Wnt pathways play essential roles in cell proliferation, morphogenesis, and cell 
fate specification during embryonic development. According to the consensus 
view, the Wnt pathway prevents the degradation of the key signaling component 
β-catenin by the protein complex containing the negative regulators Axin and 
glycogen synthase kinase 3 (GSK3). Stabilized β-catenin associates with TCF 
proteins and enters the nucleus to promote target gene expression. This study 
examines the involvement of HIPK2 (homeodomain-interacting protein kinase 2) in 
the regulation of different TCF proteins in Xenopus embryos in vivo. We show 
that the TCF family members LEF1, TCF4, and TCF3 are phosphorylated in embryonic 
ectoderm after Wnt8 stimulation and HIPK2 overexpression. We also find that TCF3 
phosphorylation is triggered by canonical Wnt ligands, LRP6, and dominant 
negative mutants for Axin and GSK3, indicating that this process shares the same 
upstream regulators with β-catenin stabilization. HIPK2-dependent 
phosphorylation caused the dissociation of LEF1, TCF4, and TCF3 from a target 
promoter in vivo. This result provides a mechanistic explanation for the 
context-dependent function of HIPK2 in Wnt signaling; HIPK2 up-regulates 
transcription by phosphorylating TCF3, a transcriptional repressor, but inhibits 
transcription by phosphorylating LEF1, a transcriptional activator. Finally, we 
show that upon HIPK2-mediated phosphorylation, TCF3 is replaced with positively 
acting TCF1 at a target promoter. These observations emphasize a critical role 
for Wnt/HIPK2-dependent TCF phosphorylation and suggest that TCF switching is an 
important mechanism of Wnt target gene activation in vertebrate embryos.