubiquinol to cytochrome c of mitochondrial membranes was diminished in all mutants. The substitution G137 leads to E decreases Vmax/Km by one order of magnitude, indicating a reduced catalytic efficiency for ubiquinol. The amino acid exchange at position 256 to a positively charged lysine residue or to a hydrophobic isoleucine residue resulted mainly in a diminished specific activity. The iron-sulfur subunit and the 8.5-kDa subunit were detectable in all mutants at normal levels in immunoblots of membrane preparations, indicating proper assembly of the complex. However, after purification, the mutant bc1 complex lacked the iron-sulfur subunit and the 8.5-kDa subunit. In contrast, the iron-sulfur subunit can only be dissociated from the parental bc1 complex by harsh treatment. These data suggest that residues 137 and 256 in cytochrome b are crucial for cytochrome-b/ iron-sulfur protein interaction.