In the Wnt/β-catenin signaling pathway, β-catenin activates target genes
through its interactions with the T-cell factor/lymphoid enhancer-binding factor
(TCF/Lef) family of transcription factors. The crystal structures of complexes
between the β-catenin armadillo domain and the Lef-1 N-terminal domain show
that the overall conformation and many of the interactions are similar to other
published structures of TCFs bound to β-catenin. However, a second salt bridge
in other TCF-β-catenin structures is absent in the structure of
β-catenin-Lef-1 complex, indicating that this feature is not obligatory for
β-catenin binding. Casein kinase II (CK2) has been shown to act as a positive
regulator of Wnt signaling, and Lef-1 is a substrate of CK2. In vitro
phosphorylation of purified Lef-1 was used to examine the effect of CK2 on the
interaction of Lef-1 with β-catenin. Mass spectrometry data show that CK2
phosphorylation of Lef-1 N-terminal domain results in a single phosphorylation
site at Ser40. Isothermal titration calorimetry revealed that β-catenin binds
to nonphosphorylated or CK2-phosphorylated Lef-1 with the same affinity, which
is consistent with the absence of phospho-Ser40 interactions in the crystal
structure of phosphorylated Lef-1 N-terminal domain bound to β-catenin. These
data indicate that the effect of CK2 on the Wnt/β-catenin pathway does not
appear to be at the level of the Lef-1-β-catenin interaction.
