The Fas-FADD death domain complex structure reveals the basis of DISC assembly and disease mutations

Area of Science:

• Molecular Biology
• Immunology
• Structural Biology

Background:

• The extrinsic apoptotic pathway is initiated by the death-inducing signaling complex (DISC).
• DISC formation involves the death receptor Fas, adaptor protein FADD, and caspase-8.
• Mutations in Fas leading to DISC disruption cause autoimmune lymphoproliferative syndrome (ALPS).

Purpose of the Study:

• To elucidate the structural basis of the Fas-FADD interaction within the DISC.
• To understand how ALPS-associated mutations affect Fas-FADD complex formation and function.
• To correlate structural insights with Fas signaling mechanisms and disease pathology.

Main Methods:

• X-ray crystallography to determine the structure of the Fas-FADD death domain (DD) complex.
• Site-directed mutagenesis to create and analyze ALPS-associated mutations.
• In vitro biochemical assays and cell-based signaling assays to assess protein interactions and apoptotic signaling.

Main Results:

• The Fas-FADD DD complex forms an asymmetric oligomer of 5-7 Fas DD and 5 FADD DD units.
• ALPS-associated mutations are located at the interfaces of this oligomeric structure.
• Structure-based mutations impair Fas-FADD interaction in vitro and in cells, with severity correlating to mutation frequency in the structure.
• The oligomeric structure explains Fas ligand (FasL) requirements and predicts dominant-negative effects of Fas mutations.

Conclusions:

• The determined oligomeric structure of the Fas-FADD DD complex provides a mechanistic understanding of DISC formation.
• Structural insights explain the pathogenesis of ALPS and the impact of specific Fas mutations.
• The structure highlights optimal positioning for caspase-8 recruitment, essential for initiating apoptosis.