Differential mechanisms of LEF/TCF family-dependent transcriptional activation by beta-catenin and plakoglobin.
beta-Catenin and plakoglobin are highly homologous components of cell-cell adherens junctions linking cadherin receptors to the actin cytoskeleton. beta-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 beta-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 beta-catenin-LEF-DNA. Moreover, in plakoglobin-transfected cells plakoglobin-LEF/TCF-DNA complexes were not formed; rather, the endogenous beta-catenin, whose level is elevated by plakoglobin transfection, formed a beta-catenin-LEF-DNA complex. Removal of the N- and C-terminal domains of both beta-catenin and plakoglobin (leaving the armadillo repeat domain intact) induced plakoglobin-LEF-DNA complex formation and also enhanced beta-catenin-LEF-DNA complexing, both with in vitro-translated components and in transfected cells. Transfection with these truncated catenins increased endogenous beta-catenin levels, but the truncated catenins acted as dominant-negative inhibitors of beta-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 beta-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.
Zhurinsky
J
J
Department of Molecular Cell Biology, The Weizmann Institute of Science, Rehovot, 76100, Israel.
Shtutman
M
M
Ben-Ze'ev
A
A
D001692:Biological Transport / Q000235:genetics; D002460:Cell Line; D003598:Cytoskeletal Proteins / Q000235:genetics*; D004268:DNA-Binding Proteins / Q000235:genetics*; D051180:Desmoplakins; D006801:Humans; D009154:Mutation; D015534:Trans-Activators; D014157:Transcription Factors / Q000235:genetics*; D015533:Transcriptional Activation; D051176:beta Catenin; D051185:gamma Catenin
C495270:CTNNB1 protein, human; D003598:Cytoskeletal Proteins; D004268:DNA-Binding Proteins; D051180:Desmoplakins; D015534:Trans-Activators; D014157:Transcription Factors; D051176:beta Catenin; D051185:gamma Catenin