Signal-induced ubiquitination of IκBα by the F-box protein Slimb/β-TrCP

Signal-induced phosphorylation of I kappa B alpha targets this inhibitor of NF-kappa B for ubiquitination and subsequent degradation, thus allowing NF-kappa B to enter the nucleus to turn on its target genes. We report here the identification of an I kappa B-ubiquitin (Ub) ligase complex containing the F-box/WD40-repeat protein, beta-TrCP, a vertebrate homolog of Drosophila Slimb. beta-TrCP binds to I kappa B alpha only when the latter is specifically phosphorylated by an I kappa B kinase complex. Moreover, immunopurified beta-TrCP ubiquitinates phosphorylated I kappa B alpha 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 I kappa B alpha and subsequent activation of NF-kappa B-dependent transcription. Furthermore, Drosophila embryos deficient in slimb fail to activate twist and snail, two genes known to be regulated by the NF-kappa B homolog, Dorsal. These biochemical and genetic data strongly suggest that Slimb/beta-TrCP is the specificity determinant for the signal-induced ubiquitination of I kappa B alpha.