Nuclear localization of IκBα is mediated by the second ankyrin repeat:: the IκBα ankyrin repeats define a novel class of cis-acting nuclear import sequences

The ability of the I kappa B alpha protein to sequester dimeric NF-kappa B/Rel proteins in the cytoplasm provides an effective mechanism for regulating the potent transcriptional activation properties of NF-kappa B/Rel family members. I kappa B alpha can also act in the nucleus as a postinduction repressor of NF-kappa B/Rel proteins. The mechanism by which I kappa B alpha enters the nucleus is not known, as I kappa B alpha lacks a discernible classical nuclear localization sequence (NLS). We now report that nuclear localization of I kappa B alpha is mediated by a novel nuclear import sequence within the second ankyrin repeat. Deletion of the second ankyrin repeat or alanine substitution of hydrophobic residues within the second ankyrin repeat disrupts nuclear localization of I kappa B alpha. Furthermore, a region encompassing the second ankyrin repeat of I kappa B alpha is able to function as a discrete nuclear import sequence. The presence of a discrete nuclear import sequence in I kappa B alpha suggests that cytoplasmic sequestration of the NF-kappa B/Rel-I kappa B alpha complex is a consequence of the mutual masking of the NLS within NF-kappa B/Rel proteins and the import sequence within I kappa B alpha. Nuclear import may be a conserved property of ankyrin repeat domains (ARDs), as the ARDs from two other ARD-containing proteins, 53BP2 and GABP beta, are also able to function as nuclear import sequences. We propose that the I kappa B alpha ankyrin repeats define a novel class of cis-acting nuclear import sequences.