Mitochondrial structure and distribution in Saccharomyces cerevisiae

Mitochondria play diverse roles in the physiology and metabolism of eukaryotic cells. Like most membrane bounded organelles, they cannot be synthesized de novo but grow and split into distinct organelles and must be inherited to daughter cells upon cell division. The structure of the highly dynamic mitochondrial network is adjusted to fit cellular needs by orchestrating mitochondrial movement, fusion, and fission. All three processes are important for the maintenance of functional mitochondria. The core components of the transport, fusion, and division machineries have been identified in baker’s yeast. However, the mechanisms controlling mitochondrial dynamics remain poorly understood. The synoptic aim of this work was to characterize the molecular function of three genes that are involved in maintaining structural integrity of mitochondria: NUM1, MDM33, and UPS1. Yeast cells lacking NUM1 or MDM33 show defects in mitochondrial fission, whereas UPS1 has been reported to be involved in mitochondrial fusion and the biosynthesis of the mitochondrial signature lipid cardiolipin. This work assigns a specific process to each of the three genes and provides evidence how these processes influence mitochondrial behavior. In summary, this study elucidates how various processes influence the fusion and fission of a double membrane bounded organelle. First, Num1 was identified as key component of a tethering complex that anchors mitochondria at the mother cell cortex. The tethering complex serves to counteract bud-directed mitochondrial movement and assures that a part of the mitochondria remains in the mother cell upon cell division. It acts antagonistically to a known mitochondrial anchor containing Mmr1 at the tip of the daughter cell. Thus, Num1 in the mother and Mmr1 in the bud form two separate cortical tethers to ensure proper distribution of mitochondria by generating opposing forces at spatially distinct and exclusive locations. Strikingly, the tethering of mitochondria at the mother cell cortex was identified as a prerequisite for efficient mitochondrial division. Second, it was shown that Mdm33 orchestrates mitochondrial fission and phospholipid biosynthesis. Genetic analysis revealed a tight association of MDM33 and genes affecting mitochondrial phospholipid metabolism. Consistently, Mdm33 overexpression alters mitochondrial lipid composition and directly influences mitochondrial phospholipid biosynthesis. Mutants lacking Mdm33 show reduced mitochondrial fission activity, indicating that Mdm33 promotes mitochondrial division but is no essential component of the fission machinery. Furthermore, Mdm33 was found to act upstream of mitochondrial fission and fusion and to be required to keep mitochondria in a fission competent shape. The results suggest an intriguing connection between mitochondrial fission and phospholipid homeostasis. Third, it was investigated by electron microscopy whether cells lacking Ups1 or other cardiolipin biosynthesis factors show aberrant mitochondrial ultrastructure. Intriguingly, reduction of cardiolipin levels only affected the shape of the mitochondrial inner membrane when it was accompanied by an increase in mitochondrial cytidine diphosphate-diacylglycerol. A genetic epistasis analysis with focus on mitochondrial ultrastructure revealed that Ups1 acts prior to the first enzymatic reaction of the cardiolipin biosynthesis. This pointed to a role of Ups1 in supplying the CL biosynthesis machinery with precursor lipids. Thus, Ups1 mainly functions in cardiolipin biosynthesis and it is conceivable that reduced cardiolipin levels in ∆ups1 mutants cause mitochondrial fragmentation.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2014

Author dc:creator

• Klecker, Till

Contributors dc:contributor

• Westermann, Benedikt

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/1707/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:1707