ADP/ATP carrier activity and mitochondrial translation-dependent regulation of oxidative phosphorylation in Saccharomyces cerevisiae
Ogunbona, Oluwaseun Basit
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The mitochondrial carrier family is a group of transport proteins that are mostly localized to the inner mitochondrial membrane where they facilitate the movements of various solutes across the membrane. Although these carriers represent potential targets for therapeutic application, research on the mitochondrial carrier family has not progressed commensurately. The ADP/ATP carrier is the most studied of mitochondrial carriers and is referred to as the paradigm of the family. Since it was first reported about two decades ago, a long-standing mystery in the field is how the absence of the major mitochondrial ADP/ATP carrier in yeast, Aac2p, results in a specific defect in cytochrome c oxidase (COX; complex IV) activity. In light of the recent demonstration that Aac2p physically associates with respiratory supercomplexes (RSCs), the possibility emerged that the activity of the complex may be dependent on its association with Aac2p. Therefore, this work was done to understand the mechanism of regulation of complex IV activity by the ADP/ATP carrier in yeast (Saccharomyces cerevisiae). By using a transport-dead pathogenic AAC2 point mutant discovered in a patient with hypertrophic cardiomyopathy and mild myopathy, we sought to determine whether the reduced COX activity is due to absence of the interaction between Aac2p and components of the respiratory chain and/or the absence of Aac2p function (ADP/ATP transport). Using a battery of biochemical and molecular biology techniques, we uncovered a translation-dependent regulation of cytochrome c oxidase biogenesis that is provided by Aac2p function. Importantly, the steady state levels of mitochondrial genome-encoded complex IV subunits, which form the catalytic core of the complex, are significantly reduced when Aac2p function is missing, irrespective of the status of its association with respiratory supercomplexes. This decrease in COX subunit amounts is not caused by a reduction in the mitochondrial DNA copy number or the level of its transcripts and does not reflect a defect in complex IV assembly. Instead, the absence of Aac2p activity results in an aberrant pattern of the mitochondrial translation. These results point to a direct functional link between Aac2p mediated transport and normal mitochondrial translation that is critical for the biogenesis of cytochrome c oxidase and optimal oxidative phosphorylation by extension.