NF-κB dictates the degradation pathway of IκBα

I kappa B proteins are known as the regulators of NF-kappa B activity. They bind tightly to NF-kappa B dimers, until stimulus-responsive N-terminal phosphorylation by IKK triggers their ubiquitination and proteasomal degradation. It is known that I kappa B alpha is an unstable protein whose rapid degradation is slowed upon binding to NF-kappa B, but it is not known what dynamic mechanisms control the steady-state level of total I kappa B alpha. Here, we show clearly that two degradation pathways control the level of I kappa B alpha. Free I kappa B alpha degradation is not controlled by IKK or ubiquitination but intrinsically, by the C-terminal sequence known as the PEST domain. NF-kappa B binding to I kappa B alpha masks the PEST domain from proteasomal recognition, precluding ubiquitin-independent degradation; bound I kappa B alpha then requires IKK phosphorylation and ubiquitination for slow basal degradation. We show the biological requirement for the fast degradation of the free I kappa B alpha protein; alteration of free I kappa B alpha degradation dampens NF-kappa B activation. In addition, we find that both free and bound I kappa B alpha are similar substrates for IKK, and the preferential phosphorylation of NF-kappa B-bound I kappa B alpha is due to stabilization of I kappa B alpha by NF-kappa B.