Human TopBP1 ensures genome integrity during normal S phase

Promyelocytic leukemia nuclear bodies are predetermined processing sites for damaged DNA

The promyelocytic leukemia protein (PML) participates in several cellular functions, including transcriptional regulation, apoptosis and maintenance of genomic stability. A key feature of this protein is its ability to induce the assembly of nuclear compartments termed PML-nuclear bodies (PML-NBs). Here we show that these nuclear structures recruit single-stranded DNA (ssDNA) molecules in response to exogenous DNA damage. ssDNA was readily detected in PML-NBs within 1 hour following exposure of cells to UV light. Confocal real-time imaging of cells expressing YFP-tagged PML did not reveal de novo formation of new PML-NBs following UV-irradiation, which shows that ssDNA focus formation occurred within pre-existing PML-NBs. Moreover, siRNA-mediated depletion of PML prevented ssDNA focus formation and sensitized cells to UV-induced apoptosis. PML-dependent ssDNA focus formation was found to be particularly efficient during S-phase of the cell cycle, and PML-depleted cells became retarded in S-phase upon growth in the presence of etoposide. In addition, we found that caffeine and the poly(ADP-ribose) polymerase (PARP) inhibitor NU1027 enhanced UV-induced recruitment of ssDNA to PML-NBs. Together, our results show that PML-NBs have the capacity to accommodate DNA metabolic activities that are associated with processing of damaged DNA.

Dpb11, the budding yeast homolog of TopBP1, functions with the checkpoint clamp in recombination repair

Dpb11 is required for the loading of DNA polymerases α and ɛ on to DNA in chromosomal DNA replication and interacts with the DNA damage checkpoint protein Ddc1 in Saccharomyces cerevisiae. The interaction between the homologs of Dpb11 and Ddc1 in human cells and fission yeast is thought to reflect their involvement in the checkpoint response. Here we show that dpb11-1 cells, carrying a mutated Dpb11 that cannot interact with Ddc1, are defective in the repair of methyl methanesulfonate (MMS)-induced DNA damage but not in the DNA damage checkpoint at the permissive temperature. Epistatic analyses suggested that Dpb11 is involved in the Rad51/Rad52-dependent recombination pathway. Ddc1 as well as Dpb11 were required for homologous recombination induced by MMS. Moreover, we found the in vivo association of Dpb11 and Ddc1 with not only the HO-induced double-strand break (DSB) site at MAT locus but also the donor sequence HML during homologous recombination between MAT and HML. Rad51 was required for their association with the HML donor locus, but not with DSB site at the MAT locus. In addition, the association of Dpb11 with the MAT and HML locus after induction of HO-induced DSB was dependent on Ddc1. These results indicate that, besides the involvement in the replication and checkpoint, Dpb11 functions with Ddc1 in the recombination repair process itself.

Chk1 is required to maintain claspin stability

Claspin is a Chk1-interacting protein that participates in the DNA replication checkpoint. Expression of Claspin fluctuates in a cell cycle-dependent manner, but the mechanisms involved in the regulation of Claspin protein levels have not been explored. In this study, we show that Claspin expression is downregulated by the proteasome-mediated degradation pathway and that Chk1 is required to maintain Claspin stability. Downregulation of Chk1 expression by siRNA or inhibition of Chk1 activity by UCN01 decreases Claspin levels in cells. Conversely, overexpression of Chk1 increases Claspin levels. These data indicate a role of Chk1 in regulating Claspin stability in the cell. Since Claspin has also been shown to participate in Chk1 activation following DNA damage, we further explored the exact role of Claspin during Chk1 activation following replication stress. We observed that while Rad17 is required for early Chk1 activation after hydroxyurea treatment, Claspin is only required to sustain Chk1 activation. Based on these findings, we propose that Claspin functions at late stages of Chk1 activation following DNA damage. Once Chk1 is activated, it stabilizes Claspin, which in turn helps to maintain Chk1 activation during replication stress. In summary, these data indicate that the interaction between Claspin and Chk1 is complex. These proteins regulate each other and thus ensure the proper cell cycle progression and replication checkpoint control.