Phosphorylation and destabilization of human period I clock protein by human casein kinase Iε

Period (PER),a central component of the circadian clock in drosophila, undergoes daily oscillation in abundance and phosphorylation state. Here we report that human casein kinase le (hCKle) can phosphorylate human PER I (hPER I). Puridied recombinant hCKle (but not a kinase negative mutant of hCKle, hCKle-K38R) phosphorylated hPER I in vitro. When co-transfected with wild-type hCKle, in 293T cells, hPER I shoxed a significant increase in phosporylation as evidenced by a shift in molecular mass. Furthermore, phosphorylation of hPER I by hCKle caused a decrease in protein stability in hPER I. Whereas phosphorylated hPER I had a half-life of approximately 12 h, unphosphorylated hPER I remained stable in the cell for > 24 h. hPER I protein could also be co-immunoprecipitated with transfected hCKle as well as endogenous hCKle, indicating physical association between hPER I and hCKle proteins in vivo.