Spatial control of the APC/C ensures the rapid degradation of Cyclin B1

The proper control of mitosis depends on the ubiquitin-mediated degradation of the right mitotic regulator at the right time. This is under the control of the anaphase promoting complex/cyclosome (APC/C) ubiquitin ligase that is regulated by the Spindle Assembly Checkpoint (SAC). The Checkpoint prevents the APC/C from recognizing Cyclin B1, the essential anaphase and cytokinesis inhibitor, until all chromosomes are attached to the spindle. Once chromosomes are attached, Cyclin B1 is rapidly degraded to enable chromosome segregation and cytokinesis. We have a good understanding of how the SAC inhibits the APC/C, but relatively little is known about how the APC/C recognises Cyclin B1 as soon as the SAC is turned off. Here, by combining live cell imaging, in vitro reconstitution, biochemistry, and structural analysis by cryo-electron microscopy, we provide evidence that the rapid recognition of Cyclin B1 in metaphase requires spatial regulation of the APC/C. Using fluorescence cross correlation spectroscopy, we find that Cyclin B1 and the APC/C primarily interact at the mitotic apparatus. We further show that this is because Cyclin B1, like the APC/C, binds to nucleosomes, and identify an arginine-anchor in the N-terminus as necessary and sufficient for binding to the nucleosome. Mutating the nucleosome binding motif on Cyclin B1 reduces its interaction with APC/C and delays its degradation, and cells with the mutant, non-nucleosome-binding Cyclin B1 become aneuploid, demonstrating the physiological relevance of our findings. Together, our data demonstrate that mitotic chromosomes constitute a platform to promote the efficient interaction between Cyclin B1 and APC/C and ensure the timely degradation of Cyclin B1 and genomic stability.