DECOUPLING OF THE BC1 COMPLEX IN SACCHAROMYCES-CEREVISIAE - POINT MUTATIONS AFFECTING THE CYTOCHROME-B GENE BRING NEW INFORMATION ABOUT THESTRUCTURAL ASPECT OF THE PROTON TRANSLOCATION

Four mutations in the mitochondrial cytochrome b of S. cerevisiae have been characterized with respect to growth capacities, catalytic prope rties, ATP/2e(-) ratio, and transmembrane potential. The respiratory-d eficient 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 Rage et al., 1990a). The mutant G137E i s unable to grow on respiratory substrates but its electron transfer a ctivity 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 w ild type strain, and result in a slight increase in the various electr on transfer activities at the level of the whole respiratory chain. Th e catalytic efficiency for ubiquinol was slightly (G137E) or not affec ted (E137 + I147F, E137 + C133S, and E137 + N256K) in these mutants. M utation 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 the bc 1 level, whereas the energetic capacity of the cytochrome oxidase is c onserved. Secondary mutations I147F, C133S, and N256K partly restore t he ATP/2e(-) ratio and the transmembrane potential at the bc1 complex level. The results suggest that a partial decoupling of the bc1 comple x is induced by the cytochrome b point mutation G137E. In the framewor k 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 typ e bc1 complex which may be amplified or uncovered by the G137E mutatio n when the bc1 complex is functioning.