SUBUNIT-8 OF THE SACCHAROMYCES-CEREVISIAE CYTOCHROME BC(1) COMPLEX INTERACTS WITH SUCCINATE-UBIQUINONE REDUCTASE COMPLEX

We have investigated the function of subunit 8 of the cytochrome bc(1) complex by generating six site-directed mutants, F46C, R51S, P62V, G6 4A, R91N, and W69-stop, in the cloned QCR8 gene and expressing the mut ated genes in a Saccharomyces cerevisiae strain in which the chromosom al copy of QCR8 is deleted. The W69-stop mutation impairs assembly of the bc(1) 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 asse mbly and/or the stability of the bc(1) complex. The F46C, R51S, P62V, G64A, and R91N mutations do not affect the growth of yeast on nonferme ntable carbon sources, not do they lower the activity or alter the inh ibitor sensitivity of the bc(1) complex. Rather, some of the mutations increase the cytochrome c reductase activity of the bc(1) complex by as much as 40%. However, succinate-ubiquinone reductase activity was c onsistently reduced 40-60% in mitochondrial membranes from these mutan ts, while NADH-ubiquinone reductase activity was not affected. In addi tion, the activation of succinate-ubiquinone reductase activity by suc cinate was diminished by the F46C, R51S, P62V, and G64A mutations. The se results indicate that the cytochrome bc(1) complex participates in electron transfer from succinate to ubiquinone in situ and also sugges t an interaction between succinate-ubiquinone reductase and cytochrome bc(1) complex which involves subunit 8 of the bc(1) complex.