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#spindle checkpoint

3 public questions tagged with this topic.

Cell Cycle Regulation

Which of the following prevents mitotic exit if chromosomes are not properly aligned?

Accurate mitosis demands anaphase initiation strictly after all chromosomes achieve stable amphitelic attachment generating tension. The spindle assembly checkpoint monitors this criterion. Unattached kinetochores catalyze assembly of Mitotic Checkpoint Complex composed of Mad2 locked as C-Mad2, BubR1 ortholog of yeast Mad3, Bub3 and Cdc20 coactivator of APC/C. Within MCC, BubR1 provides pseudosubstrate motif inserting KEN boxes into Cdc20 binding pockets plus direct inhibition of APC/C central cavity, thereby preventing ubiquitination of securin and cyclin B. BubR1 also functions as protein kinase and scaffold recruiting PP2A-B56 phosphatase counteracting Aurora B at tensionless attachments and promoting establishment of correct bioriented connections. As long as even one kinetochore remains unattached, MCC production continues, suppressing mitotic exit and cytokinesis. Upon microtubule capture, dynein-mediated stripping of Mad1-Mad2 and p31comet-TRIP13 driven MCC disassembly terminates signal, permitting APC/C activation. Persistence of BubR1 mediated inhibition therefore prevents premature exit, safeguarding against aneuploidy, chromosome missegregation and cancer progression. This regulatory circuit illustrates integration of checkpoint kinases, ubiquitin ligases, phosphatases and structural proteins coordinating accurate cell division and preventing aneuploidy associated with tumorigenesis.

Ref: Musacchio, Spindle Assembly Checkpoint Structure of BubR1-MCC, Curr Biol 2015; NCBI Bookshelf, SAC component BubR1.

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Cell Cycle Regulation

What is the function of p31 comet in checkpoint silencing?

The spindle assembly checkpoint generates the Mitotic Checkpoint Complex containing closed Mad2, BubR1, Bub3 and Cdc20, which potently inhibits APC/C until all kinetochores attach. Checkpoint silencing requires active disassembly of this inhibitor once alignment completes. p31comet, also called MAD2L1BP and Trip13 adaptor, functions as dedicated MCC antagonist. Structurally mimicking Mad2, p31comet selectively binds the closed conformation of Mad2, termed C-Mad2, occupying the same interface that normally contacts BubR1 and therefore competitively extracting BubR1 from MCC. Together with AAA+ ATPase TRIP13, p31comet catalyzes the energetic conversion of C-Mad2 back to open O-Mad2, a conformer incapable of sequestering Cdc20. The resulting p31comet-C-Mad2-Cdc20 intermediate is recognized by TRIP13 which hydrolyzes ATP to unfold Mad2 safety belt. Released Cdc20 then activates APC/C for securin and cyclin B ubiquitination. p31comet also caps the Mad1-Mad2 core at kinetochores, preventing de novo generation of closed Mad2, ensuring rapid, switch-like APC/C activation and anaphase onset while preventing prolonged metaphase arrest and chromosome missegregation risks associated with persistent checkpoint signaling in mitosis. Additional feedback loops involving polo-like kinases, phosphatases and SCF-mediated degradation reinforce irreversibility and protect against premature progression that would compromise genome integrity and viability.

Ref: Alfieri et al., TRIP13-p31comet mechanism in MCC disassembly, Nature Communications 2018; Musacchio, SAC silencing, Curr Biol.

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Cell Cycle Regulation

Which protein complex silences the spindle checkpoint once all chromosomes are attached?

Silencing of spindle assembly checkpoint once all chromosomes achieve correct bi-orientation requires active disassembly of mitotic checkpoint complex and removal of checkpoint proteins from attached kinetochores. Central regulator is p31comet, also called MAD2L1BP, adaptor that binds closed conformation of Mad2 within MCC and recruits ATPase TRIP13 (Pch2 in yeast). TRIP13 uses ATP hydrolysis to convert closed Mad2 back to open inactive form, promoting disassociation of BubR1-Bub3-Cdc20 complex and freeing Cdc20 to activate APC/C. p31comet also competes with Mad1 for Mad2 binding, preventing new MCC generation and promoting stripping of Mad2 from kinetochores via dynein-mediated transport along microtubules. Overexpression of p31comet overrides checkpoint, causing premature anaphase even in presence of unattached chromosomes, while depletion prolongs mitotic arrest. Mps1 kinase, Mad1-Mad2 template, and Bub3-BubR1 instead propagate wait signal; they do not extinguish it. Thus p31comet-TRIP13 module serves as dedicated silencing machinery ensuring timely transition from checkpoint-on to checkpoint-off state, coupling attachment status to APC/C activation and mitotic progression.

Ref: Mapelli & Musacchio, EMBO J 2007, p31comet and TRIP13; Eytan et al., PNAS 2014, MCC Disassembly.

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