Sequential Activation and Inactivation of Dishevelled in the Wnt/beta-Catenin Pathway by Casein Kinases
Dishevelled (Dvl) is a key component in the Wnt/beta-catenin signaling pathway. Dvl can multimerize to form dynamic protein aggregates, which are required for the activation of downstream signaling. Upon pathway activation by Wnts, Dvl becomes phosphorylated to yield phosphorylated and shifted (PS) Dvl. Both activation of Dvl in Wnt/beta-catenin signaling and Wnt-induced PS-Dvl formation are dependent on casein kinase 1 (CK1) delta/epsilon activity. However, the overexpression of CK1 was shown to dissolve Dvl aggregates, and endogenous PS-Dvl forms irrespective of whether or not the activating Wnt triggers the Wnt/beta-catenin pathway. Using a combination of gain-of-function, loss-of-function, and domain mapping approaches, we attempted to solve this discrepancy regarding the role of CK1 epsilon in Dvl biology. We analyzed mutual interaction of CK1 delta/epsilon and two other Dvl kinases, CK2 and PAR1, in the Wnt/beta-catenin pathway. We show that CK2 acts as a constitutive kinase whose activity is required for the further action of CK1 epsilon. Furthermore, we demonstrate that the two consequences of CK1 epsilon phosphorylation are separated both spatially and functionally; first, CK1 epsilon-mediated induction of TCF/LEF-driven transcription (associated with dynamic recruitment of Axin1) is mediated via a PDZ-proline-rich region of Dvl. Second, CK1 epsilon-mediated formation of PS-Dvl is mediated by the Dvl3 C terminus. Furthermore, we demonstrate with several methods that PS-Dvl has decreased ability to polymerize with other Dvls and could, thus, act as the inactive signaling intermediate. We propose a multistep and multikinase model for Dvl activation in the Wnt/beta-catenin pathway that uncovers a built-in de-activation mechanism that is triggered by activating phosphorylation of Dvl by CK1 delta/epsilon.