Catalytic assembly of the mitotic checkpoint inhibitor BubR1-Cdc20 by a Mad2-induced functional switch in Cdc20

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• The mitotic checkpoint is crucial for maintaining genomic stability during cell division.
• Unattached kinetochores trigger signaling pathways that prevent premature anaphase onset.
• Key proteins involved include Mad2, Cdc20, BubR1, and the Anaphase-Promoting Complex/Cyclosome (APC/C).

Purpose of the Study:

• To elucidate the molecular mechanism by which the mitotic checkpoint generates a diffusible anaphase inhibitor.
• To define the roles of Mad2, Cdc20, and BubR1 in the formation and function of the mitotic checkpoint complex.
• To understand how APC/C-mediated ubiquitination of cyclin B is regulated by the mitotic checkpoint.

Main Methods:

• Biochemical assays to study protein-protein interactions between Mad2, Cdc20, BubR1, and APC/C.
• Analysis of conformational changes in Mad2 and Cdc20 upon binding.
• In vitro assays to measure ubiquitination activity of APC/C(Cdc20) in the presence of checkpoint proteins.

Main Results:

• Mad2 binding induces a conformational change in Cdc20, enabling BubR1 binding.
• BubR1, but not Mad2, binding to APC/C(Cdc20) inhibits cyclin B ubiquitination.
• Closed Mad2 catalytically amplifies the production of the inhibitory BubR1-Cdc20 complex.
• The mitotic checkpoint operates via a two-step catalytic cascade involving Mad2-Cdc20 and BubR1-Cdc20 intermediates.

Conclusions:

• The study reveals a detailed catalytic cascade mechanism for mitotic checkpoint function.
• Mad2 acts as a catalyst to amplify the production of the APC/C inhibitor, BubR1-Cdc20.
• This mechanism ensures robust and efficient inhibition of cyclin B ubiquitination, maintaining chromosome integrity.