Role for Slimb in the degradation of Drosophila Period protein phosphorylated by Doubletime

Protein phosphorylation has a key role in modulating the stabilities of circadian clock proteins in a manner specific to the time of day 1 . A conserved feature of animal clocks is that Period (Per) proteins undergo daily rhythms in phosphorylation and levels 2 , 3 , events that are crucial for normal clock progression 4 , 5 , 6 , 7 . Casein kinase Iε (CKIε) has a prominent role in regulating the phosphorylation and abundance of Per proteins in animals 8 . This was first shown in Drosophila with the characterization of Doubletime (Dbt), a homologue of vertebrate casein kinase Iε 4 , 6 . However, it is not clear how Dbt regulates the levels of Per. Here we show, using a cell culture system, that Dbt promotes the progressive phosphorylation of Per, leading to the rapid degradation of hyperphosphorylated isoforms by the ubiquitin–proteasome pathway. Slimb, an F-box/WD40-repeat protein functioning in the ubiquitin–proteasome pathway 9 , 10 interacts preferentially with phosphorylated Per and stimulates its degradation. Overexpression of slimb or expression in clock cells of a dominant-negative version of slimb disrupts normal rhythmic activity in flies. Our findings suggest that hyperphosphorylated Per is targeted to the proteasome by interactions with Slimb.