Phosphorylated CtIP Functions as a Co-factor of the MRE11-RAD50-NBS1 Endonuclease in DNA End Resection
- Mol Cell. 2016 Dec 1;64(5):940-950. doi: 10.1016/j.molcel.2016.10.017.
- 1. Institute of Molecular Cancer Research, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland.
- 2. Institute of Molecular Cancer Research, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland. Electronic address: [email protected].
To repair a DNA double-strand break (DSB) by homologous recombination (HR), the 5'-terminated strand of the DSB must be resected. The human MRE11-RAD50-NBS1 (MRN) and CtIP proteins were implicated in the initiation of DNA end resection, but the underlying mechanism remained undefined. Here, we show that CtIP is a co-factor of the MRE11 Endonuclease activity within the MRN complex. This function is absolutely dependent on CtIP phosphorylation that includes the key cyclin-dependent kinase target motif at Thr-847. Unlike in yeast, where the Xrs2/NBS1 subunit is dispensable in vitro, NBS1 is absolutely required in the human system. The MRE11 Endonuclease in conjunction with RAD50, NBS1, and phosphorylated CtIP preferentially cleaves 5'-terminated DNA strands near DSBs. Our results define the initial step of HR that is particularly relevant for the processing of DSBs bearing protein blocks.