Genetic and cellular basis of mitochondrial genome inheritance and maintenance in yeast
General Genetics and Functional Genome Biology
Final Report Abstract
Mitochondria are the 'powerhouses' of eukaryotic cells. They liberate energy from metabolites taken up with the food to convert it to the universal energy currency, ATP. This process, also called oxidative phosphorylation, is performed by the respiratory chain in the mitochondrial inner membrane. Two genomes contribute to the biogenesis of mitochondria: The nuclear genome encodes about 1,000 mitochondrial proteins, whereas the mitochondrial genome (mtDNA) encodes only a handful of proteins required for oxidative phosphorylation (13 in humans, 8 in yeast). It has been estimated that more than 200 nuclear encoded proteins are required for the expression of the few mitochondria-encoded proteins. Budding yeast, Saccharomyces cerevisiae, is an excellent model organism to study this process. It is viable in the absence of oxidative phosphorylation, as long as fermentable carbon sources such as glucose are present. And, importantly, comprehensive deletion mutant collections are available that cover all 4,800 non-essential yeast genes. We performed comprehensive genetic screens of the yeast deletion library to define with high confidence the sets of nuclear genes required for respiratory growth, maintenance of mtDNA, and expression of mitochondria-encoded proteins. We found that about 250 proteins are indispensable for respiration in yeast. About 79% of these are mitochondrial proteins. Furthermore, we found that respiratory deficiency is a highly variable phenotype in many deletion mutants. This effect can be largely – but not solely – explained by an instability of mtDNA. Furthermore, our data suggest that 89 proteins are specifically required for the maintenance of mtDNA. These include factors known to be involved in mtDNA metabolism, mitochondrial fusion, and subunits of the mitochondrial ribosome. 87 additional proteins are specifically required for mitochondrial protein synthesis. These are mostly mitochondrial transcription factors, tRNA synthetases and ribosomal proteins. Taken together, these results define the complement of genes required for respiration and maintenance and expression of the mitochondrial genome with high resolution. Moreover, we confirmed earlier observations that the vacuole plays an important role in maintenance of respiratory activity in yeast. The activity of a vacuolar proton pump, the vATPase, is important for ion homeostasis in the cytosol. Respiration defects of vATPase mutants can be rescued by the addition of iron or copper to the growth medium. Also, it has been known for a long time that Mgm1, a large dynamin-related GTPase associated with the mitochondrial inner membrane, is required for maintenance of mtDNA. Mgm1 is required for fusion of the mitochondrial inner membrane and the formation of cristae, which are invaginations of the inner membrane housing the respiratory chain complexes. Our data show that mitochondrial membrane defects occur in mgm1 mutants before the loss of mtDNA. This suggests that Mgm1 primarily functions in remodelling of the mitochondrial inner membrane, and loss of mtDNA is a secondary consequence of loss of this activity.
Publications
- (2014). Mitochondrial inheritance in yeast. Biochim. Biophys. Acta 1837:1039-1046
Westermann, B.
(See online at https://doi.org/10.1016/j.bbabio.2013.10.005) - (2016). An evidence based hypothesis on the existence of two pathways of mitochondrial cristae formation. eLife, e18853
Harner, M.E., Unger, A.K., Geerts, W.J.C., Mari, M., Izawa, T., Stenger, M., Geimer, S., Reggiori, F., Westermann, B., and Neupert, W.
(See online at https://doi.org/10.7554/eLife.18853)
