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

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