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 yeas t cells contain numerous (approximately 80) small vacuoles. When S. pombe c ells are placed in water, vacuoles fuse in response to osmotic stress [4]. Fission yeast possess two type V myosin genes, myo51(+) and myo52(+) [5]. I n a rnyo51 Delta strain, vacuoles were distributed throughout the cell, and mean vacuole diameter was identical to that seen in wild-type cells. When myo51 Delta and wild-type cells were placed in water, vacuoles enlarged by fusion. In myo52 Delta cells, by contrast, vacuoles were smaller and mostly clustered around the nucleus, and fusion in water was largely inhibited. W hen cells containing GFP-Myo52 were placed in water, Myo52 was seen to redi stribute from the cell poles to the surface of the fusing vacuoles. Vacuole fusion in fission yeast was inhibited by the microtubule drug thiabendazol e (TBZ) but not by the actin inhibitor latrunculin B. This is the first dem onstration of the involvement of a type V myosin, possibly via an interacti on with microtubules, in homotypic membrane fusion.