Sirt3 - A stress regulator of OXPHOS function and mitochondrial translation

The mitochondrial oxidative phosphorylation system (OXPHOS) provides the majority of cellular ATP supply and is hence essential for cellular survival. OXPHOS derives from dual genetic origin: Both nuclear and mitochondrial (mt) DNA encode for essential structural subunits of the individual OXPHOS enzymes. Mitochondria house a protein synthesis machinery specialized for the production of the mtDNA encoded proteins. Many aspects of mitochondrial proteins synthesis are not understood. One open question is how mitochondrial protein synthesis can be regulated to ultimately modulate OXPHOS function and ATP supply. Posttranslational modifications of the main component of the the mitochondrial ribosome (mitoribosome), emerge as a possible regulatory mechanism. The mitoribosome is a target for the mitochondrial NAD+-dependent deacetylase Sirt3. We subjected Sirt3 deficient, Sirt3 overexpressing and control cells to different kinds of stresses and we analyzed OXPHOS function. Proteins that interact with the mitoribosome were identified by immunoprecipitation and sucrose gradient centrifugation. Additionally we monitored mitochondrial protein synthesis by in organello metabolic labeling of mitochondrial protein synthesis in order to understand how absence and presence of Sirt3 affects mitochondrial translation. We were able to identify several proteins interacting with the mitoribosome, and we obtained data showing how Sirt3 expression levels mediate adaptation of OXPHOS and of the mitoribosome to these environmental changes. Our data contribute to a further understanding of the molecular significance of Sirt3 on regulating OXPHOS function in response to different stress conditions and its impact on modulating mitochondrial protein synthesis.