The separation of daughter cells at mitosis is driven by constriction of a cytokinetic actomyosin ring (CAR) by myosin motors such as fission yeast Myo2. Clearly, this must occur only after the spindle has formed and sister chromatids have separated. How exactly anaphase onset and CAR formation are coordinated has been unclear, however. Daniel Mulvihill and Jeremy Hyams have approached the problem by using a Myo2-GFP fusion protein to monitor the timing and regulation of CAR formation in fission yeast (see). They show that recruitment of Myo2 to the CAR at the onset of anaphase A is significantly delayed by microtubule-depolymerizing drugs but that this delay does not occur if cells lack the spindle-assembly checkpoint component Mad2. Mulvihill and Hyams also show that full recruitment of the polo-related kinase (Plo1) to spindle pole bodies (SPBs) is similarly compromised by microtubule depolymerization and, again, this depends on Mad2. The authors conclude that recruitment of Plo1 to SPBs normally provides at least part of the signal for CAR formation but that activation of the spindle checkpoint can block this and thereby delay cytokinesis.
