[Effect of antimycin A on succinate oxidase activity in potato tuber mitochondria, and the behavior of cytochromes b]

QO Site Deficiency Can Be Compensated by Extragenic Mutations in the Hinge Region of the Iron-Sulfur Protein in the bc1 Complex of Saccharomyces cerevisiae

The mitochondrial bc(1) complex catalyzes the oxidation of ubiquinol and the reduction of cytochrome (cyt) c. The cyt b mutation A144F has been introduced in yeast by the biolistic method. This residue is located in the cyt b cd(1) amphipathic helix in the quinol-oxidizing (Q(O)) site. The resulting mutant was respiration-deficient and was affected in the quinol binding and electron transfer rates at the Q(O) site. An intragenic suppressor mutation was selected (A144F+F179L) that partially alleviated the defect of quinol oxidation of the original mutant A144F. The suppressor mutation F179L, located at less than 4 A from A144F, is likely to compensate directly the steric hindrance caused by phenylalanine at position 144. A second set of suppressor mutations was obtained, which also partially restored the quinol oxidation activity of the bc(1) complex. They were located about 20 A from A144F in the hinge region of the iron-sulfur protein (ISP) between residues 85 and 92. This flexible region is crucial for the movement of the ISP between cyt b and cyt c(1) during enzyme turnover. Our results suggested that the compensatory effect of the mutations in ISP was due to the repositioning of this subunit on cyt b during quinol oxidation. This genetic and biochemical study thus revealed the close interaction between the cyt b cd(1) helix in the quinol-oxidizing Q(O) site and the ISP via the flexible hinge region and that fine-tuning of the Q(O) site catalysis can be achieved by subtle changes in the linker domain of the ISP.

Structure-function relationships of the mitochondrial bc1 complex in temperature-sensitive mutants of the cytochrome b gene, impaired in the catalytic center N

Seven new structures of cytochrome b have been recently identified by isolating and sequencing revertants from cytochrome b respiratory deficient mutants (Coppée, J. Y., Brasseur, G., Brivet-Chevillotte, P., and Colson, A. M. (1994) J. Biol. Chem. 269, 4221-4226). These mutations are located in the center N domain (QN). All the revertants exhibited a modified heme b562 maximum, confirming that part of the NH2-terminal region is in the vicinity of the extramembranous loop between helices IV-V and heme b562. Based on measurements performed on the maximal activities occurring in each segment of the respiratory chain, the decrease observed in the NADH oxidase activities of several revertants was correlated with some bc1 complex activity impairments; this may also explain why a moderate decrease in bc1 complex activity does not limit the succinate oxidase activity. The decrease in the rate of reduction of cytochrome b via the center N pathway is responsible for the impairment of the bc1 complex activity of these revertants. The three double-mutated revertants (S206L/N208K or -Y; S206L/W30C) are temperature-sensitive in vivo, and their mitochondria like that of the original mutant S206L are thermosensitive in vitro. Isolating the W30C mutation does not yield a thermosensitive phenotype: the replacement of serine 206 by leucine is therefore responsible for the thermoinstability of these strains; this temperature sensitivity is reinforced by additional mutations N208K or N208Y, and not by W30C. These data suggest that serine 206 and asparagine 208 are involved in the thermostability of the protein. When bc1 complex activity is lost after incubating mitochondria at a nonpermissive temperature (37 degrees C), heme b is still present, but can no longer be reduced by physiological substrate. The progressive loss of bc1 complex activity seems to be initially linked to a change in the tertiary structure of cytochrome b, which occurs drastically at center N and much more slowly at center P, as shown by kinetic study on the two cytochrome b redox pathways.

Is cytochrome b really the antimycin-binding component of the cytochrome b--c1 complex of yeast mitochondria

1. The antimycin binding sites were found to be independent of cytochrome(s) b synthesis in pseudo-wild-type revertants of cytochrome b mutants. These revertants, whose primary mutation is located in the introns of the cob-box gene of mitochondrial DNA, have modified contents of cytochromes b-562 and b-565 and fully functional respiratory chain. 2. Missense mutations in three genetic loci allocated to the exons of the cytochrome b gene, abolish the strong affinity binding of antimycin. 3. It is proposed that the antimycin-binding component is not cytochrome b itself, but interacts with it in such a way that an alteration of the cytochrome b structure affects the antimycin-binding site.

Cytochrome b-565 in Saccharomyces cerevisiae: use of mutants in the cob- box region of the mitochondrial DNA to study the functional role of this spectral species of cytochrome b. 2. Relationship between energetic data and cytochrome b-565 content

A wild-type strain of Saccharomyces cerevisiae and temperature-dependent revertants of isonuclear box mutants partially or completely devoid of cytocheomr b-565, were used to study the role of this cytochrome in oxidative phosphorylation. At the mitochondrial level, the phosphorylating data and the succinate oxidase activity at three different temperatures (16 degrees C, 28 degrees C and 36 degrees C) were measured in these strains showing various cytochrome b-565 contents. It is concluded that this cytochrome b-565 is not in the main pathway of the electron transfer chain. At the optimum growth temperature (28 degrees C), the measurements of the P/O ratio for the wild phenotype strains, with ethanol as substrate, led to the conclusions that two phosphorylation sites of the respiratory chain are functional in these strains. The growth yields for wild phenotype strains and revertants grown in vivo in complex media, with ethanol or galactose as the energy source (at 16 degrees C, 28 degress C and 36 degrees C), were compared with the cytochrome b-565 content. The growth yields showed small variations if compared to the reference strain, when the cytochrome b-565 content was greatly diminished or absent. Thus the ATP production at side II is independent of the cytochrome b-565 content. Cytochrome b-565 does not play an essential role in oxidative phosphorylation.

Cytochrome b-565 in Saccharomyces cerevisiae: use of mutants in the cob-box region of the mitochondrial DNA to study the functional role of this spectral species of cytochrome b. 1. Measurements of cytochromes b-562 and b-565 and selection of revertants devoid of cytochrome b-565

In order to study the functional role of the spectral species of cytochrome b-565 observed in mitochondria, genetically manipulated strains of Saccharomyces cerevisiae have been used. Strains have been found which are devoid of cytochrome b-565 under certain conditions and which nevertheless are able to grow on a respirable substrate. Two different methods have been used to determine the cytochrome b-565 content: anaerobic titrations and antimycin-A-induced reduction of cytochrome b-565. Both yield the same results.