A systems biology analysis of apoptosome formation and apoptosis execution supports allosteric procaspase-9...

Area of Science:

• Cellular biology
• Biochemistry
• Molecular signaling

Background:

• The apoptosome is a protein complex crucial for initiating apoptosis.
• Caspase-9 activation on the apoptosome is a critical step, but its precise mechanism remains unclear.
• Current models often propose procaspase-9 homodimerization for activation.

Purpose of the Study:

• To investigate the mechanism of caspase-9 activation during apoptosis.
• To quantitatively and kinetically analyze apoptosome assembly and caspase-9 activation using mathematical simulations.
• To challenge or confirm existing models of initiator caspase activation.

Main Methods:

• Integration of quantitative data on molecular interactions and proteolytic processes.
• Development and application of mathematical simulations for biochemical signaling analysis.
• Comparison of simulation results with experimental data, including XIAP threshold effects and processing half-times.

Main Results:

• Simulations based on procaspase-9 homodimerization failed to replicate experimental kinetics of apoptosis and caspase-3 activation.
• Simulations assuming allosteric activation of procaspase-9 upon binding to the apoptosome backbone accurately reproduced experimental data.
• The model successfully predicted XIAP threshold concentrations and the apoptosome's molecular timer function.

Conclusions:

• Procaspase-9 is likely activated allosterically by binding to the apoptosome backbone, not through homodimerization.
• This finding challenges the prevailing dogma that all initiator procaspases require homodimerization for activation.
• Provides novel mechanistic insights into apoptosome-dependent apoptosis execution.