Identification and characterization of cytochrome bc 1 subcomplexes in mitochondria from yeast with single and double deletions of genes encoding cytochrome bc 1 subunits

We have examined the status of the cytochrome  bc 1 complex in mitochondrial membranes from yeast mutants in which genes for one or more of the cytochrome  bc 1 complex subunits were deleted. When membranes from wild‐type yeast were resolved by native gel electrophoresis and analyzed by immunodecoration, the cytochrome  bc 1 complex was detected as a mixed population of enzymes, consisting of cytochrome  bc 1 dimers, and ternary complexes of cytochrome  bc 1 dimers associated with one and two copies of the cytochrome  c oxidase complex. When membranes from the deletion mutants were resolved and analyzed, the cytochrome  bc 1 dimer was not associated with the cytochrome  c oxidase complex in many of the mutant membranes, and membranes from some of the mutants contained a common set of cytochrome  bc 1 subcomplexes. When these subcomplexes were fractionated by SDS/PAGE and analyzed with subunit‐specific antibodies, it was possible to recognize a subcomplex consisting of cytochrome  b , subunit 7 and subunit 8 that is apparently associated with cytochrome  c oxidase early in the assembly process, prior to acquisition of the remaining cytochrome  bc 1 subunits. It was also possible to identify a subcomplex consisting of subunit 9 and the Rieske protein, and two subcomplexes containing cytochrome  c 1 associated with core protein 1 and core protein 2, respectively. The analysis of all the cytochrome  bc 1 subcomplexes with monospecific antibodies directed against Bcs1p revealed that this chaperone protein is involved in a late stage of cytochrome  bc 1 complex assembly.