1. Protein phosphatase 1γ is responsible for dephosphorylation of histone H3 at Thr 11 after DNA damage

2. INTRODUCTION Systems to respond to DNA damage - cell cycle arrest mechanism - DNA repair pathways - apoptotic response >> partly regulated by transcriptional activation and repression Histone H3-Thr 11 phosphorylation - new transcriptional marker - rapidly decreases after DNA damage - phosphorylated by Chk1 Mammalian Ser/Thr-specific protein phosphatases - 8 prototypes : PP1, PP2A, PP2B, PP2C, PP4, PP5, PP6 and PP7 ⇒Histone phosphatase

3. Fig 1 The phosphatase responsible for H3-pThr11 dephosphoryltaion is sensitive to okadaic acid. RESULT Dephosphorylation of pThr 11 is okadaic acid sensitive OA (okadaic acid) : PP1, 2A and 2B inhibitor MI (mitotic index) : the percentage of cells with condensed chromosomes RO3306 : inhibits Cdk1 and prevents mitotic entry Fostriecin : a specific inhibitor of PP2A and PP4, 6 isotypes

4. RESULT PP1γ is a H3-pThr 11 phosphatase Fig 2 Protein phosphatase 1γ is responsible for DNA-damage-induced dephosphorylation of H3-pThr11.

5. RESULT Regulation of PP1γ activity in response to DNA damage Fig 3 Protein phosphatase 1γ-pThr311 on chromatin is decreased after DNA damage.

6. RESULT ATR–Chk1 axis regulates PP1γ activity Fig 4 Cdk-dependent phosphorylation of protein phosphatase 1γ at Thr311 is involved in DNA-damage-induced transcriptional repression. Caffeine : an inhibitor of ATR and ATM Pur A (purvalanol A) : a specific Cdk inhibitor Cdk1AF : constitutively active Cdk1 mutant

7. DISCUSSION Fig 5 Schematic model of DNA-damage-induced dephosphorylation of H3-pThr11 on the promoters of cell cycle regulatory genes. DNA damage ↓ ATR activation ↓ Chk1 phosphorylation ↓ Cdk activity suppression ↓ Reduction in PP1γ-Thr311 phosphorylation ↓ PP1γ activation ↓ H3-pThr11 dephosphorylation ↓ Transcription repression ↓ Cell cycle arrested