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

beta-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 desmosome s where it anchors intermediate filaments to the desmosomal plaques. beta-C atenin, on the other hand, is a component of the Wnt signaling pathway, whi ch is involved in embryonic morphogenesis and tumorigenesis. A key step in the regulation of this pathway involves modulation of beta-catenin stabilit y. A multiprotein complex, regulated by Wnt, directs the phosphorylation of beta-catenin and its degradation by the ubiquitin-proteasome system. Plako globin can also associate with members of this complex, but inhibition of p roteasomal degradation has little effect on its levels while dramatically i ncreasing the levels of beta-catenin. beta-TrCP, an F-box protein of the SC F E3 ubiquitin ligase complex, was recently shown to play a role in the tur nover of beta-catenin. To elucidate the basis for the apparent differences in the turnover of beta-catenin and plakoglobin we compared the handling of these two proteins by the ubiquitin-proteasome system. We show here that a deletion mutant of beta-TrCP, lacking the F-box, can stabilize the endogen ous beta-catenin leading to its nuclear translocation and induction of beta -catenin/LEF-1-directed transcription, without affecting the levels of plak oglobin. However, when plakoglobin was overexpressed, it readily associated with beta-TrCP, efficiently competed with beta-catenin for binding to beta -TrCP and became polyubiquitinated. Fractionation studies revealed that abo ut 85% of plakoglobin in 293 cells, is Triton X-100-insoluble compared to 5 0% of beta-catenin. These results suggest that while both plakoglobin and b eta-catenin can comparably interact with beta-TrCP and the ubiquitination s ystem, the sequestration of plakoglobin by the membrane-cytoskeleton system renders it inaccessible to the proteolytic machinery and stabilizes it.