Differential Mechanisms of LEF/TCF Family-Dependent Transcriptional Activation by β-Catenin and Plakoglobin

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Abstract
β-Catenin and plakoglobin are highly homologous components of cell-cell adherens junctions linking cadherin receptors to the actin cytoskeleton. β-Catenin, in addition, activates transcription by forming a complex with LEF/TCF family transcription factors in the nucleus. Plakoglobin can also bind to LEF-1 and, when overexpressed in mammalian cells, enhances LEF-1-directed transcription. Plakoglobin overexpression, however, results in the elevation and nuclear translocation of endogenous β-catenin. We show here, by DNA mobility shift analysis, that the formation of a plakoglobin-LEF/TCF-DNA complex in vitro is very inefficient compared to a complex containing β-catenin-LEF-DNA. Moreover, in plakoglobin-transfected cells plakoglobin-LEF/TCF-DNA complexes were not formed; rather, the endogenous β-catenin, whose level is elevated by plakoglobin transfection, formed a β-catenin–LEF–DNA complex. Removal of the N- and C-terminal domains of both β-catenin and plakoglobin (leaving the armadillo repeat domain intact) induced plakoglobin-LEF-DNA complex formation and also enhanced β-catenin–LEF–DNA complexing, both with in vitro-translated components and in transfected cells. Transfection with these truncated catenins increased endogenous β-catenin levels, but the truncated catenins acted as dominant-negative inhibitors of β-catenin-driven transcription by forming transcriptionally inactive complexes with LEF-1. When these catenin mutants were prevented from entering the nucleus, by their fusion to the connexin transmembrane domain, they indirectly activated transcription by increasing endogenous β-catenin levels. These results suggest that overexpression of plakoglobin does not directly activate transcription and that formation of catenin-LEF-DNA complexes is negatively regulated by the catenin N- and C-terminal domains.