ACIDIC REGIONS OF CYTOCHROME C1 ARE ESSENTIAL FOR UBIQUINOL-CYTOCHROME-C-1 REDUCTASE-ACTIVITY IN YEAST-CELLS LACKING THE ACIDIC QCR6 PROTEIN

It has been suggested that the two acidic regions around residue 70 an d residue 170 in yeast cytochrome c(1) a subunit of ubiquinol-cytochro me c reductase (complex III), interact with cytochrome c in the electr on transfer reaction and that the QCR6 protein, the acidic subunit of yeast complex III, enhances this interaction. In order to determine th e roles of the acidic regions of cytochrome c(1) more precisely, we in troduced several mutations in the two acidic regions and examined thei r effects on the ability of modified cytochrome c(1) to complement the respiration deficiency of yeast cells lacking only cytochrome c(1) or both cytochrome c(1) and the QCR6 protein. The mutant cytochrome c(1) with the deletion of the first acidic region (Delta 68-80) was still functional in the cytochrome c(1)-deficient strain. Mutant cytochrome c(1) with the deletion of the second acidic region (Delta 168-179) cau sed a decrease in the complementing ability, but this is probably due to failure in its proteolytic maturation and/or correct assembly into complex III. Mutant cytochrome c(1) with altered charge distribution i n the acidic regions (Asp(170)Asp(171)-->Asn(170)Asn(171) or Asp(170)A sp(171)-->Asn(170)Lys(171)) made the cytochrome c(1)-deficient cells r espiration-competent. On the other hand, mutant cytochrome c(1) with t he deletion of the first acidic region (Delta 68-80) or altered charge distribution in the second region (Asp(170)Asp(171)-->Asn(170)Lys(171 )) did not restore the respiration deficiency of the cells lacking not only cytochrome c(1) but also the QCR6 protein. These results indicat e that the acidic regions in cytochrome c(1) are essential for the ubi quinol-cytochrome c reductase activity in yeast cells in the absence o f the QCR6 protein, and suggest the acidic regions of cytochrome c(1) may promote binding of complex III to cytochrome c in cooperation with the QCR6 protein.