Differential interaction of plakoglobin and β-catenin with the ubiquitin-proteasome system

Abstract

b-Catenin and plakoglobin are closely related armadillo family proteins with shared and distinct properties; Both are associated with cadherins in actin-containing adherens junctions. Plakoglobin is also found in desmosomes where it anchors intermediate ®laments to the desmosomal plaques. b-Catenin, on the other hand, is a component of the Wnt signaling pathway, which is involved in embryonic morphogenesis and tumorigenesis. A key step in the regulation of this pathway involves modulation of b-catenin stability. A multiprotein complex, regulated by Wnt, directs the phosphorylation of bcatenin and its degradation by the ubiquitin-proteasome system. Plakoglobin can also associate with members of this complex, but inhibition of proteasomal degradation has little e ect on its levels while dramatically increasing the levels of b-catenin. b-TrCP, an F-box protein of the SCF E3 ubiquitin ligase complex, was recently shown to play a role in the turnover of b-catenin. To elucidate the basis for the apparent di erences in the turnover of bcatenin and plakoglobin we compared the handling of these two proteins by the ubiquitin-proteasome system. We show here that a deletion mutant of b-TrCP, lacking the F-box, can stabilize the endogenous b-catenin leading to its nuclear translocation and induction of b-catenin/ LEF-1-directed transcription, without a ecting the levels of plakoglobin. However, when plakoglobin was overexpressed, it readily associated with b-TrCP, e ciently competed with b-catenin for binding to b-TrCP and became polyubiquitinated. Fractionation studies revealed that about 85% of plakoglobin in 293 cells, is Triton X-100-insoluble compared to 50% of b-catenin. These results suggest that while both plakoglobin and b-catenin can comparably interact with b-TrCP and the ubiquitination system, the sequestration of plakoglobin by the membrane-cytoskeleton system renders it inaccessible to the proteolytic machinery and stabilizes it.

Overexpression of human βTrCP1 deleted of its F box induces tumorigenesis in transgenic mice

“…This mutant acts as a negative transdominant by sequestering b-catenin and inhibiting its ubiquitination and degradation by proteasomes even in the presence of bTrCP2 expression (Hart et al, 1999;Kitagawa et al, 1999;Latres et al, 1999). Overexpressed wild-type bTrCP1 can also act as a dominant negative of the SCF complex and causes b-catenin stabilization instead of degradation (Sadot et al, 2000). In the current study, we show that the overexpression of DFbTrCP1 or bTrCP1 in mice results in the accumulation of b-catenin and tumorigenesis.…”

Protein evolution: structure‐function relationships of the oncogene beta‐catenin in the evolution of multicellular animals

“…Motifs that are conserved in beta-catenin and plakoglobin include the GSK3b-phosphorylation site/ubiquitination site (motif 1), the alpha-catenin binding site (motif 3), and all 12 armadillo-repeats. Interestingly, the GSK-3 phosphorylation/ubiquitination site (motif 1) is highly conserved in plakoglobin, despite plakoglobin's demonstrated minor role in the wnt-signaling pathway (Sadot et al, 2000;Williams et al, 2000;Zhurinsky et al, 2000a). The conservation therefore suggests an important, undiscovered function of this site in the regulation of plakoglobin that is different from the regulation by this site in the beta-catenin protein.…”

“…This rapid sequence evolution appears related to a relaxation of stabilizing selection on an ancestral function and the evolution of a novel function. Differences in plakoglobin's cellular sequestration, its accessibility to the protein degradation machinery, its affinity to TCF/LEF-1, and its level of transactivating ability suggest that plakoglobin is unlikely to play a beta-catenin-like role in the canonical wnt-signaling pathway (Simcha et al, '98;White et al, '98;Sadot et al, 2000;Williams et al, 2000). However, plakoglobin may indirectly modify or attenuate beta-catenin mediate wntsignaling (Zhurinsky et al, 2000a(Zhurinsky et al, , 2000b.…”

Non-canonical NF-κB signaling pathway

“…One possibility is that some critical factors mediating constitutive p100 processing are located in the nucleus. For example, a number of ubiquitin ligases, including SCF βTrCP , are predominantly localized in the nucleus [27][28][29][30][31]. Although SCF βTrCP is dispensable for constitutive processing of p100 [15], the possibility for the involvement of another nuclear ubiquitin ligase(s) in this molecular event cannot be excluded.…”