Myosin V-mediated vacuole distribution and fusion in fission yeast.

The class V myosins are actin-based motors that move a variety of cellular cargoes [1]. In budding yeast, their activity includes the relocation of a portion of the vacuole from the mother cell to the bud [2, 3]. Fission yeast cells contain numerous (approximately 80) small vacuoles. When S. pombe cells are placed in water, vacuoles fuse in response to osmotic stress [4]. Fission yeast possess two type V myosin genes, myo51(+) and myo52(+) [5]. In a myo51Delta strain, vacuoles were distributed throughout the cell, and mean vacuole diameter was identical to that seen in wild-type cells. When myo51Delta and wild-type cells were placed in water, vacuoles enlarged by fusion. In myo52Delta cells, by contrast, vacuoles were smaller and mostly clustered around the nucleus, and fusion in water was largely inhibited. When cells containing GFP-Myo52 were placed in water, Myo52 was seen to redistribute from the cell poles to the surface of the fusing vacuoles. Vacuole fusion in fission yeast was inhibited by the microtubule drug thiabendazole (TBZ) but not by the actin inhibitor latrunculin B. This is the first demonstration of the involvement of a type V myosin, possibly via an interaction with microtubules, in homotypic membrane fusion.