The cytochrome b carboxyl-terminal region is necessary for mitochondrial Complex III assembly

Mitochondrial bc1 complex from yeast has ten subunits, but only Cytochrome b (Cytb) subunit is encoded in the mitochondrial genome. Cytb has eight transmembrane helices containing two hemes b for electron transfer. Cbp3 and Cbp6 assist Cytb synthesis, and together with Cbp4 induce Cytb hemylation. Subunits Qcr7/Qcr8 participate in the first steps of assembly, and lack of Qcr7 reduces Cytb synthesis through an assembly-feedback mechanism involving Cbp3/Cbp6. Since Qcr7 resides near the Cytb carboxyl-region, we wondered whether this region is important for Cytb synthesis/assembly. Although deletion of the Cytb C-region did not abrogate Cytb synthesis, the assembly-feedback regulation was lost, so Cytb synthesis was normal even if Qcr7 was missing. Mutants lacking the Cytb C-terminus were non-respiratory due to absence of fully assembled bc1 complex. By performing complexome profiling, we showed the existence of aberrant early-stage subassemblies in the mutant. In this work we demonstrate that the C-terminal region of Cytb is critical for regulation of Cytb synthesis and bc1 complex assembly.