The APC/C subunit Mnd2/Apc15 promotes Cdc20 autoubiquitination and spindle assembly checkpoint inactivation

Cohesion fatigue explains why pharmacological inhibition of the APC/C induces a spindle checkpoint-dependent mitotic arrest

The Spindle Assembly Checkpoint (SAC) delays the onset of anaphase in response to unattached kinetochores by inhibiting the activity of the Anaphase-Promoting Complex/Cyclosome (APC/C), an E3 ubiquitin ligase. Once all the chromosomes have bioriented, SAC signalling is somehow silenced, which allows progression through mitosis. Recent studies suggest that the APC/C itself participates in SAC silencing by targeting an unknown factor for proteolytic degradation. Key evidence in favour of this model comes from the use of proTAME, a small molecule inhibitor of the APC/C. In cells, proTAME causes a mitotic arrest that is SAC-dependent. Even though this observation comes at odds with the current view that the APC/C acts downstream of the SAC, it was nonetheless argued that these results revealed a role for APC/C activity in SAC silencing. However, we show here that the mitotic arrest induced by proTAME is due to the induction of cohesion fatigue, a phenotype that is caused by the loss of sister chromatid cohesion following a prolonged metaphase. Under these conditions, the SAC is re-activated and APC/C inhibition is maintained independently of proTAME. Therefore, these results provide a simpler explanation for why the proTAME-induced mitotic arrest is also dependent on the SAC. While these observations question the notion that the APC/C is required for SAC silencing, we nevertheless show that APC/C activity does partially contribute to its own release from inhibitory complexes, and importantly, this does not depend on proteasome-mediated degradation.

APC15 mediates CDC20 autoubiquitylation by APC/C(MCC) and disassembly of the mitotic checkpoint complex

The anaphase-promoting complex/cyclosome bound to CDC20 (APC/C^CDC20) initiates anaphase by ubiquitylating B-type cyclins and securin. During chromosome bi-orientation, CDC20 assembles with MAD2, BUBR1 and BUB3 into a mitotic checkpoint complex (MCC) which inhibits substrate recruitment to the APC/C. APC/C activation depends on MCC disassembly, which has been proposed to require CDC20 auto-ubiquitylation. Here we characterized APC15, a human APC/C subunit related to yeast Mnd2. APC15 is located near APC/C’s MCC binding site, is required for APC/C^MCC-dependent CDC20 auto-ubiquitylation and degradation, and for timely anaphase initiation, but is dispensable for substrate ubiquitylation by APC/C^CDC20 and APC/C^CDH1. Our results support the view that MCC is continuously assembled and disassembled to enable rapid activation of APC/C^CDC20 and that CDC20 auto-ubiquitylation promotes MCC disassembly. We propose that APC15 and Mnd2 negatively regulate APC/C coactivators, and report the first generation of recombinant human APC/C.