1. ¹Immunoregulation Unit, Autoimmunity Branch, NIAMS, NIH, Bethesda, MD 20892, USA
2. ²Department of Pharmacology, Toxicology and Therapeutics, University of Kansas Medical Center, Kansas City, KS 66160, USA
3. ³HHMI-NIH, Research Scholoars Program, Bethesda, MD 20814, USA
4. ⁴Department of Biochemistry, Weill Medical College of Cornell University, New York, NY 10021, USA

Correspondence: RM Siegel, NIAMS, NIH, 9000 Rockville pk, Bldg 10, Room 9N238, Bethesda, MD 20892, USA. Tel: +1 301 496 3761; Fax: +1 301 480 3880; E-mail: rsiegel@nih.gov

Received 5 August 2005; Revised 18 November 2005; Accepted 2 December 2005; Published online 20 January 2006.

Abstract

Death receptors in the TNF receptor superfamily signal for apoptosis via the ordered recruitment of FADD and caspase-8 to a death-inducing signaling complex (DISC). However, the nature of the protein–protein interactions in the signaling complex is not well defined. Here we show that FADD self-associates through a conserved RXDLL motif in the death effector domain (DED). Despite exhibiting similar binding to both Fas and caspase-8 and preserved overall secondary structure, FADD RDXLL motif mutants cannot reconstitute FasL- or TRAIL-induced apoptosis and fail to recruit caspase-8 into the DISC of reconstituted FADD-deficient cells. Abolishing self-association can transform FADD into a dominant-negative mutant that interferes with Fas-induced apoptosis and formation of microscopically visible receptor oligomers. These findings suggest that lateral interactions among adapter molecules are required for death receptor apoptosis signaling and implicate self-association into oligomeric assemblies as a key function of death receptor adapter proteins in initiating apoptosis.