Apoptbsis plays critical roles in mammalian biology including embryonic development, cellular homeostasis and immune regulation. Genetic mutations or abnormal expression of apoptosis pathway proteins are associated with many human diseases. Apoptosis is initiated via assembly of multimeric caspase-activating signaling complexes. In the extrinsic cell death pathway, death receptors (members of the tumor necrosis factor (TNF) receptor superfamily) such as Fas (also known as CD95 or APO-1) assemble into an oligomeric death inducing signaling complex (DISC) upon ligand stimulation. Similarly, in the caspase-2 activation pathway, the p53-inducible protein PIDD assembles into an oligomeric complex known as the PIDDosome. Death domain (DD) and death effector domains (DED) play important roles in both DISC and PIDDosome formation. For DISC assembly, the intracellular DD of Fas interacts with the C-terminal DD of the adapter protein FADD and the N-terminal DED of FADD interacts with the tandem DED in caspase-8 (or caspase-10) to form the ternary complex of Fas, FADD and caspase-8. In addition to caspase-8 and -10, cellular and viral FLIPs (cFLIPs and vFLIPs) are also tandem DED-containing proteins that interact with FADD DED and inhibit the function of the DISC. For PIDDosome assembly, one important interaction is between the DD of PIDD and the DD of the adapter protein RAIDD. Caspase recruitment into these complexes initiates proteolytic auto-processing and caspase activation. In this application, we propose to use a combination of biochemical and cell biological approaches study the molecular mechanisms that govern the assembly of these complexes. The specific aims are: 1) to reconstitute these apoptotic signaling complexes in vitro and to characterize them in cellular systems; 2) to elucidate the molecular basis of the Fas DD:FADD DD interaction; 3) to elucidate the molecular basis of the PIDD DD:RAIDD DD interaction; 4) to elucidate the interaction of FADD DED with tandem DEDs.