The Fas–FADD death domain complex structure unravels signalling by receptor clustering

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Abstract
The crystal structure of a complex between the cell surface receptor protein Fas and the Fas-associated death domain (FADD) protein - a central feature of the so-called death-inducing signalling complex of apoptosis-inducing cellular receptors - has been determined at 2.7 Å resolution. The structure reveals a previously unknown type of death domain interaction that allows four FADD and four Fas proteins to aggregate in the one complex. Surprisingly, a conformational change opens up the Fas death domain, which creates binding surfaces for FADD as well as Fas-Fas 'bridging' interactions. Only when a sufficient number of Fas molecules are in close proximity - as is the case when Fas ligand binds - can the open form of Fas be stabilized. This study presents the crystal structure of a Fas-FADD complex, a central feature of the so-called death inducing signalling complex. The structure reveals a new mode of death domain interactions that allows four FADD and four Fas proteins in one complex. The death inducing signalling complex (DISC) formed by Fas receptor, FADD (Fas-associated death domain protein) and caspase 8 is a pivotal trigger of apoptosis1,2,3. The Fas–FADD DISC represents a receptor platform, which once assembled initiates the induction of programmed cell death. A highly oligomeric network of homotypic protein interactions comprised of the death domains of Fas and FADD is at the centre of DISC formation4,5. Thus, characterizing the mechanistic basis for the Fas–FADD interaction is crucial for understanding DISC signalling but has remained unclear largely because of a lack of structural data. We have successfully formed and isolated the human Fas–FADD death domain complex and report the 2.7 Å crystal structure. The complex shows a tetrameric arrangement of four FADD death domains bound to four Fas death domains. We show that an opening of the Fas death domain exposes the FADD binding site and simultaneously generates a Fas–Fas bridge. The result is a regulatory Fas–FADD complex bridge governed by weak protein–protein interactions revealing a model where the complex itself functions as a mechanistic switch. This switch prevents accidental DISC assembly, yet allows for highly processive DISC formation and clustering upon a sufficient stimulus. In addition to depicting a previously unknown mode of death domain interactions, these results further uncover a mechanism for receptor signalling solely by oligomerization and clustering events.