| Abstrakt: |
The biological activity of the transcription factor NF-κB is controlled mainly by the IκBα and IκBβ proteins, which restrict NF-κB to the cytoplasm and inhibit its DNA binding activity. Here, we carried out experiments to determine and compare the mechanisms by which IκBα and IκBβ inhibit NF-κB-dependent transcriptional activation. First, we found that in vivo IκBα is a stronger inhibitor of NF-κB than is IκBβ. This difference is directly correlated with their abilities to inhibit NF-κB binding to DNA in vitro and in vivo. Moreover, IκBα, but not IκBβ, can remove NF-κB from functional preinitiation complexes in in vitro transcription experiments. Second, we showed that both IκBs function in vivo not only in the cytoplasm but also in the nucleus, where they inhibit NF-κB binding to DNA. Third, the inhibitory activity of IκBβ, but not that of IκBα, is facilitated by phosphorylation of the C-terminal PEST sequence by casein kinase II and/or by the interaction of NF-κB with high-mobility group protein I (HMG I) on selected promoters. The unphosphorylated form of IκBβ forms stable ternary complexes with NF-κB on the DNA either in vitro or in vivo. These experiments suggest that IκBα works as a postinduction repressor of NF-κB independently of HMG I, whereas IκBβ functions preferentially in promoters regulated by the NF-κB/HMG I complexes. |
