Mathes Erika, O'Dea Ellen L, Hoffmann Alexander, Ghosh Gourisankar
Department of Chemistry & Biochemistry, University of California, San Diego, La Jolla, CA 92093-0375, USA.
EMBO J. 2008 May 7;27(9):1357-67. doi: 10.1038/emboj.2008.73. Epub 2008 Apr 10.
IkappaB proteins are known as the regulators of NF-kappaB activity. They bind tightly to NF-kappaB dimers, until stimulus-responsive N-terminal phosphorylation by IKK triggers their ubiquitination and proteasomal degradation. It is known that IkappaBalpha is an unstable protein whose rapid degradation is slowed upon binding to NF-kappaB, but it is not known what dynamic mechanisms control the steady-state level of total IkappaBalpha. Here, we show clearly that two degradation pathways control the level of IkappaBalpha. Free IkappaBalpha degradation is not controlled by IKK or ubiquitination but intrinsically, by the C-terminal sequence known as the PEST domain. NF-kappaB binding to IkappaBalpha masks the PEST domain from proteasomal recognition, precluding ubiquitin-independent degradation; bound IkappaBalpha then requires IKK phosphorylation and ubiquitination for slow basal degradation. We show the biological requirement for the fast degradation of the free IkappaBalpha protein; alteration of free IkappaBalpha degradation dampens NF-kappaB activation. In addition, we find that both free and bound IkappaBalpha are similar substrates for IKK, and the preferential phosphorylation of NF-kappaB-bound IkappaBalpha is due to stabilization of IkappaBalpha by NF-kappaB.
IκB蛋白是已知的NF-κB活性调节因子。它们与NF-κB二聚体紧密结合，直到IKK介导的刺激响应性N端磷酸化引发其泛素化和蛋白酶体降解。已知IκBα是一种不稳定蛋白，其与NF-κB结合后快速降解会减缓，但尚不清楚何种动态机制控制总IκBα的稳态水平。在此，我们明确表明有两条降解途径控制IκBα的水平。游离IκBα的降解不受IKK或泛素化控制，而是内在地由称为PEST结构域的C端序列控制。NF-κB与IκBα结合会使PEST结构域免受蛋白酶体识别，从而阻止不依赖泛素的降解；结合的IκBα随后需要IKK磷酸化和泛素化才能进行缓慢的基础降解。我们展示了游离IκBα蛋白快速降解的生物学需求；游离IκBα降解的改变会减弱NF-κB激活。此外，我们发现游离和结合的IκBα都是IKK的相似底物，NF-κB结合的IκBα的优先磷酸化是由于NF-κB对IκBα的稳定作用。
