APC 3×15 β-catenin-binding domain potentiates β-catenin association to TBP and upregulates TCF-4 transcriptional activity

β-Catenin plays a dual role as a regulatory component of adherens junctions and as a transcriptional cofactor. The nuclear activity of this protein is controlled by adenomatous polyposis coli (APC) protein. We have analyzed the effect on β-catenin-dependent transcription of a β-catenin binding domain present in APC, consisting in three 15-amino acid repeats (APC 3×15). Association of this fragment prevents the interaction of β-catenin with E-cadherin but not with TCF-4. Transfection of this fragment to several cell lines increases the transcriptional activity of the β-catenin-TCF-4 complex and promotes the translocation of β-catenin to the nucleus. Moreover, previous binding of APC 3×15 facilitates the association of β-catenin to the TATA box-associated protein. Therefore, APC 3×15 domain plays a positive role in the control of transcriptional activity of β-catenin-TCF-4 and can contribute to explain the role of the truncated forms of APC in colon tumorigenesis.