Tumor necrosis factor-related apoptosis inducing ligand (TRAIL)-induced apoptosis is mediated by its receptors DR4 (TRAIL-RI) and DR5 (TRAIL-R2) and the adapter protein Fas-associated death domain protein (FADD). Recently, an adapter function for death-associated protein 3 (DAP3) between DR4/DR5 and FADD has been proposed. However, DAP3 has been reported to be a ribosomal protein localized to the mitochondrial matrix. To address these discrepancies, the intracellular localization of DAP3 after apoptosis induction in human T-lymphocytes with recombinant TRAIL was analyzed. DAP3, in contrast to cytochrome c, remained intra-mitochondrial during apoptosis. No interaction between FADD and DAP3 after cell fractionation could be detected as long as subcellular compartments remained intact. Only whole cell lysate co-immunoprecipitation revealed an ex vivo interaction between DAP3 and FADD. Therefore, DAP3 and FADD interact only in vitro after disruption of the cellular compartments. TRAIL-induced and DR4-mediated apoptosis in Jurkat cells is independent of DAP3. (C) 2002 Elsevier Science (USA). All rights reserved.