Casein kinase 1γ couples Wnt receptor activation to cytoplasmic signal transduction

Signalling by Wnt proteins ( Wingless in Drosophila) has diverse roles during embryonic development and in adults, and is implicated in human diseases, including cancer(1,2). LDL-receptor-related proteins 5 and 6 (LRP5 and LRP6; Arrow in Drosophila) are key receptors required for transmission of Wnt/beta-catenin signalling in metazoa(3). Although the role of these receptors in Wnt signalling is well established, their coupling with the cytoplasmic signalling apparatus remains poorly defined. Using a protein modification screen for regulators of LRP6, we describe the identification of Xenopus Casein kinase 1 gamma (CK1 gamma), a membrane-bound member of the CK1 family. Gain-of-function and loss-of-function experiments show that CK1 gamma is both necessary and sufficient to transduce LRP6 signalling in vertebrates and Drosophila cells. In Xenopus embryos, CK1 gamma is required during anterio-posterior patterning to promote posteriorizing Wnt/beta-catenin signalling. CK1 gamma is associated with LRP6, which has multiple, modular CK1 phosphorylation sites. Wnt treatment induces the rapid CK1 gamma-mediated phosphorylation of these sites within LRP6, which, in turn, promotes the recruitment of the scaffold protein Axin. Our results reveal an evolutionarily conserved mechanism that couples Wnt receptor activation to the cytoplasmic signal transduction apparatus.