A nuclear export signal in the N-terminal regulatory domain of I kappa B alpha controls cytoplasmic localization of inactive NF-kappa B/I kappa B alpha complexes

Appropriate subcellular localization is crucial for regulation of NF-kappa B function, Herein, we show that latent NF-kappa B complexes can enter and exit the nucleus in preinduction states. The nuclear export inhibitor lepto mycin B (LMB) sequestered NF-kappa B/I kappa B alpha complexes in the nucle us. Using deletion and site-directed mutagenesis, we identified a previousl y uncharacterized nuclear export sequence in residues 45-54 of I kappa B al pha that was required for cytoplasmic localization of inactive complexes. T his nuclear export sequence also caused nuclear exclusion of heterologous p roteins in a LMB-sensitive manner. Importantly, a LMB-insensitive CRM1 muta nt (Crm1-K1) abolished LMB-induced nuclear accumulation of the inactive com plexes. Moreover, a cell-permeable p50 NF-kappa B nuclear localization sign al peptide also blocked these LMB effects, These results suggest that NF-ka ppa B/I kappa B alpha complexes shuttle between the cytoplasm and nucleus b y a nuclear localization signal-dependent nuclear import and a CRM1-depende nt nuclear export. The LMB-induced nuclear complexes could not bind DNA and were inaccessible to signaling events, because LMB inhibited NF-kappa B ac tivation without affecting the subcellular localization of upstream kinases IKK beta and NIK, Our findings indicate that the dominant nuclear export o ver nuclear import contributes to the largely cytoplasmic localization of t he inactive complexes to achieve efficient NF-kappa B activation by extrace llular signals.