Tumorigenesis is the result of uncontrolled cell growth due to the deregulation of cell cycle checkpoints 1. Period 2 (Per2) is a tumor suppressor that oscillate in expression in a 24-hour cycle 2, 3. Here, we show that Per2 interacts with the tumor suppressor protein p53. Both G1 and G2 checkpoint pathways involve a p53 dependent pathway which can trigger the cell to go through cell arrest or programmed cell death4. Understanding all the mitigating factors involved in regulating cell cycle progression under DNA damage can offer a better idea in how cells become immortal.

Initially discovered through screening of a human liver cDNA library, the novel interaction between p53-Per2 was further documented using co-precipitation. Interestingly, under genotoxic stress conditions, p53 and Per2 were not found to bind which leads us to suspect that Per2 does not affect active p53 which may possibly be due to post translational modifications of its active state. Furthermore we investigated p53's ability to act as a transcription factor in the presence of Per2, showing that the Per2-p53 complex prevents p53 from binding to DNA. This implies that the tetramerization of p53 may also be another factor in Per2's ability to bind to p53. A truncated p53 lacking the last 30 amino acids that theoretically increase p53's ability to form a tetramer showed a drastic reduction in binding to Per2 5, 6. On the other hand, p53 lacking the tetramerization domain showed binding similar to wildtype. Consequently we speculate that the ability of Per2 to modulate p53 and act as a tumor suppressor protein may be dependent on either the post translational modifications of p53 or its oligomeric state.