Cryo-EM structural analysis of FADD:Caspase-8 complexes defines the catalytic dimer architecture for co-ordinated...

Area of Science:

• Cellular Biology
• Molecular Biology
• Biochemistry

Background:

• Regulated cell death is crucial for development and tissue homeostasis.
• Multi-protein signaling platforms, such as the Death-Inducing Signaling Complex (DISC), govern cell fate decisions.
• The DISC comprises a core FADD:Caspase-8 complex and regulatory proteins like c-FLIP.

Purpose of the Study:

• To structurally elucidate the FADD:Caspase-8 complex and its regulation by c-FLIP.
• To understand the mechanism by which the tandem death effector domain (tDED) filament orients procaspase-8 for activation.
• To determine how c-FLIP inhibits Caspase-8 activity within these signaling platforms.

Main Methods:

• Electron microscopy was used to visualize full-length procaspase-8 in complex with FADD.
• Structural analysis of the FADD-nucleated tandem death effector domain (tDED) helical filament.
• Investigation of c-FLIP recruitment and its impact on complex architecture.

Main Results:

• Structural analysis revealed how the FADD-nucleated tDED helical filament orients procaspase-8 catalytic domains for activation via anti-parallel dimerization.
• Recruitment of c-FLIP_S was shown to inhibit Caspase-8 activity by altering tDED triple helix architecture.
• This alteration leads to steric hindrance, preventing Caspase-8 catalytic domain assembly and tDED helical filament elongation.

Conclusions:

• The FADD:Caspase-8 complex architecture is critical for regulating cell death.
• c-FLIP acts as an inhibitor by disrupting the tDED filament structure, thereby preventing Caspase-8 activation.
• These findings illuminate how complex plasticity and composition dictate life/death decisions across multiple signaling platforms.