The SCFbeta -TRCP-ubiquitin ligase complex associates specifically with phosphorylated destruction motifs in Ikappa Balpha and beta -catenin and stimulates Ikappa Balpha ubiquitination in vitro

Abstract

Ubiquitin-mediated proteolysis has a central role in controlling the intracellular levels of several important regulatory molecules such as cyclins, CKIs, p53, and IκBα. Many diverse proinflammatory signals lead to the specific phosphorylation and subsequent ubiquitin-mediated destruction of the NF-κB inhibitor protein IκBα. 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 Skp1, Cdc53, and theF-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, SCF^β-TRCP, that specifically recognizes a 19-amino-acid destruction motif in IκBα (residues 21–41) in a phosphorylation-dependent manner. This SCF complex also recognizes a conserved destruction motif in β-catenin, a protein with levels also regulated by phosphorylation-dependent ubiquitination. Endogenous IκBα–ubiquitin ligase activity cofractionates with SCF^β-TRCP. Furthermore, recombinant SCF^β-TRCPassembled in mammalian cells contains phospho-IκBα-specific ubiquitin ligase activity. Our results suggest that an SCF^β-TRCP complex functions in multiple transcriptional programs by activating the NF-κB pathway and inhibiting the β-catenin pathway.