Mitochondrial inheritance, the process whereby these organelles are transmitted from mother cells to developing daughter cells, is an integral part of the cell division cycle. Since mitochondria are essential organelles that can only be produced from pre-existing mitochondria, this transfer process is necessary for cellular proliferation because it insures that each cell that is produced during cell division contains this indispensable organelle. Here, we propose to use the budding yeast, Saccharomyces cerevisiae, to study one aspect of mitochondrial inheritance: the determinants that control mitochondrial position and motility during cell growth and development. Several findings indicate that mitochondrial distribution and movement are under tight cellular control. In sperm, for example, mitochondrial tubules are wrapped around flagella and are concentrated at critical sites of high ATP utilization. Our working model is that the cytoskeleton acts as a scaffold to immobilize mitochondria and as a track for directed mitochondrial movement. Recent studies indicate that motor molecules, a family of cytoskeleton-associated ATPases, utilize the energy of ATP binding and hydrolysis to direct and drive movement of organelles and vesicles along cytoskeletal fibers. Consistent with this, we find that yeast mitochondria are associated with actin and with a protein that resembles the actin-associated motor molecule, myosin. Studies carried out in living cells suggest that mitochondrial movements are track-dependent and regulated, and that expression of actin mutations results in defects in mitochondrial positional control and movement. Together, these findings suggest that the actin cytoskeleton controls and directs mitochondrial motility. Here, we propose to examine the mechanisms responsible for actin-mediation of mitochondrial motility during cell division and differentiation. The mitochondrial myosin-like protein that has been uncovered in our studies appears to be distinct from any of the other myosin gene products that have been identified in yeast. Therefore, we plan to characterize this protein at the molecular level. Initial efforts will be directed towards purifying, cloning and sequencing the mitochondrial myosin-like protein. Mutagenesis analysis of the cloned gene will reveal the structural features responsible for l) interactions between myosin and mitochondria, 2) the actin-binding, ATPase and organelle motor activities of the protein, and 3) regulation of myosin activities. Ultimately, our goal is to use the knowledge obtained from studies in yeast to understand similar processes in higher eukaryotes during normal growth and under conditions where defects in mitochondrial function have been linked to human diseases such as mitochondrial myopathies and neoplasia.
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