Characterization of I kappa B alpha nuclear import pathway

I kappa B alpha controls the transcriptional activity of nuclear factor (NF)-kappa B by retaining it in the cytoplasm; but, when expressed in the nucleus, it can also inhibit the interaction of NF-kappa B with DNA and promote the export of NF-kappa B from the nucleus to the cytoplasm, Here, we report that I kappa B alpha, when not bound to NF-kappa B, is constitutively transported to the nucleus, and we confirm that the interaction of NF-kappa B with NF-kappa B retains I kappa B alpha in the cytoplasm, Nuclear import of I kappa B alpha does not result from passive diffusion but from a specific energy-dependent transport process that requires the ankyrin repeats of I kappa B alpha. Nuclear accumulation of I kappa B alpha is dependent on importins alpha and beta as well as the small GTPase Ran, which are also responsible for the nuclear import mediated by basic nuclear localization sequences (NLS). However, these proteins are not sufficient to promote I kappa B alpha nuclear translocation. Factor(s) can be removed selectively from cell extracts with ankyrin repeats of I kappa B alpha which strongly reduce import of I kappa B alpha but not of proteins containing basic NLS. These findings indicate that I kappa B alpha is imported in the nucleus by a piggy-back mechanism that involves additional protein(s) containing a basic NLS and able to interact with ankyrin repeats of I kappa B alpha.