The Zinc Finger Domain of NEMO Is Selectively Required for NF-κB Activation by UV Radiation and Topoisomerase Inhibitors

Exposure of mammalian cells to UV radiation was proposed to stimulate the transcription factor NF-κB by a unique mechanism. Typically, rapid and strong inducers of NF-κB, such as tumor necrosis factor alpha (TNF-α) and bacterial lipopolysaccharide (LPS), lead to rapid phosphorylation and proteasomal degradation of its inhibitory protein, IκBα. In contrast, UV, a relatively slower and weaker inducer of NF-κB, was suggested not to require phosphorylation of IκBα for its targeted degradation by the proteasome. We now provide evidence to account for this peculiar degradation process of IκBα. The phospho-IκBα generated by UV is only detectable by expressing a ΔF-box mutant of the ubiquitin ligase β-TrCP, which serves as a specific substrate trap for serine 32 and 36 phosphorylated IκBα. In agreement with this finding, we also find that the IκB kinase (IKK) phospho-acceptor sites on IκBα, core components of the IKK signalsome, and IKK catalytic activity are all required for UV signaling. Furthermore, deletion and point mutation analyses reveal that both the amino-terminal IKK-binding and the carboxy-terminal putative zinc finger domains of NEMO (IKKγ) are critical for UV-induced NF-κB activation. Interestingly, the zinc finger domain is also required for NF-κB activation by two other slow and weak inducers, camptothecin and etoposide. In contrast, the zinc finger module is largely dispensable for NF-κB activation by the rapid and strong inducers LPS and TNF-α. Thus, we suggest that the zinc finger domain of NEMO likely represents a point of convergence for signaling pathways initiated by slow and weak NF-κB-activating conditions.