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

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