Wildsmith, GC; (2015) Structural and functional investigation of the cytoplasmic domain of the Fas death receptor. Doctoral thesis , UCL (University College London).

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Abstract

Activation of the transmembrane death receptor Fas (CD95/APO-1) by a membrane bound ligand (FasL/CD95L) activates the extrinsic pathway of apoptosis. Intracellular Fas death domains (DDs) are induced to oligomerise enabling binding to the adaptor protein FADD, thereby leading to the recruitment of procaspase 8 and other proteins to form the death inducing signalling complex (DISC).This thesis describes an investigation of the structure and function of the cytoplasmic Fas-DD. A model for the solution structure of the Fas-DD was published in 1996, it has since been reported that the death domain can form at least one other conformation when in complex with FADD. As a foundation to the work in this thesis, modern multidimensional NMR techniques have been used to solve the structure of the FasDD, to further probe the potential for alternative conformations. It has previously been reported that Fas can be phosphorylated at Tyr291, providing a platform for the recruitment of binding partners that can affect non-apoptotic signalling. The second part of this thesis details the development of an expressed protein ligation methodology to prepare a Tyr291 phosphorylated Fas DD to provide a basis for in vitro studies of the structural, dynamic and functional effects of phosphorylation. It is widely accepted that Fas is palmitoylated at Cys199 and recognised by the membrane cytoskeletal protein, ezrin. Fas palmitoylation is important for clathrinmediated internalisation of the DISC, and amplification of the caspase cascade. There are multiple reports detailing the binding of ezrin to Fas, but it is not clear whether this interaction occurs in a palmitoylation-dependent manner. Efforts to characterise an interaction between bacterially expressed intracellular Fas and ezrin proteins were carried out using a number of biophysical assays, described in the third part of this thesis. Building upon this, the fourth section explores the preparation of a palmitoylated Fas construct suitable for biophysical analysis by incubating recombinant Fas with palmitoyl-CoA.

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