Positive and Negative Regulation of Nuclear Factor-κB-mediated Transcription by IκB-ζ, an Inducible Nuclear Protein

I Kappa B- zeta is an inducible nuclear protein that interacts with nuclear factor- Kappa B (NF- Kappa B) via its carboxyl-terminal ankyrin-repeats. Previous studies using an NF- Kappa B reporter have shown that I Kappa B- zeta inhibits the activity of NF- Kappa B. In the present study, we dissected the amino-terminal region of I Kappa B- zeta , which shows no homology to any other proteins. Indirect immunofluorescence studies demonstrated the presence of a bipartite nuclear localization signal spanning amino acids 163-178. Using GAL4 fusion proteins, we found that internal fragments containing amino acids 329-402 possessed intrinsic transcriptional activation activity. Interestingly, the activity was not detected in GAL4 fusion proteins of the full-length I Kappa B- zeta . On the other hand, the GAL4-dependent transcriptional activity was generated by co-expression of the GAL4-NF- Kappa B p50 subunit fusion protein and the full-length I Kappa B- zeta , neither of which exhibited the activity on their own. A new splicing variant, I Kappa B- zeta (D), with a deletion of amino acids 236-429, was found to lack transactivation activity. Forced expression of I Kappa B- zeta , but not I Kappa B- zeta (D), augmented interleukin-6 production, indicating the functional significance of the transactivation activity. In contrast, tumor necrosis factor-alpha production was inhibited by expression of I Kappa B- zeta , highlighting the dual functions of this molecule. These results indicate that I Kappa B- zeta harbors latent transcriptional activation activity, and that the activity is expressed upon interaction with the NF- Kappa B p50 subunit. In addition to the inhibitory activity on NF- Kappa B-mediated transcription, the transcriptional activation activity of I Kappa B- zeta should be crucial for the regulation of inflammation.