Spencer E, Jiang J, Chen Z J
Department of Molecular Biology and Oncology, University of Texas Southwestern Medical Center, Dallas, Texas 75235-9148 USA.
Genes Dev. 1999 Feb 1;13(3):284-94. doi: 10.1101/gad.13.3.284.
Signal-induced phosphorylation of IkappaBalpha targets this inhibitor of NF-kappaB for ubiquitination and subsequent degradation, thus allowing NF-kappaB to enter the nucleus to turn on its target genes. We report here the identification of an IkappaB-ubiquitin (Ub) ligase complex containing the F-box/WD40-repeat protein, beta-TrCP, a vertebrate homolog of Drosophila Slimb. beta-TrCP binds to IkappaBalpha only when the latter is specifically phosphorylated by an IkappaB kinase complex. Moreover, immunopurified beta-TrCP ubiquitinates phosphorylated IkappaBalpha at specific lysines in the presence of Ub-activating (E1) and -conjugating (Ubch5) enzymes. A beta-TrCP mutant lacking the F-box inhibits the signal-induced degradation of IkappaBalpha and subsequent activation of NF-kappaB-dependent transcription. Furthermore, Drosophila embryos deficient in slimb fail to activate twist and snail, two genes known to be regulated by the NF-kappaB homolog, Dorsal. These biochemical and genetic data strongly suggest that Slimb/beta-TrCP is the specificity determinant for the signal-induced ubiquitination of IkappaBalpha.
信号诱导的IκBα磷酸化使这种NF-κB抑制剂靶向泛素化并随后降解，从而使NF-κB进入细胞核以开启其靶基因。我们在此报告鉴定了一种包含F-box/WD40重复蛋白β-TrCP的IκB-泛素（Ub）连接酶复合物，β-TrCP是果蝇Slimb的脊椎动物同源物。只有当IκBα被IκB激酶复合物特异性磷酸化时，β-TrCP才会与之结合。此外，在泛素激活（E1）和结合（Ubch5）酶存在的情况下，免疫纯化的β-TrCP会在特定赖氨酸处使磷酸化的IκBα泛素化。缺乏F-box的β-TrCP突变体抑制信号诱导的IκBα降解以及随后NF-κB依赖性转录的激活。此外，缺乏slimb的果蝇胚胎无法激活twist和snail，这两个基因已知受NF-κB同源物Dorsal调控。这些生化和遗传数据强烈表明，Slimb/β-TrCP是信号诱导的IκBα泛素化的特异性决定因素
