In Apoptosis intiation

Ref Robbins 9/e p56 The Death Receptor (Extrinsic) Pathway of Apoptosis Many cells express surface molecules, called death recep- tors, that trigger apoptosis. Most of these are members of the tumor necrosis factor (TNF) receptor family, which contain in their cytoplasmic regions a conserved "death domain," so named because it mediates interaction with other proteins involved in cell death. The prototypic death receptors are the type I TNF receptor and Fas (CD95). Fas ligand (FasL) is a membrane protein expressed mainly on activated T lymphocytes. When these T cells recognize Fas-expressing targets, Fas molecules are cross-linked by FasL and bind adaptor proteins the death domain. These in turn recruit and activate caspase-8. In many cell types caspase-8 may cleave and activate a pro-apoptotic member of the Bcl-2 family called Bid, thus feeding into the mitochondrial pathway. The combined activation of both pathways delivers a lethal blow to the cell. Cellular proteins, notably a caspase antagonist called FLIP, block activation of caspases downstream of death receptors. Interestingly, some viruses produce homologues of FLIP, and it is suggested that this is a mechanism that viruses use to keep infected cells alive. The death receptor pathway is involved in elimination of self-reactive lympho- cytes and in killing of target cells by some cytotoxic T lymphocytes.