CtIP tetramer assembly is required for DNA-end resection and repair

Mammalian CtIP protein plays major roles in DNA double-strand break (DSB) repair. While it is well-established that CtIP promotes DNA-end resection in preparation for homology-dependent DSB repair, the molecular basis for this function remains unknown. Here we show by biophysical and X-ray crystallographic analyses that the N-terminal domain of human CtIP exists as a stable homotetramer. Tetramerization results from interlocking interactions between the N-terminal extensions of CtIP’s coiled-coil region, leading to a ‘dimer-of-dimers’ architecture. Through interrogation of the CtIP structure, we identify a point mutation that abolishes tetramerization of the N-terminal domain while preserving dimerization in vitro. Importantly, we establish that this mutation abrogates CtIP oligomer assembly in cells, leading to strong defects in DNA-end resection and gene conversion. These findings indicate that the CtIP tetramer architecture described here is essential for effective DSB repair by homologous recombination.