The transcription factor nuclear factor-kappaB (NF-kappaB) plays crucial roles in a wide variety of cellular functions and its activity is strictly regulated by cytosolic inhibitors known as IkappaBs. We here report a new member of the IkappaB protein family, IkappaB-zeta, harboring six ankyrin repeats at its carboxyl terminus. IkappaB-zeta mRNA is strongly induced after stimulation by lipopolysaccharide. The induction of IkappaB-zeta is also observed by stimulation with interleukin-1beta but not by tumor necrosis factor-alpha. In contrast to cytosolic IkappaB-alpha, -beta, and -epsilon, the induced IkappaB-zeta localizes in the nucleus via its amino-terminal region, which shows no homology with other proteins. Transiently expressed IkappaB-zeta inhibits the NF-kappaB activity without affecting the nuclear translocation of NF-kappaB upon stimulation. The expressed IkappaB-zeta preferentially associates with the NF-kappaB subunit p50 rather than p65 and recombinant IkappaB-zeta proteins inhibit the DNA binding of the p65/p50 heterodimer and the p50/p50 homodimer. Thus, IkappaB-zeta negatively regulates NF-kappaB activity in the nucleus, possibly in order to prevent excessive inflammation. Moreover, transfection of IkappaB-zeta renders cells more susceptible to apoptosis induced by tumor necrosis factor-alpha. The proapoptotic activity of IkappaB-zeta further suggests that it might be one of key regulators for inflammation and other biologically relevant processes.
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