Isoform-specific C-terminal phosphorylation drives autoinhibition of Casein Kinase 1.

Casein kinase 1 δ (CK1δ) controls essential biological processes including circadian rhythms and Wnt signaling, but how its activity is regulated is not well understood. CK1δ is inhibited by autophosphorylation of its intrinsically disordered C-terminal tail. Two CK1 splice variants, δ1 and δ2, are known to have very different effects on circadian rhythms. These variants differ only in the last 16 residues of the tail, referred to as the extreme C-termini (XCT), but with marked changes in potential phosphorylation sites. Here we test if the XCT of these variants have different effects in autoinhibition of the kinase. Using NMR and HDX-MS, we show that the δ1 XCT is preferentially phosphorylated by the kinase and the δ1 tail makes more extensive interactions across the kinase domain. Mutation of δ1-specific XCT phosphorylation sites increases kinase activity both Subtle control of kinase activity is critical to physiologic modulation of multiple physiological processes including circadian rhythms. CK1δ and the closely related CK1ε regulate circadian rhythms by phosphorylation of PER2, but how kinase activity itself is controlled is not clear. Building on the prior observation that two splice isoforms of CK1δ regulate the clock differently, we show that the difference maps to three phosphorylation sites in the variably spliced region (XCT) that cause feedback inhibition of the kinase domain. More broadly, the data suggest a general model where CK1 activity on diverse substrates can be controlled by signaling pathways that alter tail phosphorylation. These inhibitory phosphorylation sites could also be targets for new therapeutic interventions.