Co-ordination
of spindle orientation and mitotic progression in fission yeast.
Sylvie Tournier (1), Yannick Gachet (2), Jeremy Hyams (2), Jonathan Millar
(1)
(1) Division of Yeast Genetics, Nat Inst for Medical Research, Mill Hill,
London, NW7 1AA, England (jmillar@nimr.mrc.ac.uk); (2) Department of Biology,
University College London, Gower Street, London
We have previously shown that the actin depolymerising agent latrunculin B (Lat B) activates a mitotic checkpoint in fission yeast that delays the onset of anaphase (Gachet et al., 2001; Nature 412, 352-355). We show that, by live imaging of ndc80-GFP cdc11-CFP cells in the presence of Lat B, cells arrest with short, mis-oriented spindles and unseparated sister chromatids. [Ndc80 is a component of the fission yeast centromere and Cdc11 is a component of the outer plaque of the spindle pole body]. By live imaging of cdc13-GFP and cut2-GFP tagged cells, we find that a fraction of both Cdc13 (Cyclin B) and Cut2 (Securin) remain associated with the short, mis-oriented spindles in Lat B arrested cells. We find that the dissappearance of spindle associated Cdc13 and Cut2 requires a functional Anaphase Promoting Complex (APC). However we find that the delay in sister separation and destruction of Cdc13 and Cut2 imposed by Lat B is not altered by inactivation of the Mad1 or Mad2 spindle assembly checkpoint proteins. Instead cells lacking Atf1, or other components of the fission yeast stress activated MAP kinase pathway, fail to arrest in mitosis in the presence of Lat B. We conclude that depolymerisation of the actin cytoskeleton in fission yeast causes mis-orientation of mitotic spindles and activates a novel cell cycle checkpoint that inhibits APC-mediated destruction of Cdc13 and Cut2 and the onset of anaphase.