Department of Cellular Signal Integration Helmholtz Zentrum München, German Research Center for Environmental Health, Institute of Toxicology, Neuherberg, Germany.
J Biol Chem. 2011 Jul 22;286(29):26107-17. doi: 10.1074/jbc.M111.233163. Epub 2011 May 26.
The IκB kinase (IKK) complex acts as a gatekeeper of canonical NF-κB signaling in response to upstream stimulation. IKK activation requires sensing of ubiquitin chains by the essential IKK regulatory subunit IKKγ/NEMO. However, it has remained enigmatic whether NEMO binding to Lys-63-linked or linear ubiquitin chains is critical for triggering IKK activation. We show here that the NEMO C terminus, comprising the ubiquitin binding region and a zinc finger, has a high preference for binding to linear ubiquitin chains. However, immobilization of NEMO, which may be reminiscent of cellular oligomerization, facilitates the interaction with Lys-63 ubiquitin chains. Moreover, selective mutations in NEMO that abolish association with linear ubiquitin but do not affect binding to Lys-63 ubiquitin are only partially compromising NF-κB signaling in response to TNFα stimulation in fibroblasts and T cells. In line with this, TNFα-triggered expression of NF-κB target genes and induction of apoptosis was partially compromised by NEMO mutations that selectively impair the binding to linear ubiquitin chains. Thus, in vivo NEMO interaction with linear and Lys-63 ubiquitin chains is required for optimal IKK activation, suggesting that both type of chains are cooperating in triggering canonical NF-κB signaling.
IKK 激酶（IKK）复合物作为经典 NF-κB 信号通路对上游刺激的门控因子发挥作用。IKK 的激活需要必需的 IKK 调节亚基 IKKγ/NEMO 识别泛素链。然而，NEMO 与 Lys-63 连接的或线性泛素链的结合对于触发 IKK 激活是否至关重要，这仍然是个谜。我们在此表明，包含泛素结合区和锌指的 NEMO C 端对线性泛素链具有很高的偏好。然而，NEMO 的固定化（可能类似于细胞的寡聚化）促进了与 Lys-63 泛素链的相互作用。此外，选择性突变 NEMO 使其与线性泛素的结合，但不影响与 Lys-63 泛素的结合，仅部分削弱了成纤维细胞和 T 细胞中 TNFα 刺激时的 NF-κB 信号转导。与此一致的是，TNFα 触发的 NF-κB 靶基因的表达和细胞凋亡的诱导部分受到 NEMO 突变的损害，这些突变选择性地损害了与线性泛素链的结合。因此，体内 NEMO 与线性和 Lys-63 泛素链的相互作用对于最佳的 IKK 激活是必需的，这表明这两种类型的链都协同触发经典的 NF-κB 信号转导。
