A role for casein kinase 2α in the Drosophila circadian clock

Circadian clocks drive rhythmic behaviour in animals and are regulated by transcriptional feedback loops1,2. For example, the Drosophila proteins Clock (Clk) and Cycle (Cyc) activate transcription of period (per) and timeless (tim). Per and Tim then associate, translocate to the nucleus, and repress the activity of Clk and Cyc. However, post-translational modifications are also critical to proper timing. Per and Tim undergo rhythmic changes in phosphorylation1, and evidence supports roles for two kinases in this process: Doubletime (Dbt) phosphorylates Per3,4, whereas Shaggy (Sgg) phosphorylates Tim5. Yet Sgg and Dbt often require a phosphoserine in their target site6,7, and analysis of Per phosphorylation in dbt mutants3,8 suggests a role for other kinases. Here we show that the catalytic subunit of Drosophila casein kinase 2 (CK2α) is expressed predominantly in the cytoplasm of key circadian pacemaker neurons. CK2α mutant flies show lengthened circadian period, decreased CK2 activity, and delayed nuclear entry of Per. These effects are probably direct, as CK2α specifically phosphorylates Per in vitro. We propose that CK2 is an evolutionary link between the divergent circadian systems of animals, plants and fungi.