The cytochrome b carboxyl terminal region is necessary for mitochondrial complex III assembly.

Mitochondrial bc 1 complex from yeast has 10 subunits, but only cytochrome b (Cyt b ) subunit is encoded in the mitochondrial genome. Cyt b has eight transmembrane helices containing two hemes b for electron transfer. Cbp3 and Cbp6 assist Cyt b synthesis, and together with Cbp4 induce Cyt b hemylation. Subunits Qcr7/Qcr8 participate in the first steps of assembly, and lack of Qcr7 reduces Cyt b synthesis through an assembly-feedback mechanism involving Cbp3/Cbp6. Because Qcr7 resides near the Cyt b carboxyl region, we wondered whether this region is important for Cyt b synthesis/assembly. Although deletion of the Cyt b C-region did not abrogate Cyt b synthesis, the assembly-feedback regulation was lost, so Cyt b synthesis was normal even if Qcr7 was missing. Mutants lacking the Cyt b C-terminus were non-respiratory because of the absence of fully assembled bc 1 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 Cyt b is critical for regulation of Cyt b synthesis and bc 1 complex assembly.