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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 lε (hCKIε) can phosphorylate human PERI (hPERI). Purified recombinant hCKIε (but not a kinase negative mutant of hCKIε, hCKIε-K38R) phosphorylated hPERI in vitro. When co-transfected with wild-type hCKIε, in 293T cells, hPERI showed a significant increase in phosphorylation as evidenced by a shift in molecular mass. Furthermore, phosphorylation of hPERI by hCKIε caused a decrease in protein stability in hPERI. Whereas phosphorylated hPERI had a half-life of approximately 12 h, unphosphorylated hPERI remained stable in the cell for > 24 h. hPERI protein could also be co-immunoprecipitated with transfected hCKIε as well as endogenous hCKIε, indicating physical association between hPERI and hCKIε proteins in vivo.