Science for Health
15 February 2012
A recent paper by NIMR scientists and collaborators in Oxford has revealed new molecular links between DNA double-stranded break repair and the cell-cycle machinery through interplay between the Rad51 recombinase, the Mre11/Rad50/Nbs1 complex and two important but poorly understood protein kinases. The research is published in Molecular Cell.
Homologous recombination plays an important role in the maintenance of genome integrity, enabling the precise repair of DNA double-strand breaks that are caused during DNA replication and by exogenous stresses such as ionizing radiation. Homologous recombination processes are controlled by the central cell-cycle regulators, cyclin-dependent kinases (CDKs), but a complete picture of cell cycle-dependent homologous recombination regulation remains elusive. In addition to CDKs, Polo-like kinase 1 (Plk1) is recognised as an essential cell-cycle regulator. It is vital for cell proliferation and is frequently upregulated in cancer cells. Plk1 is structurally characterized by the polo-box domain at the carboxyl terminus, which mediates its binding to phosphorylated proteins at specific intracellular locations. Subsequently, Plk1 phosphorylates binding partners and/or other local proteins and, hence, coordinates phosphorylation in a spatiotemporal manner. It is unknown, however, whether and how Plk1 that binds to DNA damage responsive proteins may regulate DNA repair.
Steve Smerdon (pictured), from NIMR 's Division of Molecular Structure, in collaboration with Fumiko Esashi from Oxford, has shown that Plk1 phosphorylates Rad51 recombinase during the cell cycle and in response to DNA damage. This initial phosphorylation licenses subsequent Rad51 phosphorylation by casein kinase 2 at an adjacent site, which in turn triggers direct binding to the Nijmegen breakage syndrome gene product, Nbs1. This mechanism facilitates Rad51 recruitment to damage sites, thus enhancing cellular resistance to genotoxic stresses. These results uncover a role for Plk1 in linking DNA damage recognition with homologous recombination repair and suggest a molecular mechanism for cancer development associated with elevated activity of Plk1. Collectively, these data support a model for Rad51 recruitment mediated through sequential phosphorylation by Plk1 and CK2. This mechanism helps increase the Rad51 concentration at the site of DNA damage and facilitates homologous recombination.
Our findings represent a significant step toward a comprehensive understanding of homologous recombination regulation by cell-cycle regulators. The interplay between protein kinases in these processes is clearly important but the molecular mechanisms underpinning their biological readout are often unclear. Casein kinase 2 is now acknowledged as a major player in the DNA-damage response but how it is regulated following damage is largely unknown. This study shows how prior phosphorylation of a poor CK2 site by a known cell-cycle kinase, Plk1 primes its activity, generating a binding site for the Nbs1 component of the Mre11/Rad50/Nbs1 complex, itself a crucial component of the repair apparatus. Perhaps most important is the implication that inhibitors of Plk1, already a well validated anti-cancer target, may act synergistically with, for example, poly-ADP ribose polymerase (PARP) inhibitors to provide increased efficacy against breast/ovarian and potentially other cancers.
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A model for Rad51 recruitment mediated through sequential phosphorylation by Plk1 and casein kinase 2 (CK2).
Plk1 and CK2 act in concert to regulate Rad51during DNA double strand break repair
Keiko Yata, Janette Lloyd, Sarah Maslen, Jean-Yves Bleuyard, Mark Skehel, Stephen J. Smerdon, and Fumiko EsashiMolecular Cell 45:371–383. Publisher abstract
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