Dynamic shuttling of nuclear factor kappa B 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 nuc leus. Using the nuclear export inhibitor leptomycin B, we demonstrate shutt ling of the RELA subunit of NF kappaB and the inhibitory subunit I kappaB a lpha between these two compartments in unstimulated cells. Determination of the kinetics of nuclear entry shows marked differences for the two compone nts; the entry of I kappaB alpha occurs more rapidly than RELA. The shuttli ng is suggested to be a consequence of the cytoplasmic dissociation of the NF kappaB.I kappaB complex rather than its direct nuclear import or degrada tion and resynthesis of I kappaB alpha. Using previously published kinetic data, this proposition is born out by the deduction that 17% of NF kappaB i s not complexed to I kappaB alpha in a resting cell. A numerical model is p resented to validate the proposed regulation of NF kappaB subcellular local ization consequent in part on the:nuclear export function and in part on th e cytoplasmic retention function of I kappaB alpha. We suggest that the non -saturated interaction of NF kappaB with the inhibitor may enhance the spec ificity of action of I kappaB proteins on different NF kappaB dimers and al low additional modes of regulation of I kappaB function.