The role of Protein Phosphatase One beta (PP1-β) in DNA damage response

5685

Protein Phosphatase One (PP1) is one of the major protein Ser/Thr phosphatases that regulate a variety of key steps in metabolism, replication, transcription, and the cell cycle. Multiple genes encode PP1 isoforms in most eukaryotes and there are at least four mammalian PP1 gene products: PP1α, PP1β, PP1γ1 and PP1 γ2. These isoforms possess distinct tissue distributions and subcellular localizations. However, due to their 90% amino acid sequence identity in the catalytic subunits and their broad and similar substrate specificities in vitro, it is widely believed that the regulatory subunits with which the catalytic units of PP1 interact control the specificity and enormous diversity of PP1 function. Using pharmaceutical inhibitors, we have found that inhibition of PP1 induces global Histone H3 phosphorylation. Since Histone H3 phosphorylation is required for mitotic chromosome condensation and is downregulated after DNA damage, we further studied the role of PP1 in DNA damage response. Here we show that inhibition of PP1 by calyculin-A and over-expression of a phosphatase-inactive form of PP1 eliminate the activation of the ATM kinase after ionizing radiation (IR), suggesting that PP1 phosphatase activity is required for the optimal DNA damage response. Using immunofluoresence microscopy, we demonstrate that PP1-β, predominantly localized in the chromosome at the mitotic stage, is upregulated in response to IR. Furthermore, we find that PP1-β knock-down cells have reduced ATM activation and ATM-mediated cell cycle checkpoints in response to IR. Taken together, our data suggest that the beta isoform of PP1 (PP1-β) is directly involved in DNA damage response and cell cycle checkpoint regulation.