beta-Catenin and plakoglobin are two related armadillo proteins necessary for the establishment of adhesion junctions and desmosomes. Moreover, beta-catenin can also act as a transcriptional co-activator through its interaction with the members of Tcf/LEF-1 transcriptional factor family. We show here that Tcf-4 can be phosphorylated in vitro by protein kinase CK2 stoichiometrically in amino acids Ser-58-Ser-59-Ser-60. Phosphorylation of these residues does not modify the interaction of Tcf-4 with beta-catenin but reduces its association to plakoglobin. The binding sites of Tcf-4 for these two proteins were compared; whereas beta-catenin requires the N-terminal first 50 amino acids, plakoglobin interacts mainly with residues 51-80. Tcf-4-(51-80) binds plakoglobin in the region of armadillo repeats 1-6. Ternary complexes composed by beta-catenin/Tcf-4/plakoglobin could be detected in vitro, demonstrating that simultaneous binding of the two armadillo proteins to Tcf-4 is possible. Experiments performed using a Tcf-4 mutant with decreased interaction to plakoglobin demonstrated that binding to this protein negatively affected the transcriptional activity of Tcf-4. These results indicate that Tcf-4 contains two different sites for binding of beta-catenin and plakoglobin, and the interaction of the latter hinders the transcriptional activity of the complex.