The F-box protein beta-TrCP associates with phosphorylated beta-catenin and regulates its activity in the cell

Defects in beta-catenin regulation contribute to the neoplastic transformat ion of mammalian cells. Dysregulation of beta-catenin can result from misse nse mutations that affect critical sites of phosphorylation by glycogen syn thase kinase 3 beta (GSK3 beta). Given that phosphorylation can regulate ta rgeted degradation of beta-catenin by the proteasome, beta-catenin might in teract with an E3 ubiquitin ligase complex containing an F-box protein, as is the case for certain cell cycle regulators. Accordingly, disruption of t he Drosophila F-box protein Slimb upregulates the beta-catenin homolog Arma dillo. We reasoned that the human homologs of Slimb beta-TrCP and its isofo rm beta-TrCP2 (KIAA0696) - might interact with beta-catenin. We found that the binding of beta-TrCP to beta-catenin was direct and dependent upon the WD40 repeat sequences in beta-TrCP and on phosphorylation of the GSK3 beta sites in beta-catenin. Endogenous beta-catenin and beta-TrCP could be coimm unoprecipitated from mammalian cells. Overexpression of wild-type beta-TrCP in mammalian cells promoted the downregulation of beta-catenin, whereas ov erexpression of a dominant-negative deletion mutant upregulated beta-cateni n protein revels and activated signaling dependent on the transcription fac tor Tcf. In contrast, beta-TrCP2 did not associate with beta-catenin. We co nclude that beta-TrCP is a component of an E3 ubiquitin ligase that is resp onsible for the targeted degradation of phosphorylated beta-catenin.