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.