Targeting of TAK1 by the NF-κB protein Relish regulates the JNK-mediated immune response in Drosophila

The molecular circuitry underlying innate immunity is constructed of multiple, evolutionarily conserved signaling modules with distinct regulatory targets. The MAP kinases and the IKK-NF-B-K molecules play important roles in the initiation of immune effector responses. We have found that the Drosophila NF-B-K protein Relish plays a crucial role in limiting the duration of INK activation and output in response to Gram-negative infections. Relish activation is linked to proteasomal degradation of TAK1, the upstream MAP kinase kinase kinase required for INK activation. Degradation of TAK1 leads to a rapid termination of INK signaling, resulting in a transient JNK-dependent response that precedes the sustained induction of Relish-dependent innate immune loci. Because the IKK-NF-KB module also negatively regulates INK activation in mammals, thereby controlling inflammation-induced apoptosis, the regulatory cross-talk between the INK and NF-KB pathways appears to be broadly conserved.