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The molecular machinery of mitochondrial dynamics and inheritance Benedikt Westermann Introduction Each type of eukaryotic cell possesses a characteristic three-dimensional structure. Maintenance of its architecture and duplication during cell division depend on active transport of organelles along the cytoskeleton as well as continuous fission and fusion events of intracellular membranes. Mitochondria are essential organelles which are often located at sites of high energy consumption in the cell. They cannot be formed de novo, but have to be inherited from the mother to the daughter cell during cell division. There is mounting evidence that positioning and transport of mitochondria are controlled by the cytoskeleton and depend on fusion and fission of their membranes. We use the budding yeast Saccharomyces cerevisiae and other model organisms to study these processes.
First of all, because it comprises a fascinating basic cell biological problem. By studying mitochondrial morphology and inheritance we can learn a lot about many different aspects of organellar biogenesis, such as partitioning of organelles during cell division, transport of membranes along the cytoskeleton, intracellular membrane fusion, regulatory processes etc. Mitochondrial dynamics is directly important for cellular function. Just to give a few examples: Fusion of mitochondria is essential for the development of spermatids in Drosophila, mitochondrial division is important for apoptosis, and mitochondrial fusion is required in human cells to allow extensive complementation of mitochondrial genomes as a defense against the accumulation of oxidative damage during cellular ageing.
Function and regulation of the mitochondrial membrane fusion machinery (DFG We 2174/4-2; funding granted 08/2009) Mitochondria are highly dynamic organelles that continuously undergo fusion and fission in many eukaryotic cells. Their dynamics is important for a multitude of cellular processes, including dissipation of energy in the cell, apoptosis and cellular aging. As the molecular machinery of mitochondrial fusion and fission has been highly conserved in evolution it can be conveniently studied in model organisms such as baker's yeast Saccharomyces cerevisiae. In the past funding period, we have functionally characterized two novel F-box proteins that regulate mitochondrial morphology and dynamics e.g. by degradation of components of the fusion machinery, and we have identified Mdm36 as a novel protein of the mitochondrial intermembrane space where it is required to regulate the equilibrium of fusion and fission. With the continuation of the project we plan to focus on three current questions. First, we would like to analyse the role of Mdm36 as a novel regulator of mitochondrial dynamics on the molecular level; second, we plan to further develop an in vitro assay to monitor fusion activity of isolated mitochondria; and third, we plan to study the role of mitochondrial fusion as a mechanism to counteract aging in yeast. Thus, the proposed research should contribute to our understanding of the molecular mechanisms of mitochondrial membrane fusion and its regulation, and provide insights into the importance of mitochondrial fusion for cell physiology. Molecular basis of motility, intracellular positioning and morphogenesis of mitochondria (DFG We 2174/3-3; funding granted 11/2007) Mitochondria exhibit an amazingly dynamic behaviour in many eukaryotic cell types. They continuously move along the tracks of the cytoskeleton, fuse and divide. Mitochondrial dynamics plays an important role for inheritance of the organelles upon cell division, and for their positioning in intracellular regions of high energy demand. The proposed project is aimed at the elucidation of the molecular mechanisms that mediate mitochondrial motility and determine mitochondrial shape. Depending on the cell type, mitochondria move in a microtubule or actin-dependent manner. In previous work, we have identified two novel kinesin-related motor proteins in the filamentous fungus Neurospora crassa and demonstrated their function in microtubule-dependent mitochondrial transport. To study actin-dependent organelle transport, we use the yeast Saccharomyces cerevisiae as a model organism. A systematic screen of yeast mutants has identified the myosin-related motor protein, Myo2, as a key component in this process. The functional characterization of Myo2 in mitochondrial transport by genetic, biochemical and microscopic approaches will be in the centre of the proposed project. Furthermore, we will investigate the role of mitochondrial membrane protein complexes that are essential for maintenance of the tubular shape of the organelles. These complexes are thought to build a scaffold spanning both mitochondrial membranes and connecting the mitochondrial genome in the matrix to a cytoskeleton-dependent inheritance machinery in the cytosol.
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