BubR1 blocks substrate recruitment to the APC/C in a KEN-box-dependent manner

The spindle assembly checkpoint (SAC) is a signalling network that delays anaphase onset until all the chromosomes are attached to the mitotic spindle through their kinetochores. The downstream target of the spindle checkpoint is the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase that targets several anaphase inhibitors for proteolysis, including securin and cyclin B1. In the presence of unattached kinetochores, the APC/C is inhibited by the mitotic checkpoint complex (MCC), a tetrameric complex composed of three SAC components, namely BubR1, Bub3 and Mad2, and the APC/C co-activator Cdc20. The molecular mechanisms underlying exactly how unattached kinetochores catalyse MCC formation and how the MCC then inhibits the APC/C remain obscure. Here, using RNAi complementation and in vitro ubiquitylation assays, we investigate the domains in BubR1 required for APC/C inhibition. We observe that kinetochore localisation of BubR1 is required for efficient MCC assembly and SAC response. Furthermore, in contrast to previous studies, we show that the N-terminal domain of BubR1 is the only domain involved in binding to Cdc20-Mad2 and the APC/C. Within this region, an N-terminal KEN box (KEN1) is essential for these interactions. By contrast, mutation of the second KEN box (KEN2) of BubR1 does not interfere with MCC assembly or APC/C binding. However, both in cells and in vitro, the KEN2 box is required for inhibition of APC/C when activated by Cdc20 (APC/C(Cdc20)). Indeed, we show that this second KEN box promotes SAC function by blocking the recruitment of substrates to the APC/C. Thus, we propose a model in which the BubR1 KEN boxes play two very different roles, the first to promote MCC assembly and the second to block substrate recruitment to APC/C(Cdc20).