Figure 5.

Coupling the circadian clock to the cell cycle/checkpoint response. (A) Timeless (TIM) is required for Chk1 activation. HeLa cells were transfected with control or TIM siRNA twice over a period of 3 days. Seventy-two hours after the initial transfection, cells were treated with 10 mM hydroxyurea (HU) for 1.5 h or left untreated. Two hundred micrograms of cell lysates was immunoblotted with anti-Tim (α-Timeless), anti-P-Chk1 (Ser345), anti-Chk1, anti-Per2, and anti-actin antibodies.⁴⁶ (B) Model for parallel coupling of the circadian clock and the cell cycle/checkpoint response. The core circadian clock protein CRY2, in conjunction with TIM, participates in the ATR–Chk1 signaling pathway in response to UV and UV-mimetic agents.