Positioning and elongation of the fission yeast spindle by microtubule-based pushing.

In eukaryotic cells, proper position of the mitotic spindle is necessary for successful cell division and development. We explored the nature of forces governing the positioning and elongation of the mitotic spindle in Schizosaccharomyces pombe. We hypothesized that astral microtubules exert mechanical force on the S. pombe spindle and thus help align the spindle with the major axis of the cell. Microtubules were tagged with green fluorescent protein (GFP) and visualized by two-photon microscopy. Forces were inferred both from time-lapse imaging of mitotic cells and, more directly, from mechanical perturbations induced by laser dissection of the spindle and astral microtubules. We found that astral microtubules push on the spindle poles in S. pombe, in contrast to the pulling forces observed in a number of other cell types. Further, laser dissection of the spindle midzone induced spindle collapse inward. This offers direct evidence in support of the hypothesis that spindle elongation is driven by the sliding apart of antiparallel microtubules in the spindle midzone. Broken spindles recovered and mitosis completed as usual. We propose a model of spindle centering and elongation by microtubule-based pushing forces.