Unattached kinetochores result in the mitotic kinase BUB1 activating the spindle assembly checkpoint (SAC), which recruits the mitotic checkpoint complex composed of MAD2, BUBR1, BUB3 and CDC20, and delays anaphase entry. Lack of a clear mechanistic insight and emergence of conflicting studies prompted Geert Kops and colleagues to investigate (p. ) how exactly BUB1 activates SAC in human cells. By expressing full-length or truncated tagged constructs of BUB1 in BUB1-depleted HeLa cells to determine the domains and interactions that play a role in SAC activation, they found that the N-terminal region is sufficient to recruit BUBR1 to the kinetochore but, interestingly, its recruitment is not required for SAC activity. Although the interaction of BUB1 with BUB3, BUBR1 and KNL1 was confirmed here BUB1, unexpectedly, does not bind to MAD1 as is the case in lower organisms. Accordingly, MAD1 localised to kinetochores in the absence of BUB1, although loading was delayed, indicating that BUB1 somehow accelerates MAD1 recruitment. Importantly, the authors uncovered a region in BUB1 that contains an ABBA and a KEN-box motif, which is responsible for recruiting CDC20 to kinetochores and essential for SAC activity. In conclusion, this study shows that, in human cells, BUB1 is not an important kinetochore receptor for MAD1, and BUB1-mediated recruitment of BUBR1 is dispensable for SAC activation; instead SAC depends strongly on the CDC20-binding ABBA and KEN-box motifs of BUB1.
