Decoupling of thebc1 complex inS. cerevisiae; point mutations affecting the cytochromeb gene bring new information about the structural aspect of the proton translocation

Four mutations in the mitochondrial cytochromeb ofS. cerevisiae have been characterized with respect to growth capacities, catalytic properties, ATP/2e ^– ratio, and transmembrane potential. The respiratory-deficient mutant G137E and the three pseudo-wild type revertants E137 + I147F, E137 + C133S, and E137 + N256K were described previously (Tron and Lemesle-Meunier, 1990; Di Ragoet al., 1990a). The mutant G137E is unable to grow on respiratory substrates but its electron transfer activity is partly conserved and totally inhibited by antimycin A. The secondary mutations restore the respiratory growth at variable degree, with a phosphorylation efficiency of 12–42% as regards the parental wild type strain, and result in a slight increase in the various electron transfer activities at the level of the whole respiratory chain. The catalytic efficiency for ubiquinol was slightly (G137E) or not affected (E137 + I147F, E137 + C133S, and E137 + N256K) in these mutants. Mutation G137E induces a decrease in the ATP/2e ^– ratio (50% of the W.T. value) and transmembrane potential (60% of the W.T. value) at thebc1 level, whereas the energetic capacity of the cytochrome oxidase is conserved. Secondary mutations I147F, C133S, and N256K partly restore the ATP/ 2e ^– ratio and the transmembrane potential at thebc1 complex level. The results suggest that a partial decoupling of thebc1 complex is induced by the cytochromeb point mutation G137E. In the framework of the protonmotive Q cycle, this decoupling can be explained by the existence of a proton wire connecting centers P and N in the wild typebc1 complex which may be amplified or uncovered by the G137E mutation when the bc1 complex is functioning.