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

Abstract

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