Bub1 is not essential for the checkpoint response to unattached kinetochores in diploid human cells

Error-free chromosome segregation during mitosis depends on a functional spindle assembly checkpoint (SAC). The SAC is a multi-component signalling system that is recruited to unattached or incorrectly attached kinetochores to catalyse the formation of a soluble inhibitor, known as the Mitotic Checkpoint Complex (MCC), which binds and inhibits the anaphase promoting complex (APC/C) [1]. We have previously proposed that two separable pathways, composed of KNL1-Bub3-Bub1 (KBB) and Rod-Zwilch-Zw10 (RZZ), recruit Mad1-Mad2 complexes to human kinetochores to activate the SAC [2]. Although Bub1 is absolutely required for checkpoint signalling in yeast (which lack RZZ), there is conflicting evidence as to whether this is the case in human cells based on siRNA studies [2-5]. Here we show that, while Bub1 is required for recruitment of BubR1, it is not strictly required for the checkpoint response to unattached kinetochores in diploid human cells.