Phosphorylation of FADD/MORT1 at serine 194 and association with a 70-kDa cell cycle-regulated protein kinase

The adapter molecule Fas-associated death domain protein (FADD)/mediator of receptor-induced toxicity-1 (MORT1) is essential for signal transduction o f the apoptosis-inducing receptor CD95 (APO-1/Fas) as it connects the activ ated receptor with the effector caspase-8, FADD also plays a role in embryo nic development and the cell cycle reentry of T cells. FADD is phosphorylat ed at serine residues. We now show that phosphorylation exclusively occurs at serine 194, The phosphorylation of FADD was found to correlate with the cell cycle. In cells arrested at the G(2)/M boundary with nocodazole, FADD was quantitatively phosphorylated, whereas only nonphosphorylated FADD was found in cells arrested in G(1)/S with hydroxyurea, in this context, we hav e identified a 70-kDa cell cycle-regulated kinase that specifically binds t o the C-terminal half of FADD, Because CD95-mediated apoptosis is independe nt of the cell cycle, phosphorylation of FADD may regulate its apoptosis-in dependent functions.