Activate or Repress?

Abstract

Canonical Wnt signaling results in the stabilization of β-catenin, which translocates to the nucleus and binds to members of the T cell factor (TCF) family of DNA binding proteins. This interaction displaces a corepressor protein from TCF and recruits a coactivator protein to activate gene transcription. However, β-catenin is also known to directly repress the transcription of a small number of genes in a TCF-dependent manner, but how this occurs is unclear. Blauwkamp et al . investigated the mechanism by which the Drosophila β-catenin protein Armadillo (Arm) represses transcription. The authors performed microarray analyses and identified a number of genes in Drosophila Kc167 cells whose decreased expression was dependent on TCF and on signals that stabilized Arm. However, real-time reverse transcription polymerase chain reaction (RT-PCR) assays showed that, in the absence of Wnt signaling, transcription of these genes was activated in a TCF-dependent manner. The authors focused on the gene Ugt36Bc and identified a 178-base pair fragment that contained the Wnt responsive element (WRE) required for repressed transcriptional activity in reporter assays. Although the WRE did not contain the consensus TCF-binding site, chromatin immunoprecipitation (ChIP) assays showed that TCF bound to the Ugt36Bc WRE in Kc167 cells. A mutant TCF protein that cannot bind to Arm inhibited Arm-dependent repression in a reporter assay. DNAseI protection assays identified the TCF-binding sites, which contained a novel consensus sequence, mutation of which blocked reporter activity in the absence of Wnt signaling. A mutant Arm protein (DisArmed) defective for gene activation was still able to repress target gene transcription in transfected Kc167 cells, which suggests that different Arm domains repress or activate transcription. The authors suggest that distinct TCF-binding sites allosterically determine whether TCF-Arm will activate or repress transcription. T. A. Blauwkamp, M. V. Chang, K. M. Cadigan, Novel TCF-binding sites specify transcriptional repression by Wnt signalling. EMBO J . 27 , 1436-1446 (2008). [PubMed]