Nuclear localization and formation of beta-catenin-lymphoid enhancer factor 1 complexes are not sufficient for activation of gene expression.
In response to activation of the Wnt signaling pathway, beta-catenin accumulates in the nucleus, where it cooperates with LEF/TCF (for lymphoid enhancer factor and T-cell factor) transcription factors to activate gene expression. The mechanisms by which beta-catenin undergoes this shift in location and participates in activation of gene transcription are unknown. We demonstrate here that beta-catenin can be imported into the nucleus independently of LEF/TCF binding, and it may also be exported from nuclei. We have introduced a small deletion within beta-catenin (Delta19) that disrupts binding to LEF-1, E-cadherin, and APC but not axin. This Delta19 beta-catenin mutant localizes to the nucleus because it may not be efficiently sequestered in the cytoplasm. The nuclear localization of Delta19 definitively demonstrates that the mechanisms by which beta-catenin localizes in the nucleus are completely independent of LEF/TCF factors. beta-Catenin and LEF-1 complexes can activate reporter gene expression in a transformed T-lymphocyte cell line (Jurkat) but not in normal T lymphocytes, even though both factors are nuclear. Thus, localization of both factors to the nucleus is not sufficient for activation of gene expression. Excess beta-catenin can squelch reporter gene activation by LEF-1-beta-catenin complexes but not activation by the transcription factor VP16. Taken together, these data suggest that a third component is necessary for gene activation and that this third component may vary with cell type.
D000818:Animals; D019556:COS Cells; D015820:Cadherins / Q000378:metabolism; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism*; D003513:Cycloheximide / Q000494:pharmacology; D003593:Cytoplasm / Q000378:metabolism; D003598:Cytoskeletal Proteins / Q000378:metabolism* / Q000502:physiology*; D004268:DNA-Binding Proteins / Q000378:metabolism* / Q000502:physiology*; D003609:Dactinomycin / Q000494:pharmacology; D005786:Gene Expression Regulation; D006801:Humans; D015759:Ionomycin / Q000494:pharmacology; D019169:Jurkat Cells; D008214:Lymphocytes / Q000378:metabolism; D051821:Lymphoid Enhancer-Binding Factor 1; D008856:Microscopy, Fluorescence; D008957:Models, Genetic; D016296:Mutagenesis; D010957:Plasmids; D011233:Precipitin Tests; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D011993:Recombinant Fusion Proteins; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D015534:Trans-Activators; D014157:Transcription Factors / Q000378:metabolism* / Q000502:physiology*; D015533:Transcriptional Activation; D051176:beta Catenin