Further insights into the assembly of the yeast cytochrome bc₁ complex based on analysis of single and double deletion mutants lacking supernumerary subunits and cytochrome b

Abstract

The cytochrome bc1 complex of the yeast Saccharomyces cerevisiae is composed of 10 different subunits that are assembled as a symmetrical dimer in the inner mitochondrial membrane. Three of the subunits contain redox centers and participate in catalysis, whereas little is known about the function of the seven supernumerary subunits. To gain further insight into the function of the supernumerary subunits in the assembly process, we have examined the subunit composition of mitochondrial membranes isolated from yeast mutants in which the genes for supernumerary subunits and cytochrome b were deleted and from yeast mutants containing double deletions of supernumerary subunits. Deletion of any one of the genes encoding cytochrome b, subunit 7 or subunit 8 caused the loss of the other two subunits. This is consistent with the crystal structure of the cytochrome bc1 complex that shows that these three subunits comprise its core, around which the remaining subunits are assembled. Absence of the cytochrome b/subunit 7/subunit 8 core led to the loss of subunit 6, whereas cytochrome c1, iron–sulfur protein, core protein 1, core protein 2 and subunit 9 were still assembled in the membrane, although in reduced amounts. Parallel changes in the amounts of core protein 1 and core protein 2 in the mitochondrial membranes of all of the deletion mutants suggest that these can be assembled as a subcomplex in the mitochondrial membrane, independent of the presence of any other subunits. Likewise, evidence of interactions between subunit 6, subunit 9 and cytochrome c1 suggests that a subcomplex between these two supernumerary subunits and the cytochrome might exist.

Selective Oma1 Protease-mediated Proteolysis of Cox1 Subunit of Cytochrome Oxidase in Assembly Mutants

“…We focused on the mitochondrial encoded cytochrome b subunit of the bc 1 complex. Cells lacking Qcr7 stall maturation of the bc 1 complex leading to rapid cytochrome b degradation (33). To address whether Oma1 contributes to the degradation cytochrome b, we compared qcr7⌬ cells to qcr7⌬ oma1⌬ double null cells (Fig.…”

X-Ray Structure of Rhodobacter Capsulatus Cytochrome bc1: Comparison with its Mitochondrial and Chloroplast Counterparts

“…A large part of the complexity of the mitochondrial cyt bc 1 , which plays an identical electron transfer role as the bacterial enzyme, may be a result of endosymbiosis that has distributed the various components of this enzymes between organellar and nuclear genomes, consequently requiring their assembly from two different sources into the mitochondrial membrane (Zara et al 2004). These additional complexities are absent in bacteria, rendering the interpretation of genetic results more straightforward.…”

Deleterious Epistatic Interactions Between Electron Transport System Protein-Coding Loci in the Copepod Tigriopus californicus

“…Of these subunits, QCR6p (corresponding to subunit 8 in mammals) and QCR9p (corresponding to subunit 10) are close in the structure to the active sites of CYC1 and RISP (Iwata et al 1998;Zhang et al 1998;Lange and Hunte 2002). Although complex III can function without these two subunits, it appears that they are important for complex assembly and may modify functional interactions as well (Kim et al 1987;Schmitt and Trumpower 1991;Zara et al 2004). To gain a complete understanding of the evolution of interactions in this complex for these hybrid copepods, these additional subunits should be characterized as well.…”