Although casein kinase I epsilon (CKI epsilon) has been shown to regulate the Wnt signaling pathway positively, its mode of action is not clear. In this study we show that CKI epsilon activates the Wnt signaling pathway in co-operation with Dv1. CKI epsilon and Axin associated with different sites of Dv1, and CKI epsilon and Dv1 interacted with distinct regions on Axin. Therefore, these three proteins formed a ternary complex. Either low expression of Dv1 or CKI epsilon alone did not accumulate beta -catenin, but their co-expression accumulated greatly. Dv1 and CKI epsilon activated the transcriptional activity of T cell factor (Tcf) synergistically. Although the Dv1 mutant that binds to Axin but not to CKI epsilon activated Tcf, it did not synergize with CKI epsilon Another Dv1 mutant that does not bind to Axin did not activate Tcf irrespective of the presence of CKI epsilon Furthermore, Dv1 and CKI epsilon co-operatively induced axis duplication of Xenopus embryos. These results indicate that Dv1 and CKI epsilon synergistically activated the Writ signaling pathway and that the binding of the complex of Dv1 and CKI epsilon to Axin is necessary for their synergistic action.
