Regulation of Casein Kinase I Activity by Wnt Signaling*

The Wnt/ -catenin signaling pathway is important in both development and cancer. Casein kinase I (CKI ) is a positive regulator of the canonical Wnt pathway. CKI itself can be regulated in vitro by inhibitory autophosphorylation, and recent data suggest that in vivo kinase activity can be regulated by extracellular stimuli. We show here that the phosphorylation state and kinase activity of CKI are directly regulated by Wnt signaling. Coexpression of XWnt-8 or addition of soluble Wnt-3a ligand led to a significant and rapid increase in the activity of endogenous CKI . The increase in CKI activity is the result of decreased inhibitory autophosphorylation because it is abolished by preincubation of immunoprecipitated kinase with ATP. Furthermore, mutation of CKI inhibitory autophosphorylation sites creates a kinase termed CKI (MM2) that is significantly more active than CKI and is not activated further upon Wnt stimulation. Autoinhibition of CKI is biologically relevant because CKI (MM2) is more effective than CKI at activating transcription from a Lef1-dependent promoter. Finally, CKI (MM2) expression in Xenopus embryos induces both axis duplication and additional developmental abnormalities. The data suggest that Wnt signaling activates CKI by causing transient dephosphorylation of critical inhibitory sites present in the carboxyl-terminal domain of the kinase. Activation of the Wnt pathway may therefore stimulate a cellular phosphatase to dephosphorylate and activate CKI .