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

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.