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Fas-FADD Death Domain Complex

Jackie Shirreffs '11 and Mac Woods '10


I. Introduction

Apoptosis, or programmed cell death, is induced upon the formation of death inducing signaling complex (DISC). Two of the main main subunits of this complex are Fas receptor and FADD (Fas associated death domain protein complex), which make up a complex weighing 217,476 kDa. The Fas-FADD complex is the initiating factor for DISC formation by creating a receptor platform for the recruitment of procaspase 8, which begins a series of interactions that eventually lead to programmed cell death.

A conformational change in the Fas domain allows for the binding of FADD protein. While the conformational change occurs a Fas-Fas complex bridge forms, resulting in a regulatory Fas-FADD complex. This complex functions as a mechanistic switch, which  regulates accidental DISC assembly, but allows for highly processive DISC formation, which leads to cell apoptosis.

II. General Structure

The Fas-FADD death domain consists of two tetramers, Subunit A and Subunit B. Each tetramer is made up of four Fas and four FADD subunits. The Fas subunits make up the core of the tetramer and the FADD subunits make up the outer section of the tertramer. Each Fas subunit is associated with a FADD death domain subunit creating a Fas-FADD copmplex bridge. The death domain complex contains eight Fas-FADD complex bidges; 1, 2, 3, 4, 5, 6, 7, and 8. The Fas-FADD complex bridges form a dimer with the neighboring Fas-FADD complex bridge through associations between the two Fas subunits.  The structure contains 12 SO4 molecules and 8 Na molecules. 

III. Fas Death Domain

Fas subunit undergoes a conformational change when the Fas-Ligand (FasL) binds. This conformational change shifts helix six away from the deatht domain (helix 1, 2, 3 and 4). Helix 6 fuses with helix 5 to form the stem helix. As the fusion occurs the formation of the c-helix occurs.The Fas interaction interface consists of the stem helix and the c-helix. Ile-313 unbends helix 6 and allows for the fusing of helix 5 and helix 6. The binding of the Fas ligand causes the opening of helix 6. This binding is mediated by two essential residues Arg-279 and His-282 of helix 4 in the Fas complex, which hydrogen bond with Tyr of the Fas ligand. H20 and Na are found between the Fas subunits and help stabilize the hydrophobic interactions between the Leu-303, Cys-304, Ala-307, and Gln-311 with the Thr-319, Ile-813, Leu-315, Gln-311, and Ile-314.  The opening of the Fas subunit is crucial for the formation of the Fas-Fas bridge and for the formtion of the Fas-FADD death domain complex.

IV. FADD Death Domain

The FADD subunit must undergo a conformational change of the c-helix (6th helix). This conformational change exposes the death domain interface of the FADD subunit. A hydrophobic pocket between helices 3 and 5 allows for the binding of death effector domain (DED) through hydophic interactions, mainly with a Phe-101, Cys-105 and Val-141. A different interface in helix 4 allows for binding with procaspase-8, creating favorable conditions for DISC complex formation. the exact binding is not well understood but may involve residues Ala-116 and Gly-109

V. Fas-FADD Interactions

The opening of the Fas subunit allows for the binding of the FADD death domain by exposing helix 2 and helix 3. FADD undergoes a conformational change of the C-helix in order to bind to Fas. The c-helix must shift in order to avoid a steric clash with the C-helix of the Fas subunit, helix 2 and helix 3 are exposed in the process. Helices 2 and 3 of both Fas and FADD are similar. Helix 1 of Fas binds to helix 6 of the FADD complex in an anitparellel orientation through electrostatic and hydrophobic interactions. The residues involved in FADD are Asp-175, Arg-189, Leu-176, Glu-179, Ala-183 and Gln-182 and in Fas are Lys-309, Leu-306, Asn-302, Ile-313, Ile-295, and Tyr-291Helix 6 of the Fas binds with helix 1 of the FADD complex. The residues Thr-235, Lys-231, Tyr-232, and Asp-228 of the Fas complex interact with the residues Cys-98, Asn-102, and Asp-106 of the FADD complex. Once the Fas ligand opens the Fas subunit and FADD has bound to the death domain of the Fas subunit the final subunit of the death domain complex binds. Proaspase-8 binds to the death domain of FADD and completes the death domain complex. Now DISC complex formation occurs, which leads to programed cell death will occur.

VI. References

Bajoratha, J. 1999. Analysis of Fas–ligand interactions using a molecular model of the receptor–ligand interface. Journal of Computer-Aided Molecular Design, 13: 409–418

Berglund, H; Olerenshaw, D; Sankar, A; Federwisch, M; McDonald, N,Q; and Driscoll, P.C. 2000 The Three-dimensional Solution Structure and Dynamic Properties of the Human FADD Death Domain. J. Mol. Biol. 302, 171±188.

Hill JM, Morisawa G, Kim T, Huang T, Wei Y, Wei Y et al. 2004. Identification of an expanded binding surface on the FADD death domain responsible for interaction with CD95/Fas. J Biol Chem; 279: 1474–1481.

Jeong EJ, Bang S, Lee TH, Park YI, Sim WS, Kim KS. 1999. The Solution Structure of FADD Death Domain: Strucutral basis of the Death Domain interactions of Fas and FADD. J. Biol Chem;274(23):16337– 16342.

Scott FL, Stec B, Pop C, Dobaczewska MK, Lee JJ, Monosov E, Robinson H, Salvesen GS, Schwarzenbacher R, Riedl SJ. 2009. The fas-FADD death domain complex structure unravels signalling by receptor clustering. Nature 457(7232):1019-22.

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