title: Structure and function of the mitochondrial bc1 complex. Properties of the complex in temperature-sensitive cor1 mutants.
abstract: The properties of the ubiquinol-cytochrome c reductase complex (bc1 complex) have been studied in respiratory defective mutants of Saccharomyces cerevisiae bearing lesions in the core 1 subunit. All the cor1 mutants examined have greatly reduced concentrations of mitochondrial cytochrome b and display succinate-cytochrome c reductase activities near the limits of detection. Two mutants (E576 and C7), however, had 5% of wild type activity when the cells were grown at 23 degrees C, but not at 37 degrees C. The temperature-sensitive phenotype was determined to result from substitution of either Arg or Glu for Gly68 of the core 1 subunit. The respiratory competent revertants E576/R8 and C7/R4 derived from E576 and C7 retain the temperature sensitivity of the original mutants. Both revertants are temperature sensitive in vivo, but only mitochondria isolated from E576/R8 are temperature sensitive in vitro. The bc1 complex of mitochondria isolated from this revertant displays a normal value of the ratio Kcat/Km for cytochrome c and four times higher than the wild type for duroquinol. The succinate-cytochrome c reductase activity of E576/R8 is almost completely abolished after incubation at 37 degrees C for 90 min. It is inferred that the quaternary structure of ubiquinol-cytochrome c reductase complex is more labile at the nonpermissive temperature in the mutant and undergoes an alteration such that cytochrome b is no longer able to receive electrons through either the "o" or the "i" site pathway. The temperature lability and kinetic properties of the mutant enzyme point to a requirement of the core 1 not only for assembly but also for the catalytic activity of the complex.
authors.affiliation: Department of Biological Sciences, Columbia University, New York, New York 10027.
authors.forename: D L
authors.initials: DL
authors.lastname: Gatti
authors.forename: A
authors.initials: A
authors.lastname: Tzagoloff
mesh_terms: D015153:Blotting, Western; D014450:Electron Transport Complex III / Q000737:chemistry / Q000235:genetics* / Q000276:immunology; D005656:Fungal Proteins / Q000378:metabolism; D007700:Kinetics; D008928:Mitochondria / Q000201:enzymology*; D009154:Mutation; D010101:Oxygen Consumption; D012441:Saccharomyces cerevisiae / Q000201:enzymology* / Q000235:genetics; D013057:Spectrum Analysis; D013696:Temperature
chemical_list: D005656:Fungal Proteins; D014450:Electron Transport Complex III