REGULATION OF BETA-CATENIN LEVELS AND LOCALIZATION BY OVEREXPRESSION OF PLAKOGLOBIN AND INHIBITION OF THE UBIQUITIN-PROTEASOME SYSTEM

beta-Catenin and plakoglobin (gamma-catenin) are closely related molec ules of the armadillo family of proteins. They are localized at the su bmembrane plaques of cell-cell adherens junctions where they form inde pendent complexes with classical cadherins and alpha-catenin to establ ish the link with the actin cytoskeleton. Plakoglobin is also found in a complex with desmosomal cadherins and is involved in anchoring inte rmediate filaments to desmosomal plaques. In addition to their role in junctional assembly, beta-catenin has been shown to play an essential role in signal transduction by the Wnt pathway that results in its tr anslocation into the nucleus. To study the relationship between plakog lobin expression and the level of beta-catenin, and the localization o f these proteins in the same cell, we employed two different tumor cel l lines that express N-cadherin, and alpha- and alpha-catenin, but no plakoglobin or desmosomal components. Individual clones expressing var ious levels of plakoglobin were established by stable transfection. Pl akoglobin overexpression resulted in a dose-dependent decrease in the level of beta-catenin in each clone. Induction of plakoglobin expressi on increased the turnover of beta-catenin without affecting RNA levels , suggesting posttranslational regulation of beta-catenin. In plakoglo bin overexpressing cells, both beta-catenin and plakoglobin were local ized at cell-cell junctions. Stable transfection of mutant plakoglobin molecules showed that deletion of the N-cadherin binding domain, but not the alpha-catenin binding domain, abolished beta-catenin downregul ation. Inhibition of the ubiquitin-proteasome pathway in plakoglobin o verexpressing cells blocked the decrease in beta-catenin levels and re sulted in accumulation of both beta-catenin and plakoglobin in the nuc leus. These results suggest that (a) plakoglobin substitutes effective ly with beta-catenin for association with N-cadherin in adherens junct ions, (b) extrajunctional beta-catenin is rapidly degraded by the prot easome-ubiquitin system but, (c) excess beta-catenin and plakoglobin t ranslocate into the nucleus.