IQGAP1-mediated stimulation of transcriptional co-activation by beta-catenin is modulated by calmodulin.
Beta-catenin, an oncoprotein integral to cell-cell adhesion and proliferative signaling, is increased in several malignancies. The recently discovered calmodulin-binding protein IQGAP1 binds stoichiometrically to beta-catenin and regulates the association of beta-catenin with the cell-cell adhesion complex. In the present work, we investigated the role of IQGAP1 on the transcriptional co-activator ... function of beta-catenin, and whether calmodulin modulated the functional interaction between IQGAP1 and beta-catenin. In vitro competition assays revealed that both Ca(2+)/calmodulin and apo-calmodulin, when pre-bound to IQGAP1, prevented binding of beta-catenin to IQGAP1, and partially displaced beta-catenin pre-bound to IQGAP1 when added subsequently. Conversely, beta-catenin partially displaced apo-calmodulin, but not Ca(2+)/calmodulin, from IQGAP1. Overexpression of IQGAP1 in SW480 colon carcinoma cells enhanced beta-catenin-mediated transcriptional co-activation by 1.72-fold, and this stimulation was significantly attenuated upon antagonism of calmodulin using the cell-permeable antagonist CGS9343B. Moreover, an IQGAP1 mutant that does not bind calmodulin was unable to stimulate beta-catenin transcriptional function. Results of pulse-chase analyses suggested that IQGAP1 slowed the turnover of soluble, but not total, beta-catenin. Immunocytochemistry revealed that IQGAP1 overexpression increased the amount of beta-catenin located in the nucleus, whereas incubation of cells with CGS9343B blocked this accumulation. Together, our results imply that IQGAP1 enhances the function of beta-catenin in the nucleus and that calmodulin regulates this stimulation.
Mesh Terms:
Antidiarrheals, Benzimidazoles, Binding Sites, Binding, Competitive, Calmodulin, Cell Line, Cell Nucleus, Cytoskeletal Proteins, Glutathione Transferase, Green Fluorescent Proteins, Humans, Immunohistochemistry, Luciferases, Luminescent Proteins, Methionine, Microfilament Proteins, Microscopy, Fluorescence, Models, Biological, Plasmids, Promoter Regions, Genetic, Protein Binding, Time Factors, Trans-Activators, Transcription, Genetic, Transcriptional Activation, Transfection, Tumor Cells, Cultured, Up-Regulation, beta Catenin, ras GTPase-Activating Proteins
Antidiarrheals, Benzimidazoles, Binding Sites, Binding, Competitive, Calmodulin, Cell Line, Cell Nucleus, Cytoskeletal Proteins, Glutathione Transferase, Green Fluorescent Proteins, Humans, Immunohistochemistry, Luciferases, Luminescent Proteins, Methionine, Microfilament Proteins, Microscopy, Fluorescence, Models, Biological, Plasmids, Promoter Regions, Genetic, Protein Binding, Time Factors, Trans-Activators, Transcription, Genetic, Transcriptional Activation, Transfection, Tumor Cells, Cultured, Up-Regulation, beta Catenin, ras GTPase-Activating Proteins
J. Biol. Chem.
Date: Mar. 01, 2002
PubMed ID: 11734550
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