A MAP kinase-dependent actin checkpoint ensures proper spindle orientationin fission yeast

The accurate segregation of chromosomes at mitosis depends on a correctly a ssembled bipolar spindle that exerts balanced forces on each sister chromat id(1,2). The integrity of mitotic chromosome segregation is ensured by the spindle assembly checkpoint (SAC) that delays mitosis in response to defect ive spindle organisation or failure of chromosome attachment(2,3). Here we describe a distinct mitotic checkpoint in the fission yeast, Schizosaccharo myces pombe, that monitors the integrity of the actin cytoskeleton and dela ys sister chromatid separation, spindle elongation and cytokinesis until sp indle poles have been properly oriented. This mitotic delay is imposed by a stress-activated mitogen-activated protein (MAP) kinase pathway but is ind ependent of the anaphase-promoting complex (APC)(4,5).