Structural Basis for Fas-mediated Apoptosis and Mechanism of Inhibition
University of Alabama, Birgmingham
The extrinsic apoptotic pathway is initiated by cell surface death receptors such as Fas. Engagement of Fas by Fas ligand triggers a conformational change that allows Fas to interact with adaptor protein Fas-associated death domain (FADD) via the death domain, which recruits downstream signaling proteins to form the death-inducing signaling complex (DISC). Previous studies have shown that calmodulin (CaM) is recruited into the DISC in cholangiocarcinoma cells, suggesting a novel role of CaM in Fas-mediated signaling. CaM antagonists induce apoptosis through a Fas-related mechanism in cholangiocarcinoma and other cancer cell lines possibly by inhibiting Fas-CaM interactions. The structural determinants of Fas-CaM interaction and the underlying molecular mechanisms of inhibition, however, are unknown. We employ NMR, biochemical and biophysical techniques to elucidate these mechanisms. We have shown that CaM binds to the death domain of Fas (FasDD) with 2:1 CaM:FasDD stoichiometry. The interactions between FasDD and CaM are endothermic and entropically driven, suggesting that hydrophobic contacts are critical for binding. Proteolytic digestion experiments coupled with mass spectrometry and NMR methods facilitated the identification of the two regions involved in CaM binding. We provide the first structural evidence for Fas-CaM interactions and provide new insight into the molecular basis for a novel role of CaM in regulating Fasmediated apoptosis.