The role of actin in spindle orientation changes during the Saccharomyces cerevisiae cell cycle

In the budding yeast Saccharomyces cerevisiae, the mitotic spindle must ali gn along the mother-bud axis to accurately partition the sister chromatids into daughter cells. Previous studies showed that spindle orientation requi red both astral microtubules and the actin cytoskeleton, We now report that maintenance of correct spindle orientation does not depend on F-actin duri ng G2/M phase of the cell cycle. Depolymerization of F-actin using Latruncu lin-A did not perturb spindle orientation after this stage. Even an early s tep in spindle orientation, the migration of the spindle pole body (SPB), b ecame actin-independent if it was delayed until late in the cell cycle. Early in the cell cycle, both SPB migration and spindle orientation were ve ry sensitive to perturbation of F-actin. Selective disruption of actin cabl es using a conditional tropomyosin double-mutant also led to defects in spi ndle orientation, even though cortical actin patches were still polarized. This suggests that actin cables are important for either guiding astral mic rotubules into the bud or anchoring them in the bud. In addition, F-actin w as required early in the cell cycle for the development of the actin-indepe ndent spindle orientation capability later in the cell cycle. Finally, neit her SPB migration nor the switch from actin-dependent to actin-independent spindle behavior required B-type cyclins.