Dynein-dependent movements of the mitotic spindle in Saccharomyces cerevisiae do not require filamentous actin

In budding yeast, the mitotic spindle is positioned in the neck between the mother and the bud so that both cells inherit one nucleus. The movement of the mitotic spindle into the neck can be divided into two phases: (1) Kip3p-dependent movement of the nucleus to the neck and alignment of the short spindle, followed by (2) dynein-dependent movement of the spindle into the neck and oscillation of the elongating spindle within the neck. Actin has been hypothesized to be involved in all these movements. To test this hypothesis, we disrupted the actin cytoskeleton with the use of mutations and latrunculin A (latrunculin). We assayed nuclear segregation in synchronized cell populations and observed spindle movements in individual living cells. In synchronized cell populations, no actin cytoskeletal mutant segregated nuclei as poorly as cells lacking dynein function, furthermore, nuclei segregated efficiently in latrunculin-treated cells. Individual living cell analysis revealed that the preanaphase spindle was mispositioned and misaligned in latrunculin-treated cells and that astral microtubules were misoriented, confirming a role for filamentous actin in the early, Kip3p-dependent phase of spindle positioning. Surprisingly, mispositioned and misaligned mitotic spindles moved into the neck in the absence of filamentous actin, albeit less efficiently. Finally, dynein-dependent sliding of astral microtubules along the cortex and oscillation of the elongating mitotic spindle in the neck occurred in the absence of filamentous actin.