Regulation of RelA subcellular localization by a putative nuclear export signal and p50

Nuclear factor kappa B (NF-kappa B) represents a family of dimeric DNA bind ing proteins, the pleotropic form of which is a heterodimer composed of Rel A and p50 subunits. The biological activity of NF-kappa B is controlled thr ough its subcellular localization. Inactive NF-kappa B is sequestered in th e cytoplasm by physical interaction with an inhibitor, I kappa B alpha, Sig nal-mediated I kappa B alpha degradation triggers the release and subsequen t nuclear translocation of NF-kappa B. It remains unknown whether the NF-ka ppa B shuttling between the cytoplasm and nucleus is subjected to additiona l steps of regulation. In this study, we demonstrated that the RelA subunit of NF-kappa B exhibits strong cytoplasmic localization activity even in th e absence of I kappa B alpha inhibition. The cytoplasmic distribution of Re lA is largely mediated by a leucine-rich sequence homologous to the recentl y characterized nuclear export signal (NES). This putative NES is both requ ired and sufficient to mediate cytoplasmic localization of RelA as well as that of heterologous proteins. Furthermore, the cytoplasmic distribution of RelA is sensitive to a nuclear export inhibitor, leptomycin B, suggesting that RelA undergoes continuous nuclear export. Interestingly; expression of p50 prevents the cytoplasmic expression of RelA, leading to the nuclear ac cumulation of both RelA and p50. Together, these results suggest that the n uclear and cytoplasmic shuttling of RelA is regulated by both an intrinsic NES-like sequence and the p50 subunit of NF-kappa B.