Separase is a protease that triggers separation of sister chromatids at mitosis by cleaving the cohesin complex that holds them together. In interphase, it is held in check by securin; this is then degraded at anaphase when separase activity is required. On , Christian Lehner and co-workers unveil additional roles for separase and securin in flies and cast doubt on previous suggestions (based on work in budding yeast) that they are required for completion of mitosis. The authors have used live-cell imaging of Drosophila embryos to investigate the effects of mutations in fly securin (PIM) and the separase regulatory subunit THR. Predictably, they observe that sister chromatids do not separate in pim and thr mutant epidermal cells. Nevertheless, these cells do exit mitosis and complete cytokinesis despite failing to segregate their DNA correctly. The most surprising finding, however, is that the mutants develop profound defects in epithelial tissue organization. Lehner and co-workers therefore conclude that separase regulates tissue integrity as well as genetic stability. Since separase has been shown to affect microtubule stability in yeast, it might control epithelial organization through such a mechanism.
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