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Abstract

The spindle assembly checkpoint (SAC) is a crucial surveillance mechanism within the eukaryotic cell cycle that ensures proper chromosome segregation. Despite a wealth of cell biological and biochemical data about the SAC, its complex in vivo dynamics are still only fragmentarily understood. To obtain insight into this intricate signaling mechanism, we have combined mathematical modeling with quantitative experiments in fission yeast. As an accurate foundation for our mathematical models, we have quantified the abundance of SAC proteins in vivo in single cells. Furthermore, we have modulated the protein abundance of single SAC proteins in steps between 0 and about 200 % of the endogenous level. This enabled us to determine the range of SAC protein abundance that allows checkpoint activity, and enabled us to test models for the signaling mechanism. We find that the range of expression that allows SAC activity varies greatly between different SAC proteins. Interestingly, subtle reduction of some SAC proteins leads to a bimodal distribution of the SAC response. This implies the presence of an ultrasensitive step in the signaling mechanism, which could explain how the SAC can robustly respond to even a single misattached chromosome.