Ionizing radiation (IR) and other DNA damaging agents can lead to instability of the genetic information and cellular transformation. To lessen the deleterious effects of such stress, eukaryotic cells have evolved various checkpoint mechanisms that halt the progression of the cell cycle at the G1/S and G2/M boundaries, and within S phase. Cells thus have the opportunity to repair the damage or, if the cell judges the damage too severe, to undergo apoptosis. My laboratory has been studying the intra-S phase checkpoint, a poorly understood control mechanism that serves to inhibit DNA replication at the level of origin firing after various types of cell stress.
We have found that nucleolin, a key factor in ribosome biogenesis, also acts to repress origin firing following stress. Stresses such as IR cause a dramatic redistribution of nucleolin from the nucleolus to the nucleoplasm. Nucleolin relocalization requires complex formation with the tumor suppressor p53 protein, and this interaction can also regulate p53 activity. The relocalization of nucleolin then leads to a great increase in complex formation with RPA, the eukaryotic single-stranded DNA-binding protein. Nucleolin significantly inhibits the ability of RPA to support origin firing by sequestering RPA away from chromosomal DNA replication foci. Among other experimental goals, we are defining the signal transduction pathways that regulate the nucleolin-p53 and nucleolin-RPA interactions, and examining the effects of nucleolin on p53 and RPA activity.