The IκB family of proteins regulates NF-κB-dependent transcription by inhibiting DNA binding and localizing these factors to the cell cytoplasm. IκBα does this by shifting the balance between nuclear import of Rel proteins and their export from the nucleus. Here we show that, unlike IκBα, IκBβ and IκBε appear to sequester p65 or c-Rel in the cytoplasm by inhibiting nuclear import. Furthermore, because IκBβ does not undergo nucleocytoplasmic shuttling, it cannot remove nuclear proteins like IκBα does. We conclude that the mechanism of action differs among IκB family members.
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