NF-jB dictates the degradation pathway of IjBa

IjB proteins are known as the regulators of NF-jB activity. They bind tightly to NF-jB dimers, until stimulus-responsive N-terminal phosphorylation by IKK triggers their ubiquitination and proteasomal degradation. It is known that IjBa is an unstable protein whose rapid degradation is slowed upon binding to NF-jB, but it is not known what dynamic mechanisms control the steady-state level of total IjBa. Here, we show clearly that two degradation pathways control the level of IjBa. Free IjBa degradation is not controlled by IKK or ubiquitination but intrinsically, by the C-terminal sequence known as the PEST domain. NF-jB binding to IjBa masks the PEST domain from proteasomal recognition, precluding ubiquitin-independent degradation; bound IjBa then requires IKK phosphorylation and ubiquitination for slow basal degradation. We show the biological requirement for the fast degradation of the free IjBa protein; alteration of free IjBa degradation dampens NF-jB activation. In addition, we find that both free and bound IjBa are similar substrates for IKK, and the preferential phosphorylation of NF-jB-bound IjBa is due to stabilization of IjBa by NF-jB. The EMBO Journal (2008) 27, 1357–1367. doi:10.1038/ emboj.2008.73; Published online 10 April 2008 Subject Categories: signal transduction; proteins