CD95 stimulation results in the formation of a novel death effector domain protein-containing complex

Stimulation of CD95 (APO-1/Fas) by its natural ligand CD95L (APO-1L/FasL) leads to the formation of the death-inducing signaling complex. Here we report that upon CD95 stimulation in several T and B cell lines, a novel signaling complex is formed, which we term complex II. Complex II is composed of the death effector domain proteins as follows: procaspase-8a/b, three isoforms of c-FLIP (c-FLIP(L), c-FLIP(S), c-FLIP(R)), and FADD. Notably, complex II does not contain CD95. Based on our findings we suggest that CD95 signaling includes two steps. The first step involves formation of the death-inducing signaling complex at the cell membrane. The second step involves formation of the cytosolic death effector domain protein-containing complex that may play an important role in amplification of caspase activation.