The mammalian circadian clock proteins undergo a daily cycle of accumulation followed by phosphorylation and degradation. The mechanism by which clock proteins undergo degradation has not been fully understood. Circadian clock protein PERIOD2 (PER2) is shown to be the potential target of F-box protein β-TrCP1, a component of ubiquitin E3 ligase. Here, we show that β-TrCP2 as well as β-TrCP1 target PER2 protein in vitro. We also identified β-TrCP binding site (m2) of PER2 being recognized by both β-TrCP1 and β-TrCP2. Luciferase-PER2 fusion system revealed that m2 site was responsible for the stability of PER2. The role of β-TrCP1 and β-TrCP2 in circadian rhythm generation was analysed by real-time reporter assay revealing that siRNA-mediated suppressions of β-TrCP1 and/or β-TrCP2 attenuate circadian oscillations in NIH3T3 cell. β-TrCP1-deficient mice, however, showed normal period length, light-induced phase-shift response in behaviour and normal expression of PER2, suggesting that β-TrCP1 is dispensable for the central clock in the suprachiasmatic nucleus. Our study indicates that β-TrCP1 and β-TrCP2 were involved in the cell autonomous circadian rhythm generation in culture cells, although the role of β-TrCP2 in the central clock in the suprachiasmatic nucleus remains to be elucidated.
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