A nuclear export signal in the N-terminal regulatory domain of IκBα controls cytoplasmic localization of inactive NF-κB/IκBα complexes

Appropriate subcellular localization is crucial for regulation of NF-κB function. Herein, we show that latent NF-κB complexes can enter and exit the nucleus in preinduction states. The nuclear export inhibitor leptomycin B (LMB) sequestered NF-κB/IκBα complexes in the nucleus. Using deletion and site-directed mutagenesis, we identified a previously uncharacterized nuclear export sequence in residues 45–54 of IκBα that was required for cytoplasmic localization of inactive complexes. This nuclear export sequence also caused nuclear exclusion of heterologous proteins in a LMB-sensitive manner. Importantly, a LMB-insensitive CRM1 mutant (Crm1-K1) abolished LMB-induced nuclear accumulation of the inactive complexes. Moreover, a cell-permeable p50 NF-κB nuclear localization signal peptide also blocked these LMB effects. These results suggest that NF-κB/IκBα complexes shuttle between the cytoplasm and nucleus by a nuclear localization signal-dependent nuclear import and a CRM1-dependent nuclear export. The LMB-induced nuclear complexes could not bind DNA and were inaccessible to signaling events, because LMB inhibited NF-κB activation without affecting the subcellular localization of upstream kinases IKKβ and NIK. Our findings indicate that the dominant nuclear export over nuclear import contributes to the largely cytoplasmic localization of the inactive complexes to achieve efficient NF-κB activation by extracellular signals.