β - catenin is a target for the ubiquitin – proteasome

stabilization and accumulation of hypophosphorylated β-catenin is a central component of the cadherin cell Arm/β-catenin (Peifer et al., 1994a,b), which is believed adhesion complex and plays an essential role in the to interact with downstream-acting transcription factors Wingless/Wnt signaling pathway. In the current model (Behrens et al., 1996; Huber et al., 1996b; Molenaar et al., of this pathway, the amount of β-catenin (or its inver1996). In the absence of Wg/Wnt signal, ZW3/GSK3β tebrate homolog Armadillo) is tightly regulated and its acts to cause Arm/β-catenin to be degraded, a process steady-state level outside the cadherin–catenin complex which in mammalian cells also requires the adenomatous is low in the absence of Wingless/Wnt signal. Here we polyposis coli (APC) tumor suppressor protein (reviewed show that the ubiquitin-dependent proteolysis system in Peifer, 1996). It has been reported that GSK3β binds is involved in the regulation of β-catenin turnover. to the APC–β-catenin complex and that this interaction β-catenin, but not E-cadherin, p120cas or α-catenin, reduces the amount of cytoplasmic β-catenin (Munemitsu becomes stabilized when proteasome-mediated proteoet al., 1995; Rubinfeld et al., 1996). However, little is lysis is inhibited and this leads to the accumulation of known about the identity of the protease actually involved multi-ubiquitinated forms of β-catenin. Mutagenesis in β-catenin degradation. experiments demonstrate that substitution of the serine Proteasomes play an essential role in the rapid eliminaresidues in the glycogen synthase kinase 3β (GSK3β) tion of short-lived key regulatory proteins, e.g. cell cycle phosphorylation consensus motif of β-catenin inhibits proteins (cyclins), rate-limiting enzymes (ornithine decarbubiquitination and results in stabilization of the protein. oxylase) or transcriptional activators (IκB–NFκB complex, This motif in β-catenin resembles a motif in IκB c-Jun, p53) (reviewed in Coux et al., 1996). Therefore, (inhibitor of NFκB) which is required for the phosphothe possibility that β-catenin could be turned over by rylation-dependent degradation of IκB via the ubiquithe ubiquitin–proteasome system was examined. In most tin–proteasome pathway. We show that ubiquitination cases, targeted proteins are tagged for proteolysis by the of β-catenin is greatly reduced in Wnt-expressing cells, ligation of multiple ubiquitin molecules in a multimeric providing the first evidence that the ubiquitin–prochain. Ligation involves a series of enzymatic reactions: teasome degradation pathway may act downstream of ubiquitin itself is first activated with ATP by the ubiquitinGSK3β in the regulation of β-catenin. activating enzyme (E1), and secondly transferred onto a