Function and regulation of the mitochondria upon osmotic stress in the yeast Saccharomyces cerevisiae
    Hyperosmotic stress triggers a great variety of adaptive responses in eukaryotic cells that affect many different physiological functions. Here we investigate the role of the mitochondria during osmostress adaptation in budding yeast. Mitochondria are dynamic organelles with the capacity to adapt to environmental stimuli and stress. Mitochondrial function is generally required for proper salt and osmotic stress adaptation since yeast mutants with defects in many different mitochondrial components show hypersensitivity to increased NaCl and KCl concentrations. Mitochondrial protein abundance rapidly increases upon osmoshock in a selective manner, as it affects Krebs cycle enzymes (Sdh2, Cit1) and components of the electron transport chain (Cox6), but not the ATP synthase complex (Atp5). Transcription of the SDH2, CIT1 and COX6 genes is several fold induced within the first minutes of osmotic shock, dependent to various degree on the Hog1 and Snf1 protein kinases. Mitochondrial succinate dehydrogenase enzyme activity is stimulated upon osmostress in a Snf1 dependent manner. The osmosensitivity of mitochondrial mutants is not caused by impaired stress-activated transcription or by a general depletion of the cellular ATP pool during osmostress. We finally show that the growth defect of mitochondrial mutants in high salt medium can be partially rescued by supplementation of glutathione. Additionally, mitochondrial defects cause the hyperaccumulation of reactive oxygen species during salt stress. These results indicate that the antioxidant function of the mitochondria might play an important role in the adaptation to hyperosmotic stress.
    Furthermore, we use proteomic approaches to quantify the changes in the protein composition of mitochondria in the presence of salt stress provoked by NaCl. We identified 15 proteins which were more than 2 fold overrepresented in salt adapted mitochondria. These proteins are mainly involved in the oxidative stress defense, the biosynthesis of amino acids and ubiquinone or in the metabolism of pyruvate and acetate. Loss of function of most of the up-regulated proteins did not result in a significant growth phenotype under high salt conditions. However, all identified proteins were necessary to sustain efficient growth under oxidative stress caused by hydrogen peroxide. Additionally, a subset of outer mitochondrial proteins was shown to be up-regulated upon salt stress. We furthermore identified 9 proteins which were more than 3 fold underrepresented in salt adapted mitochondria. These proteins were mainly glycolytic enzymes or proteins with a predominant localization at the endoplasmatic reticulum. Our results underline the complex nature of the stress adaptation of mitochondria and identify functional groups of proteins whose specific role in salt resistance should be revealed in the future.