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Vol. 19, Issue 9, 3713-3723, September 2008

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Latrunculin A Delays Anaphase Onset in Fission Yeast by Disrupting an Ase1-independent Pathway Controlling Mitotic Spindle Stability

John C. Meadows^*,, and Jonathan Millar^*,

*Division of Yeast Genetics, National Institute for Medical Research, London NW7 1AA, United Kingdom; and Cell Cycle Laboratory (M116), Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, United Kingdom

Submitted February 15, 2008; Revised May 23, 2008; Accepted June 11, 2008
Monitoring Editor: Mark Solomon

It has been proposed previously that latrunculin A, an inhibitor of actin polymerization, delays the onset of anaphase by causing spindle misorientation in fission yeast. However, we show that mto1 cells, which are defective in nucleation of cytoplasmic microtubules, have profoundly misoriented spindles but are not delayed in the timing of sister chromatid separation, providing compelling evidence that fission yeast does not possess a spindle orientation checkpoint. Instead, we show that latrunculin A delays anaphase onset by disrupting interpolar microtubule stability. This effect is abolished in a latrunculin A-insensitive actin mutant and exacerbated in cells lacking Ase1, which cross-links antiparallel interpolar microtubules at the spindle midzone both before and after anaphase. These data indicate that both Ase1 and an intact actin cytoskeleton are required for preanaphase spindle stability. Finally, we show that loss of Ase1 activates a checkpoint that requires only the Mad3, Bub1, and Mph1, but not Mad1, Mad2, or Bub3 checkpoint proteins.

Address correspondence to: Jonathan Millar (j.millar{at}warwick.ac.uk)