The Bub1-Plk1 kinase complex promotes spindle checkpoint signalling through Cdc20 phosphorylation

Area of Science:

• Cell Biology
• Molecular Biology
• Genetics

Background:

• The spindle checkpoint prevents chromosome missegregation by inhibiting the anaphase-promoting complex/cyclosome (APC/C).
• The mitotic checkpoint complex (MCC) is a known inhibitor of APC/C(Cdc20).
• It remains unclear if MCC is the sole inhibitor of APC/C(Cdc20).

Purpose of the Study:

• To investigate if additional mechanisms, beyond MCC, regulate APC/C(Cdc20) activity.
• To elucidate the role of human checkpoint kinase Bub1 in APC/C inhibition.
• To determine the interplay between Bub1-Plk1 phosphorylation of Cdc20 and MCC formation.

Main Methods:

• In vitro kinase assays to assess APC/C(Cdc20) inhibition.
• Cellular experiments using human cells to study checkpoint signaling.
• Analysis of Cdc20 phosphorylation by Bub1 and Plk1.
• Genetic manipulation to deplete checkpoint proteins (Mad2, BubR1) and assess mitotic arrest.

Main Results:

• Human Bub1 directly phosphorylates Cdc20 and scaffolds Plk1 for Cdc20 phosphorylation.
• Bub1-Plk1-mediated Cdc20 phosphorylation inhibits APC/C(Cdc20) in vitro and is crucial for checkpoint signaling in cells.
• This phosphorylation is regulated by upstream signals and is independent of MCC assembly.
• A phospho-mimicking Cdc20 mutant rescued mitotic arrest in cells lacking Mad2 or BubR1.

Conclusions:

• Bub1-Plk1-mediated phosphorylation of Cdc20 represents a distinct APC/C inhibitory mechanism parallel to MCC.
• Both MCC and Bub1-Plk1-dependent Cdc20 phosphorylation are essential for sustaining mitotic arrest upon spindle defects.
• This highlights a multi-layered regulatory system for ensuring accurate chromosome segregation.