Allotopic expression ofCOX6elucidates Atco-driven co-assembly of cytochrome oxidase and ATP synthase

The Cox6 subunit ofSaccharomyces cerevisiaecytochrome oxidase (COX) and the Atp9 subunit of the ATP synthase are encoded in nuclear and mitochondrial DNA, respectively. The two proteins interact to form Atco complexes that serve as the source of Atp9 for ATP synthase assembly. To determine if Atco is also a precursor of COX, we pulse-labeled Cox6 in isolated mitochondria of acox6nuclear mutant withCOX6in mitochondrial DNA. Only a small fraction of the newly translated Cox6 was found to be present in Atco, which can explain the low concentration of COX and poor complementation of thecox6mutation by the allotopic gene. This and other pieces of evidence presented in this study indicate that Atco is an obligatory source of Cox6 for COX biogenesis. Together with our finding thatatp9mutants fail to assemble COX, we propose a regulatory model in which Atco unidirectionally couples the biogenesis of COX to that of the ATP synthase to maintain a proper ratio of these two complexes of oxidative phosphorylation.