^FADDDED filaments coordinate complex IIa assembly during TNF-induced apoptosis.

Abstract

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.