Phosphorylation of FADD/MORT1 and Fas by kinases that associate with the membrane-proximal cytoplasmic domain of Fas.

Fas (Apo-1, CD95), a member of the TNFR family, is expressed on a variety of cell types and transduces an apoptotic signal. Since Fas does not possess known enzymatic activities, proteins that interact with the cytoplasmic domain of Fas regulate the death signal. Several proteins have been identified, primarily using the yeast two-hybrid system, that associate with the death domain of Fas. One of these proteins, FADD/MORT1, can be phosphorylated, although the kinase that is responsible has not been identified. Furthermore, direct signaling connections between Fas and its known activation of sphingomyelinase or NF-kappaB have not been made, suggesting that other proteins may associate with Fas. In this study, a series of glutathione S-transferase fusion proteins was constructed that contained the cytoplasmic domain of murine Fas. These proteins were used to search for additional proteins that associate with Fas. Novel proteins, including kinases, were identified that associated specifically with the membrane-proximal, cytoplasmic tail of Fas but not with the death domain. One of these kinases phosphorylates FADD/MORT1. Moreover, the membrane-proximal region of Fas itself was phosphorylated by one of the associating kinases. These findings suggest that, similar to the Fas-related p55 TNFR, the membrane-proximal region of Fas likely participates in signaling.