Dynamic shuttling of nuclear factor κB between the nucleus and cytoplasm as a consequence of inhibitor dissociation

Activation of the nuclear factor kappaB (NF kappaB) transcription factor is intimately associated with its translocation from the cytoplasm to the nucleus. Using the nuclear export inhibitor leptomycin B, we demonstrate shuttling of the RELA subunit of NF kappaB and the inhibitory subunit I kappaB alpha between these two compartments in unstimulated cells. Determination of the kinetics of nuclear entry shows marked differences for the two components; the entry of I kappaB alpha occurs more rapidly than RELA. The shuttling is suggested to be a consequence of the cytoplasmic dissociation of the NF kappaB.I kappaB complex rather than its direct nuclear import or degradation and resynthesis of I kappaB alpha. Using previously published kinetic data, this proposition is born out by the deduction that 17% of NF kappaB is not complexed to I kappaB alpha in a resting cell. A numerical model is presented to validate the proposed regulation of NF kappaB subcellular localization consequent in part on the:nuclear export function and in part on the cytoplasmic retention function of I kappaB alpha. We suggest that the non-saturated interaction of NF kappaB with the inhibitor may enhance the specificity of action of I kappaB proteins on different NF kappaB dimers and allow additional modes of regulation of I kappaB function.