The cytochromebcarboxyl terminal region is necessary for mitochondrial complex III assembly

Abstract

Mitochondrialbc1complex from yeast has 10 subunits, but only cytochromeb(Cytb) subunit is encoded in the mitochondrial genome. Cytbhas eight transmembrane helices containing two hemesbfor electron transfer. Cbp3 and Cbp6 assist Cytbsynthesis, and together with Cbp4 induce Cytbhemylation. Subunits Qcr7/Qcr8 participate in the first steps of assembly, and lack of Qcr7 reduces Cytbsynthesis through an assembly-feedback mechanism involving Cbp3/Cbp6. Because Qcr7 resides near the Cytbcarboxyl region, we wondered whether this region is important for Cytbsynthesis/assembly. Although deletion of the CytbC-region did not abrogate Cytbsynthesis, the assembly-feedback regulation was lost, so Cytbsynthesis was normal even if Qcr7 was missing. Mutants lacking the CytbC-terminus were non-respiratory because of the absence of fully assembledbc1complex. 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 Cytbis critical for regulation of Cytbsynthesis andbc1complex assembly.