Friday, March 30, 2012

Fas/FADD Death Domain Complex - Stage 3

In this post I will attempt to make my case for why you should consider the Fas/FADD death domain complex as a strong contender for protein of the year. Unlike several of our opponents, My protein has not received any favors by presented in class... (I'm looking at you RuBisCO and Nitrogenase...). To be fair, these proteins do very novel chemistry. Without RuBisCO we would find ourselves lacking fresh air to breath and without nitrogenase we would find ourselves without plants to produce RuBisCO. However, as Prof. Arnoys said today "breathing is for the weak". Assuming this to be true I would now like to present my case for protein of the year.

Apoptosis can be initiated in two ways: either a cell fails to recieve the signal to survive or the cell recieves a signal for death. One method of actively signaling for apoptosis proceeds through the formation of a signalling complex called the Fas/FADD death domain complex. This occurs through the event of a Fas trimer bonding with a FasL trimer from a neighboring cell. However, unlike cell necrosis, apoptosis does not cause the release of cellular content and no inflammatory response is initiated.

The Fas/FADD death domain complex is formed through the boding of the FADD protein domain to the Fas Domain (initiated through the binding of Fas to FasL mentioned above. These interactions are novel as the bonds which hold these complexes together are very weak. Although this protein tends to form tetramers and higher oligomers, the complex is only held together through hydrophobic interactions as can be seen below.


The interaction between Fas/FADD



The interactions between 2 Fas and 2 FADD are shown.


A Fas/FADD Death domain complex

This results in a highly oligomeric product which signals cell apoptosis in the following steps.

Simple explanation:
1.Fas trimer's on the surface of a cell bind with Fas-Ligands on the exterior of a neighboring cell.
2. The Death induced signaling complex (DISC) is formed through the incorporation of FADD.
3. Shazam
4. signaling molecules are released (more shazam)
5. A lot more intercellular-activity that was imitated through the DISC formation
6. Apoptosis

OR, for the not weak of heart, the complicated explanation:

1. A Fas Trimer associates with the adapter protein: Fas-Ligand
2. The DISC (death induced signaling complex) is formed
3. The adapter protein FADD, Fas associated death domain, binds its death domain to the death domain of the Fas
4. The DED, death effector domain, of FADD binds the DED of procaspase 8 into the DISC
5. Procaspace 8 is activated and caspase 8 dissociates from the DISC moving into the cytoplasm
6. Caspase 8 cleaves procaspase 3 into active caspase 3
7. Caspase 3 cleaves I-CAD, enabling the activity of CAD which enters the nucleus and begins to destroy DNA

Although the Fas/FADD death domain begins the process of cell apoptosis, the enzyme lacks a specific interaction site. There is no well defined active site and as such, ligands and signalling molecules bind only through being incorporated into or forming additional clusters with the DISC.

As one might imagine, a protein which can induec cellular death has potential to be employed as killer of virally infected cells. Cytotoxic T cells can kill a virally infected cell in two ways: one, through the release of toxic perforins and granzymes and two, through the association of a T cell Fas-Ligand With a Fas trimer on the infected cell. The T-cells identify a infected cell, and when the FasL bound to the T-cell binds to the Fas trimer on the surface of the infected cell the DISC forms. This then causes the cell to commit apoptosis, that is cellular death without the release inner cellular content. This then would cause the "death" of the infected cell without releasing the virus to simply move on and infect the closest target.

To recount the reasons why this protein is cool:
1. Initiates apoptosis through binding of a ligand
2. No active site. All signaling molecules released from this protein must be formed from substrates incorporated into the DISC cluster.
3. It has an awesome name
4. Can function as a way to destroy virally infected cells.
5. It has an awesome name
6. The DISC cluster is held together only through hydrophobic interactions
7. It has an awesome name

Sunday, March 11, 2012

Fas/FADD Death Domain Complex - Stage 2

The Fas-FADD death domain complex structure unravels signalling by receptor clustering.
Scott FL, Stec B, Pop C, Dobaczewska MK, Lee JJ, Monosov E, Robinson H, Salvesen GS, Schwarzenbacher R, Riedl SJ.
Nature. 2009 Feb 19;457(7232):1019-22. Epub 2008 Dec 31.

  • The Fass/Fadd death domain complex serves as a cellular switch which governs DISC (death induced signaling complex) formation in the presence of stimuli. The group responsible for this research was looking to qualify the types of interactions between this protein and it's signalling molecules as well as with the cell, and other proteins of this type. They found that this protein is weakly bound and mostly held together through hydrophobic interactions. The protein forms tetramers and higher oligomers. The Fass/FaDD death domain complex has no well defined active interaction site where a ligand or signalling molecule can bind and thus the signalling occurs only through the protein forming clusters with the signalling molecule.


The Fas-FADD death domain complex structure reveals the basis of DISC assembly and disease mutations.
Wang L, Yang JK, Kabaleeswaran V, Rice AJ, Cruz AC, Park AY, Yin Q, Damko E, Jang SB, Raunser S, Robinson CV, Siegel RM, Walz T, Wu H.
Nat Struct Mol Biol. 2010 Nov;17(11):1324-9. Epub 2010 Oct 10.

  • The goal of the research presented in the paper was to clarify structural data surrounding the interactions of Fas DD and Fad DD in the DISC. The group was able to successfully crystalize and obtain diffraction data for the mFas-hFADD complex. With more structural data, it became clear that there were three types of asymmetric interactions occurring, mediating interactions between sub-units of the protein complex. Select residues within these interaction sites as well as structural residues were mutated, proving that these interactions require specific residues to occur.



Salvesen GS, Riedl SJ. Structure of the Fas/FADD complex: a conditional death domain complex mediating signaling by receptor clustering. Cell Cycle. 2009 Sep 1;8(17):2723-7. Epub 2009 Sep 30. PubMed PMID: 19652545; PubMed Central PMCID: PMC2788920.


  • The DISC complex formed through the association of Fas, the adapter protein FADD and the recruited caspase-8 is a very important step in the initiation of apoptosis. However, the mechanism of formation of this complex is rather unknown. The research covered in this paper explains this process. It starts with the clustering of Fas proteins on lipid rafts. A Fas trimer is linked by a FAS-ligand which initiates DISC formation. Fas has a DD domain which can interact with that of FADD which can then recruit caspase-8 initiating programmed cell death.