Dissecting Fas signaling with an altered-specificity death-domain mutant: Requirement of FADD binding for apoptosis but not Jun N-terminal kinase activation

Fas is a cell surface death receptor that regulates peripheral tolerance an d lymphoid homeostasis. In many pathologic conditions, ectopic Fas activati on mediates tissue destruction. Several proteins that fan bind to the cytop lasmic death domain of Fas have been implicated in Fas signal transduction. Here we show that FADD, which couples Fas to pro-caspase-8, and, Daxx, whi ch couples Fas to the Jun N-terminal kinase pathway, bind independently to the Fas death domain. We have isolated a death domain mutant, termed Fas De lta, that selectively binds Daxx but not FADD. In tranfected tissue culture cells, Fas Delta activated Jun N-terminal kinase normally but was impaired in cell death induction. These results suggest that FADD and Daxx activate two independent pathways downstream of Fas and confirm the essential role of FADD binding in apoptosis induction.