Structural and biochemical characterization of beta-catenin and its transcription binding partners in Wnt signaling

Wnt signaling is a developmental pathway conserved among all members of the animal kingdom. This pathway plays essential roles in embryogenesis, and differentiation and proliferation of adult stem cells. Inappropriate activation of Wnt signaling by mutation of pathway components has been associated with various diseases and human cancers.;Upon activation of Wnt signaling, the transcriptional co-activator beta-catenin is translocated into the nucleus, where it activates transcription of Wnt-dependent target genes through its interactions with TCF/Lef. There is increasing evidence that Casein Kinase II (CK2) is a positive regulator of Wnt signaling, but the mechanism is not well understood at the molecular level. We used structural and biochemical methods to study the Lef-1/beta-catenin interaction, and the effect of phosphorylation by CK2 on the interaction. Crystal structures of the complexes including beta-catenin with both untreated and phosphorylated Lef-1 N-terminal domain are presented. The overall structure of Lef-1 bound to beta-catenin is very similar to other published structures of TCFs. However, the second salt bridge present in other TCF-beta-catenin structures is absent in the structures of the beta-catenin/Lef-1 complexes. Structures of the unphosphorylated and phosphorylated complexes are similar and Isothermal titration calorimetry measurements of beta-catenin with both Lef-1 ligands do not show significant differences in their binding affinity. The structural and biochemical data eliminate certain models for the role of CK2 in promoting beta-catenin-mediated gene activation, and we suggest alternative models for the role of CK2 in activating Wnt signaling.;beta-catenin seems to function as a molecular platform by recruiting both Wnt-specific and general transcription factors. Preliminary results from pulldown experiments show that beta-catenin specifically interacts with mouse TATA-binding protein (TBP) and not yeast TBP, and the presence of the Lef-1 N-terminal domain does not seem to affect the interaction. Furthermore, mapping of the minimal binding region suggests that the N-terminal domain of TBP is likely responsible for this interaction.;There is increasing evidence that beta-catenin is important in androgen signaling through its direct interaction with the ligand binding domain of androgen receptor (AR-LBD). This suggests an interesting possibility of a cross-talk between Wnt and androgen signaling. Using various biochemical experiments, we show that both the N-terminal and arm domains of beta-catenin are necessary to interact with purified AR-LBD. However, the binding affinity between the two proteins is very weak. Furthermore, the presence of E-cadherin or ICAT does not interfere with the interaction.