A schematic model of Zfra/WOX1 involvement in TNF signaling. TNF is able to initiate two counteractive pathways – one apoptotic and the other protective. (A) In the death pathway, binding of TNF to the cognate p55-TNF receptor (TNFR) results in recruitment of death domain proteins TRADD, FADD and RIP, thus generating the so called death-inducing signaling complex (DISC). Caspase 8 and downstream effectors are then activated to induce cell death at the mitochondrial and nuclear levels [16,17]. In this study, we discovered that Zfra physically interacts with TRADD and WOX1, and that WOX1 binds TRADD. Thus, Zfra and WOX1 are likely to be recruited to the DISC. WOX1 enhances TNF cytotoxicity , and Zfra counteracts the WOX1 function. Zfra either enhances or inhibits the function of death domain proteins (open arrow). Thus, the ying and yang of cell death depends upon the strength of DISC formation and the counteractive or enhancing force of Zfra. (B) In the protective pathway, p55-TNF recruits TRADD, TRAF2 and RIP, followed by activating several downstream adaptors and finally JNK1 and NF-κB. We determined that overexpressed Zfra sequesters p53, WOX1, and NF-κB in the cytoplasm. Thus, Zfra is likely to bind and block the function of these proteins during TNF signaling (see each step marked by a number). In Step 1, at the membrane level, Zfra binds TRADD in the presence of TRADD, TRAF2, RIP and WOX1. In Step 2, a trimolecular complex of Zfra/JNK1/WOX1 may form when JNK1 is activated by the upstream activated MEK. Zfra binds and counteracts the apoptotic function of JNK1. Also, JNK1 counteracts the apoptotic function of WOX1 . In Step 3, MEK activates ERK, and that Zfra may bind and sequester ERK to the cytoplasm. In Step 4, phosphorylation of IκBα by IKK causes degradation of IκBα and release of NF-κB for nuclear translocation. Again, Zfra is able to bind and sequester NF-κB in the cytoplasm. In Step 5 and 6, TNF induces NF-κB activation, and then NF-κB activates p53 . The non-ankyrin C-terminus of IκBα physically interacts with cytosolic p53 . p53 is functionally associated with WOX1, and both proteins may induce apoptosis synergistically [3,4,6,8]. Thus, an in vivo complex of Zfra with IκBα/p53/WOX1 or p53/WOX1 is likely (Chang et al., submitted).