Abstact

Extrinsic apoptosis is initiated by signaling from death receptors, leading to the assembly of RIPK1, FADD, and caspase-8 complex. Subsequently, caspase-8 forms a filamentous structure through the oligomerization of its tandem death effector domain (tDED), resulting in caspase activation and cell death. Although the DED of FADD (FADDDED) is homologous to the tDEDs of caspase-8 (casp8tDED) and both oligomerize to function, the functional form of FADDDED oligomer in extrinsic apoptosis remains unclear. Here, using cryogenic-electron microscopy, we elucidate the structure of FADDDED filaments comprising three helical chains assembled through three types of iterative interactions. Mutations disrupting FADDDED filament formation impair the recruitment of RIPK1 and caspase-8, and abrogate the cell death response, suggesting that FADDDED filamentation represents an important mechanistic step in the initiation of TNF-induced extrinsic apoptosis. Contrary to the belief that the homotypic death domains of RIPK1 and FADD are solely responsible for their interaction, we here show this interaction requires FADDDED filamentation. Furthermore, cFLIP can disrupt FADDDED filaments, uncovering an additional antiapoptotic mechanism of cFLIP beyond its disruption of caspase-8 filament. Molecular dynamics simulations reveal that FADDDED filament thermodynamically favors casp8tDED monomer over FADDDED monomer, thus explaining the hierarchy and stoichiometry of FADD/caspase-8 complex assembly. These findings highlight the hitherto unappreciated roles of FADDDED filament formation in extrinsic apoptosis.