Subunit 8 of the Saccharomyces Cerevisiae Cytochrome Bc1 Complex Interacts with Succinate-ubiquinone Reductase Complex

We have investigated the function of subunit 8 of the cytochrome bc1 complex by generating six site-directed mutants, F46C, R51S, P62V, G64A, R91N, and W69-stop, in the cloned QCR8 gene and expressing the mutated genes in a Saccharomyces cerevisiae strain in which the chromosomal copy of QCR8 is deleted. The W69-stop mutation impairs assembly of the bc1 complex and growth of yeast on nonfermentable carbon sources as does deletion of QCR8 [Maarse, A. C., De Haan, M., Schoppink, P. J., Berden J. A., and Grivell, L. A. (1988) Eur. J. Biochem. 172, 179-184], implying that the C-terminus of subunit 8 is important for assembly and/or the stability of the bc1 complex. The F46C, R51S, P62V, G64A, and R91N mutations do not affect the growth of yeast on nonfermentable carbon sources, not do they lower the activity or alter the inhibitor sensitivity of the bc1 complex. Rather, some of the mutations increase the cytochrome C reductase activity of the bc1 complex by as much as 40%. However, succinate-ubiquinone reductase activity was consistently reduced 40-60% in mitochondrial membranes from these mutants, while NADH-ubiquinone reductase activity was not affected. In addition, the activation of succinate-ubiquinone reductase activity by succinate was diminished by the F46C, R51S, P62V, and G64A mutations. These results indicate that the cytochrome bc1 complex participates in electron transfer from succinate to ubiquinone in situ and also suggest an interaction between succinate-ubiquinone reductase and cytochrome bc1 complex which involves subunit 8 of the bc1 complex.