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Abstract

The spindle assembly checkpoint is a highly conserved signaling pathway that safeguards the proper distribution of chromosomes into daughter cells. This checkpoint detects kinetochores that fail to make proper contact to microtubules and prevents anaphase as a consequence. Despite the crucial importance of this pathway, it is unknown to which extent checkpoint signaling is robust to perturbations. We varied the expression of checkpoint proteins in fission yeast, and analyzed the outcome on checkpoint activity in single cells. We created a framework for interpreting these results by accurately quantifying checkpoint and target proteins on the single cell level. We found that, for core checkpoint proteins, a mere 20 % reduction in abundance can suffice to shift the checkpoint out of the robustness zone and to provoke errors in signaling. Under such conditions, genetically identical cells split into two populations, where some cells maintain a checkpoint response, whereas others fail. Our combined in vivo imaging and in silico analysis suggests that stoichiometric inhibition of the anaphase activator Slp1 can explain both the physiological robustness as well as the population split outside the robustness zone. Our results highlight that checkpoint protein abundance is an important determinant in specifying the checkpoint response and that some checkpoint proteins need to be kept within exceptionally tight windows of abundance to ensure robust signaling.