An N-terminal nuclear export signal is required for the nucleocytoplasmic shuttling of IκBα

The potent transcriptional activities of Rel/NF-kappa B proteins are regulated in the cytoplasm and nucleus by the inhibitor, I kappa B alpha. The mechanism, by which I kappa B alpha can either sequester NF-kappa B in the cytoplasm or act as a nuclear post-induction repressor of NF-kappa B, is uncertain. We find that I kappa B alpha shuttles continuously between the nucleus and cytoplasm, This shuttling requires a previously unidentified CRM1-dependent nuclear export signal (NES) located within the N-terminal domain of I kappa B alpha at amino acids 45-55, Deletion or mutation of the N-terminal NES results in nuclear localization of I kappa B alpha. NF-kappa B (p65) association with I kappa B alpha affects steady-state localization but does not inhibit its shuttling. Endogenous complexes of I kappa B alpha-kappa B shuttle and will accumulate in the nucleus when CRM1 export is blocked. We find TNF alpha can activate the nuclear I kappa B alpha-NF-kappa B complexes by the classical mechanism of proteasome-mediated degradation of I kappa B alpha. These studies reveal a more dynamic nucleocytoplasmic distribution for I kappa B alpha and NF-kappa B suggesting previously unknown strategies for regulating this ubiquitous family of transcription activators.