Laser Microsurgery in Fission Yeast: Role of the Mitotic Spindle Midzone in Anaphase B

Introduction: During anaphase B in mitosis, polymerization and sliding of overlapping spindle microtubules (MTs) contribute to the outward movement the spindle pole bodies (SPBs). To probe the mechanism of spindle elongation, we combine fluorescence microscopy, photobleaching, and laser microsurgery in the fission yeast Schizosaccharomyces pombe. Results: We demonstrate that a green laser cuts intracellular structures in yeast cells with high spatial specificity. By using laser microsurgery, we cut mitotic spindles labeled with GFP-tubulin at various stages of anaphase B. Although cutting generally caused early anaphase spindles to disassemble, midanaphase spindle fragments continued to elongate. In particular, when the spindle was cut near a SPB, the larger spindle fragment continued to elongate in the direction of the cut. Photobleach marks showed that sliding of overlapping midzone MTs was responsible for the elongation of the spindle fragment. Spindle midzone fragments not connected to either of the two spindle poles also elongated. Equatorial microtubule organizing center (eMTOC) activity was not affected in cells with one detached pole but was delayed or absent in cells with two detached poles. Conclusions: These studies reveal that the spindle midzone is necessary and sufficient for the stabilization of MT ends and for spindle elongation. By contrast, SPBs are not required for elongation, but they contribute to the attachment of the nuclear envelope and chromosomes to the spindle, and to cell cycle progression. Laser microsurgery provides a means by which to dissect the mechanics of the spindle in yeast.