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-kappaB by a unique mechanism. Typically, rapid and strong inducers of NF-kappaB, such as tumor necrosis factor alpha (TNF-alpha) and bacterial lipopolysaccharide (LPS), lead to rapid phosphorylation and proteasomal degradation of its inhibitory protein, IkappaBalpha. In contrast, UV, a relatively slower and weaker inducer of NF-kappaB, was suggested not to require phosphorylation Of IkappaBalpha for its targeted degradation by the proteasome. We now provide evidence to account for this peculiar degradation process Of IkappaBalpha. The phospho-IkappaBalpha generated by LJV is only detectable by expressing a DeltaF-box mutant of the ubiquitin ligase beta-TrCP, which serves as a specific substrate trap for serine 32 and 36 phosphorylated IkappaBalpha. In agreement with this finding, we also find that the IKB kinase (IKK) phospho-acceptor sites on IkappaBalpha, 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 (IKKgamma) are critical for UV-induced NF-kappaB activation. Interestingly, the zinc finger domain is also required for NF-kappaB activation by two other slow and weak inducers, camptothecin and etoposide. In contrast, the zinc finger module is largely dispensable for NF-kappaB activation by the rapid and strong inducers LIPS and TNF-alpha. 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-kappaB-activating conditions.