Actin and myosin function in directed vacuole movement during cell division in Saccharomyces cerevisiae

During cell division, cytoplasmic organelles are not synthesized de novo, rather they are replicated and partitioned between daughter cells. Partitioning of the vacuole in the budding yeast Saccharomyces cerevisiae is coordinated with the cell cycle and involves a dramatic translocation of a portion of the parental organelle from the mother cell into the bud. While the molecular mechanisms that mediate this event are unknown, the vacuole's rapid and directed movements suggest cytoskeleton involvement. To identify cytoskeletal components that function in this process, vacuole inheritance was examined in a collection of actin mutants. Six strains were identified as being defective in vacuole inheritance. Tetrad analysis verified that the defect cosegregates with the mutant actin gene. One strain with a deletion in a myosin-binding region was analyzed further. The vacuole inheritance defect in this strain appears to result from the loss of a specific actin function; the actin cytoskeleton is intact and protein targeting to the vacuole is normal. Consistent with these findings, a mutation in the actin-binding domain of Myo2p, a class V unconventional myosin, abolishes vacuole inheritance. This suggests that Myo2p serves as a molecular motor for vacuole transport along actin filaments. The location of actin and Myo2p relative to the vacuole membrane is consistent with this model. Additional studies suggest that the actin filaments used for vacuole transport are dynamic, and that profilin plays a critical role in regulating their assembly. These results present the first demonstration that specific cytoskeletal proteins function in vacuole inheritance.