The SCFβ-TRCP-ubiquitin ligase complex associates specifically with phosphorylated destruction motifs in IκBα and β-catenin and stimulates IκBα ubiquitination in vitro

Ubiquitin-mediated proteolysis has a central role in controlling the intracellular levels of several important regulatory molecules such as cyclins, CKIs, p53, and I kappa B alpha. Many diverse proinflammatory signals lead to the specific phosphorylation and subsequent ubiquitin-mediated destruction of the NF-kappa B inhibitor protein I kappa B alpha. Substrate specificity in ubiquitination reactions is, in large part, mediated by the specific association of the E3-ubiquitin ligases with their substrates. One class of E3 ligases is defined by the recently described SCF complexes, the archetype of which was first described in budding yeast and contains (S) under bar kp1, (C) under bar dc53, and the (F) under bar-box protein Cdc4. These complexes recognize their substrates through modular F-box proteins in a phosphorylation-dependent manner. Here we describe a biochemical dissection of a novel mammalian SCF complex, SCFbeta-TRCP, that specifically recognizes a 19-amino-acid destruction motif in I kappa B alpha (residues 21-41) in a phosphorylation-dependent manner. This SCF complex also recognizes a conserved destruction motif in beta-catenin, a protein with levels also regulated by phosphorylation-dependent ubiquitination. Endogenous I kappa B alpha-ubiquitin ligase activity cofractionates with SCFbeta-TRCP. furthermore, recombinant SCFbeta-TRCP assembled in mammalian cells contains phospho-I kappa B alpha-specific ubiquitin ligase activity. Our results suggest that an SCFbeta-TRCP complex functions in multiple transcriptional programs by activating the NF-kappa B pathway and inhibiting the beta-catenin pathway.