Molecular mechanisms behind the functional (non)redundancy of CK1 paralogs in the Wnt pathway
The casein kinase 1 (CK1) family of serine/threonine protein kinases consists of seven isoforms in humans. CK1 family members are important regulators of the Wnt/{beta}-catenin signaling pathway. Using a comprehensive panel of CRISPR/Cas9-generated knockout cell lines, we demonstrated the opposing roles of endogenous CK1 (negative, via phosphorylation of {beta}-catenin in the destruction complex) and CK1{delta}/{epsilon} (positive, via phosphorylation of DVL in the signalosome), while no phenotype was observed for CK1{gamma}1/2/3 triple knockout cells. Using in vitro kinase assays and TurboID-based interactomics, we revealed that this functional divergence between CK1 and CK1{epsilon} is not due to the intrinsically different capacity to phosphorylate {beta}-catenin or DVL but rather due to different affinities for the destruction complex and signalosome in the cellular environment. Through functional analysis of CK1-CK1{epsilon} chimeras containing both N-terminal and C-terminal domain swaps, we identified the N-terminal lobe of CK1 and the C-terminus of CK1{epsilon} as determinants mediating increased affinity towards the degradasome and signalosome, respectively. We further show that the CK1 N-lobe not only drives cellular activity toward {beta}-catenin but also underlies the CK1-specific interaction with the scaffolding protein SACK1G (also known as FAM83G and PAWS1). Additionally, despite clearly distinct physiological roles of CK1 and CK1{delta}/{epsilon}, we provide evidence that, in the absence of CK1, CK1{delta} and CK1{epsilon} can physically and functionally substitute for CK1 in the {beta}-catenin destruction complex. This rewires CK1{delta} and CK1{epsilon} as negative regulators acting via phosphorylation of {beta}-catenin in the destruction complex. These findings resolve prior contradictions by (i) clarifying context-dependent CK1 roles, (ii) identifying mechanistic determinants navigating CK1 and CK1{epsilon} to different substrates, and (iii) defining the limitations of these affinity-based subcellular distributions that become apparent especially in the physical absence of the physiological, high-affinity kinase. An important implication of our findings is the identification of a mechanism that changes the ultimate outcome of CK1{delta}/{epsilon} inhibitor treatment from Wnt/{beta}-catenin pathway inhibition to its robust activation.