Myosin V orientates the mitotic spindle in yeast

Coordination of spindle orientation with the axis of cell division is an es sential process in all eukaryotes. In addition to ensuring accurate chromos omal segregation, proper spindle orientation also establishes differential cell fates and proper morphogenesis(1). In both animal and yeast cells, thi s process is dependent on cytoplasmic microtubules interacting with the cor tical actin-based cytoskeleton(2-5), although the motive force was unknown. Here we show that yeast Myo2, a myosin V that translocates along polarized actin cables into the bud(6), orientates the spindle early in the cell cyc le by binding and polarizing the microtubule-associated protein Kar9 (refs 7-9). The tail domain of Myo2 that binds Kar9 also interacts with secretory vesicles(12) and vacuolar elements(13), making it a pivotal component of y east cell polarization.