A Point Mutation in the Mitochondrial Cytochrome b Gene Obviates the Requirement for the Nuclear Encoded Core Protein 2 Subunit in the Cytochrome bc1 Complex in Saccharomyces cerevisiae

Abstract

A yeast mutant (cor2-45) in which approximately half of the C terminus of core protein 2 of the cytochrome bc 1 complex is lacking due to a frameshift mutation that introduces a stop at codon 197 in the COR2 gene fails to assemble the cytochrome bc 1 complex and does not grow on non-fermentable carbon sources that require respiration. The loss of respiration is more severe with this frameshift mutation than with the complete deletion of the COR2 gene, suggesting deleterious effects of the truncated core 2 protein. A search for extragenic suppressors of the nuclear cor2-45 mutation resulted (in addition to the expected nuclear suppressors) in the isolation of a suppressor mutation in the mitochondrial DNA that replaces serine 223 by proline in cytochrome b.Assembly of the cytochrome bc 1 complex and the respiratory deficient phenotype of the cor2-45 mutant are restored by the proline for serine replacement in cytochrome b. Surprisingly, this amino acid replacement in cytochrome b corrects not only the phenotype resulting from the cor2-45 frameshift mutation, but it also obviates the need for core protein 2 in the cytochrome bc 1 complex since it alleviates the respiratory deficiency resulting from the complete deletion of the COR2 gene. This is the first report of a homoplasmic missense point mutation of the mitochondrial DNA acting as a functional suppressor of a mutation located in a nuclear gene and the first demonstration that the supernumerary core protein 2 subunit is not essential for the electron transfer and energy transducing functions of the mitochondrial cytochrome bc 1 complex.

SLC25 Family Member Genetic Interactions Identify a Role for HEM25 in Yeast Electron Transport Chain Stability

“…We determined the abundance of subunit 2, Cor2, and cytochrome c1, Cyt1, for complex III of the ETC (di Rago et al 1997; Hunte et al 2000; Schneider and Guarente 1991). Cor2 and Cyt1 levels were significantly decreased in the double mutant flx1 Δ hem25 Δ cells (Figure 6C).…”

An out-of-frame cytochromeb gene deletion from a patient with parkinsonism is associated with impaired complex III assembly and an increase in free radical production

“…This was unexpected, because low levels of wild-type mtDNA are usually sufficient to restore normal oxidative phosphorylation function. 25 This complementation seems to be more efficient with tRNA mutations than with proteincoding gene mutations, and small decreases in the percentage of wild-type mtDNA are commonly reflected as a partial biochemical phenotype. 23 Although the potential differences are just starting to be explored, they may be due to increased stability and mobility of tRNA when compared with messenger RNA and hydrophobic polypeptides.…”

Structure at 2.3 Å resolution of the cytochrome bc1 complex from the yeast Saccharomyces cerevisiae co-crystallized with an antibody Fv fragment

“…The two core proteins of the yeast cytochrome bc 1 complex are not involved in cytochrome c reductase or MPP activity. Mutants with structural defects in COR1 or QCR2 fail to properly assemble the complex [28,29,31]. This might explain why these subunits were retained during evolution.…”