The dramatic impact of the PER-DBT interaction on circadian timekeeping and temperature compensation

Circadian timekeeping is orchestrated by multiple clock proteins, but mechanistic understanding is limited. AlphaFold predicts novel interactions between two helical domains of the Drosophila PERIOD (PER) protein and its major kinase Doubletime (DBT). These PER domains, SYQ and LT, are conserved with mammals and are predicted to interact identically with the DBT ortholog casein kinase 1 (CK1). SYQ and LT mutations designed to disrupt this interface result in severe period and temperature-sensitive fly phenotypes, and two LT mutations cause unprecedented free-running periods ≥ 44 hours and temperature-sensitive changes of ∼20 hours. Further characterization of a LT mutant strain shows a severe reduction in both PER phosphorylation and PER degradation. A human PER equivalent of the same LT mutation is a much less efficient in vitro substrate for CK1-mediated phosphorylation. These observations indicate that this conserved PER-DBT interaction is critical for PER phosphorylation, circadian timing and its enigmatic temperature compensation phenomena.