Deciphering DED assembly mechanisms in FADD-procaspase-8-cFLIP complexes regulating apoptosis.

Area of Science:

• Molecular and Cellular Biology
• Structural Biology
• Immunology

Background:

• Fas-associated protein with death domain (FADD), procaspase-8, and cellular FLICE-inhibitory proteins (cFLIP) are key regulators of cell death signaling.
• Their assembly via death-effector domains (DEDs) dictates apoptosis or cell survival.
• Lack of atomic structures for their ternary DED complex limited understanding of regulatory mechanisms.

Purpose of the Study:

• To determine the atomic coordinates of human FADD-procaspase-8-cFLIP complexes.
• To gain structural insights into the interactions governing death receptor signaling.
• To elucidate the mechanisms of apoptotic and necroptotic signaling regulation.

Main Methods:

• X-ray crystallography
• Cryogenic electron microscopy (cryo-EM)
• Structure-guided mutagenesis

Main Results:

• Atomic coordinates of human FADD-procaspase-8-cFLIP complexes were determined.
• Structures revealed how FADD and cFLIP orchestrate caspase-8 complex assembly.
• A helical procaspase-8-cFLIP hetero-double layer was identified, potentially promoting limited caspase-8 activation for cell survival.
• Mutagenesis supported the role of the triple-FADD complex in caspase-8 activation and RIPK1 regulation.

Conclusions:

• The study provides a unified mechanism for DED assembly and procaspase-8 activation.
• These findings offer mechanistic explanations for regulating apoptotic and necroptotic signaling.
• The results have implications for understanding cell death in development, immunity, and disease.