y tagged wild-type EGFP-Bub1 was only marginally accelerated by Bub1 inhibition . When considering this discrepancy, it is important to bear in mind that our data reflect turnover of chemically inhibited wild-type Bub1 expressed at endogenous levels, whereas Ashghar and colleagues monitored mutant versions of overexpressed Bub1, raising the possibility that their results may have been influenced by expression levels and/or mutation-induced structural alterations. We conclude that the effects of Bub1 activity on Bub1 turnover at KTs are at most minor, particularly when compared to 
the striking effects of Mps1 activity on Mps1 dynamics at KTs. As a further read-out for the effects of Bub1 inhibition on SAC activity, we used live cell imaging to monitor the responses of nocodazole-arrested HeLa and RPE1 cells to BAY-320 or BAY-524 and compared these to the responses seen in Bub1-depleted cells. Over a 24 hr observation period, the percentage of HeLa cells maintaining a SAC arrest dropped from 17% in Baron et al. eLife 2016;5:e12187. DOI: 10.7554/eLife.12187 11 of 26 Research article Cell biology controls to 4% and 2% in response to Bub1 inhibition by BAY-320 and BAY-524, respectively. These shifts in cell fates were largely compensated by increases in the percentages of cells BIRB-796 web undergoing apoptosis, from 74% in controls to 94% in Bub1-inhibited cells. In contrast, although the duration of mitosis was slightly reduced upon Bub1 inhibition, the extent of mitotic slippage remained at less than 10% under all conditions. In RPE1 cells, maintenance of SAC arrest over 24 hr was more pronounced, but again the percentage of arrested cells dropped from 61% in controls to 51%/44% in response to Bub1-inhibition, with increasing proportions of cells undergoing apoptosis or mitotic slippage. For comparison, depletion of Bub1 from either HeLa or RPE1 cells resulted in a 23 fold increase in mitotic slippage at the expense of apoptosis, while the proportion of cells sustaining an arrest remained roughly constant. Collectively, these results indicate that Bub1 activity contributes to the maintenance of maximal SAC activity, but that Bub1 protein levels are more important, most likely reflecting the observed role of Bub1 in the KT recruitment of SAC components. Importantly, we also compared the requirements for Bub1 activity and Bub1 protein in a cellular background in which SAC activity was partially compromised by the treatment of HeLa or RPE1 cells with a low dose of Reversine, a widely used inhibitor of the SAC kinase Mps1. In agreement with the results described above, Bub1 inhibition marginally reduced the time that Reversine-treated cells remained arrested before overriding nocodazole-induced arrest. Addition of Aurora B or Plk1 inhibitors, used as positive controls, led to PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19825579 the expected shortening of the duration of mitotic arrest . Similarly, Bub1 depletion also caused a drastic shortening of arrest. Taken together with previous studies, these observations demonstrate that the scaffolding function of Bub1 is required for the SAC, but its catalytic activity is largely dispensable. Bub1 inhibition does not significantly impair chromosome congression To analyze the impact of Bub1 inhibition on chromosome alignment, we treated cells with the Eg5 inhibitor Monastrol and then monitored the restoration of KT-MT attachments during spindle bipolarization in response to drug washout. While nearly 28% of Bub1-depleted cells failed to completely align all chromoso