Identification of RIP3, a RIP-like kinase that activates apoptosis and NFκB

The tumor necrosis factor receptor 1 (TNFR1) and the Fas receptor recruit complexes formed by the interactions between RIP kinase, TRADD, FADD and RAIDD — adaptor proteins that contain death domains – which in turn recruit other proteins to initiate signaling [1–5]. To identify proteins associated with the TNF signaling pathway, we performed a yeast twohybrid interaction screen using RIP as bait. We isolated a kinase, RIP3, which shares homology with the kinase domain of RIP and RIP2 (also known as Rick or CARDIAK). RIP3 could be co-immunoprecipitated with RIP, TRAF2 and TNFR1 in mammalian cells. The carboxy-terminal domain of RIP3, like that of RIP, could activate the transcription factor NFκB and induce apoptosis when expressed in mammalian cells. Interestingly, this region shares no significant sequence homology to the death domain of RIP, the caspaserecruiting domain (CARD) of RIP2 [6–8] or any other apoptosis-inducing domain. As with RIP and RIP2, the kinase domain of RIP3 was not required for either NFκB activation or apoptosis induction. Overexpression of a dominant-negative mutant of RIP3 strongly inhibited the caspase activation but not the NFκB activation induced by TNFα. Therefore, RIP3 appears to function as an intermediary in TNFα-induced apoptosis.