Control of Intracellular Dynamics of Mammalian Period Proteins by Casein Kinase I ɛ (CKIɛ) and CKIδ in Cultured Cells

Recent studies have shown that casein kinase I ɛ (CKIɛ) is an essential regulator of the mammalian circadian clock. However, the detailed mechanisms by which CKIɛ regulates each component of the circadian negative-feedback loop have not been fully defined. We show here that mPer proteins, negative limbs of the autoregulatory loop, are specific substrates for CKIɛ and CKIδ. The CKI phosphorylation of mPer1 and mPer3 proteins results in their rapid degradation, which is dependent on the ubiquitin-proteasome pathway. Moreover, CKIɛ and CKIδ are able to induce nuclear translocation of mPer3, which requires its nuclear localization signal. The mutation in potential phosphorylation sites on mPer3 decreased the extent of both nuclear translocation and degradation of mPer3 that are stimulated by CKIɛ. CKIɛ and CKIδ affected the inhibitory effect of mPer proteins on the transcriptional activity of BMAL1-CLOCK, but the inhibitory effect of mCry proteins on the activity of BMAL1-CLOCK was unaffected. These results suggest that CKIɛ and CKIδ regulate the mammalian circadian autoregulatory loop by controlling both protein turnover and subcellular localization of mPer proteins.