Identification of components required for mitophagy and mitochondrial inheritance by genetic screens in yeast

Mitochondria are membrane-bounded organelles, which are important for diverse cellular and physiological processes such as energy production by oxidative phosphorylation. Loss of functional mitochondria can lead to cell death and is associated with neurodegenerative diseases like Parkinson’s disease and amyotrophic lateral sclerosis. In order to execute their tasks, mitochondria have to communicate and interact with different cellular structures including other organelles and the cytoskeleton. In the first part of this study, it is shown that the contacts between the endoplasmic reticulum (ER) and mitochondria are important for the mitochondrial turnover by autophagy, also called mitophagy. Mitophagy is a process ensuring the appropriate quality and quantity of mitochondria by sequestering a mitochondrion within a double membrane and delivering it to degradative organelles. Failed mitophagy in neurons is supposed to result in accumulation of dysfunctional mitochondria and ultimately to neurodegeneration. By screening a collection of several hundred yeast mutants for defective mitochondrial autophagy, the four mutants lacking the ER-mitochondria encounter structure (ERMES), which connects ER and mitochondria, were found to have a decreased rate of mitophagy. Strikingly, artificial tethering of mitochondria and ER by a chimeric protein restores mitophagy in the ERMES mutants, indicating that loss of spatial proximity between the two organelles is the main cause of the mitophagy deficit. Moreover, one of the ERMES subunits interacts with the autophagic membrane expansion factor Atg8, which suggests that ERMES plays a role during growth of this membrane. Consequently, ERMES mutants show aberrant autophagic membrane structures, which can again be rescued by artificial mitochondria-ER tethering. It can thus be hypothesized that ERMES mediates the spatial proximity between mitochondria, the membrane expansion factor Atg8 and the ER, and that ERMES thereby promotes lipid flux from the ER to the autophagic membrane. In the second part, a genetic screen revealed that mitochondrial dynamics is important for the inheritance of mitochondria into the daughter cell. In yeast, mitochondria are transported along the actin cytoskeleton by the myosin V motor protein Myo2. The mutant myo2(LQ) allele carries two amino acid substitutions resulting in impaired mitochondrial motility. By introducing this allele into yeast deletion mutants on a genome-wide scale by synthetic genetic array technology, it was shown that mutants lacking fusion-competent mitochondria heavily depend on a functional transport machinery, since otherwise mitochondria are not transported into the daughter cell. However, if mitochondrial division is blocked in the myo2(LQ) mutant, mitochondrial inheritance is restored, indicating that mitochondrial dynamics regulates the amount of mitochondria that is transported into the bud. In sum, this study provides new insights into how the interplay of mitochondria with different cellular structures orchestrates mitochondrial behavior.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2015

Author dc:creator

• Böckler, Stefan

Contributors dc:contributor

• Westermann, Benedikt

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/2145/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:2145