Different thresholds of MPF inactivation are responsible for controlling different mitotic events in mammalian cell division

We present evidence for a paradigm that, during cell division, the decreasing activity of MPF acts as a master signal, which utilizes different thresholds to control the initiation of different mitotic events. The key temporal control here is the degradation of cyclin B1. Using single cell analysis, we measured the kinetics of cyclin B1 degradation and determined quantitatively the thresholds of cyclin B1 level for different mitotic events within a HeLa cell. These observed thresholds were: 1.36 +/- 0.49 mu M ( for chromosome separation), 0.75 +/- 0.08 mu M ( for cytokinesis) and 0.54 +/- 0.16 mu M ( for nuclear reassembly). By comparison, the average concentration of endogenous cyclin B1 within a prometaphase cell was found to be 2.92 +/- 1.7 mu M. We suggest that the decreasing order of these thresholds plays an important role in triggering the initiation of successive mitotic events in cell division.