Bub1 is not required 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 signaling system that is recruited to incorrectly attached kinetochores to catalyze the formation of a soluble inhibitor, known as the mitotic checkpoint complex (MCC), which binds and inhibits the anaphase promoting complex [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]. We refer to this as the dual pathway model. Although Bub1 is absolutely required for MCC formation in yeast (which lack RZZ), there is conflicting evidence as to whether this is also the case in human cells based on siRNA studies [2-5]. Here we report, using genome editing, that Bub1 is not strictly required for the SAC response to unattached kinetochores in human diploid hTERT-RPE1 cells, consistent with the dual pathway model.