ATM- and ATR-Mediated Phosphorylation of XRCC3 Regulates DNA Double-Strand Break-Induced Checkpoint Activation and Repair

Abstract

The RAD51 paralogs XRCC3 and RAD51C have been implicated in homologous recombination (HR) and DNA damage responses. However, the molecular mechanism(s) by which these paralogs regulate HR and DNA damage signaling remains obscure. Here, we show that an SQ motif serine 225 in XRCC3 is phosphorylated by ATR kinase in an ATM signaling pathway. We find that RAD51C but not XRCC2 is essential for XRCC3 phosphorylation, and this modification follows end resection and is specific to S and G₂ phases. XRCC3 phosphorylation is required for chromatin loading of RAD51 and HR-mediated repair of double-strand breaks (DSBs). Notably, in response to DSBs, XRCC3 participates in the intra-S-phase checkpoint following its phosphorylation and in the G₂/M checkpoint independently of its phosphorylation. Strikingly, we find that XRCC3 distinctly regulates recovery of stalled and collapsed replication forks such that phosphorylation is required for the HR-mediated recovery of collapsed replication forks but is dispensable for the restart of stalled replication forks. Together, these findings suggest that XRCC3 is a new player in the ATM/ATR-induced DNA damage responses to control checkpoint and HR-mediated repair.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://dx.doi.org/10.1128/MCB.01521-12.

ACKNOWLEDGMENTS

We thank Yosef Shiloh, Sandeep Burma, and K. Muniyappa for their critical comments on the manuscript. We thank Michael Kastan, Penny Jeggo, Yosef Shiloh, Titia deLange, Karlene Cimprich, K. Muniyappa, Anjali Karande, and Sathees Raghavan for providing various constructs and reagents. We also thank Meenakshi Sen and Deepti Bapat of the IISc confocal microscopy facility and Omana Joy, Pooja Pai, and Kavya Ananthswamy from the IISc FACS facility and the IISc animal facility for their help.

Funding support from the Department of Science and Technology and the Council of Scientific and Industrial Research (CSIR) is greatly acknowledged. K.S. is supported by a fellowship from CSIR and a Bristol-Myers Squibb fellowship from the United Kingdom.

There are no conflicts of interest to report.