Ctp1-dependent clipping and resection of DNA double-strand breaks by Mre11 endonuclease complex are not genetically separable.

Homologous recombination (HR) repair of programmed meiotic double-strand breaks (DSBs) requires endonucleolytic clipping of Rec12(Spo11)-oligonucleotides from 5' DNA ends followed by resection to generate invasive 3' single-stranded DNA tails. The Mre11-Rad50-Nbs1 (MRN) endonuclease and Ctp1 (CtIP and Sae2 ortholog) are required for both activities in fission yeast but whether they are genetically separable is controversial. Here, we investigate the mitotic DSB repair properties of Ctp1 C-terminal domain (ctp1-CD) mutants that were reported to be specifically clipping deficient. These mutants are sensitive to many clastogens, including those that create DSBs devoid of covalently bound proteins. These sensitivities are suppressed by genetically eliminating Ku nonhomologous end-joining (NHEJ) protein, indicating that Ctp1-dependent clipping by MRN is required for Ku removal from DNA ends. However, this rescue requires Exo1 resection activity, implying that Ctp1-dependent resection by MRN is defective in ctp1-CD mutants. The ctp1-CD mutants tolerate one but not multiple broken replication forks, and they are highly reliant on the Chk1-mediated cell cycle checkpoint arrest, indicating that HR repair is inefficient. We conclude that the C-terminal domain of Ctp1 is required for both efficient clipping and resection of DSBs by MRN and these activities are mechanistically similar.

Jensen KL ，Russell P 《-》

Release of Ku and MRN from DNA ends by Mre11 nuclease activity and Ctp1 is required for homologous recombination repair of double-strand breaks.

Langerak P ，Mejia-Ramirez E ，Limbo O ，Russell P ... -  《PLoS Genetics》

Two separable functions of Ctp1 in the early steps of meiotic DNA double-strand break repair.

Meiotic programmed DNA double-strand break (DSB) repair is essential for crossing-over and viable gamete formation and requires removal of Spo11-oligonucleotide complexes from 5' ends (clipping) and their resection to generate invasive 3'-end single-stranded DNA (resection). Ctp1 (Com1, Sae2, CtIP homolog) acting with the Mre11-Rad50-Nbs1 (MRN) complex is required in both steps. We isolated multiple S. pombe ctp1 mutants deficient in clipping but proficient in resection during meiosis. Remarkably, all of the mutations clustered in or near the conserved CxxC or RHR motif in the C-terminal portion. The mutants tested, like ctp1Δ, were clipping-deficient by both genetic and physical assays-. But, unlike ctp1Δ, these mutants were recombination-proficient for Rec12 (Spo11 homolog)-independent break-repair and resection-proficient by physical assay. We conclude that the intracellular Ctp1 C-terminal portion is essential for clipping, while the N-terminal portion is sufficient for DSB end-resection. This conclusion agrees with purified human CtIP resection and endonuclease activities being independent. Our mutants provide intracellular evidence for separable functions of Ctp1. Some mutations truncate Ctp1 in the same region as one of the CtIP mutations linked to the Seckel and Jawad severe developmental syndromes, suggesting that these syndromes are caused by a lack of clipping at DSB ends that require repair.

Ma L ，Milman N ，Nambiar M ，Smith GR ... -  《-》

Ctp1 and Exonuclease 1, alternative nucleases regulated by the MRN complex, are required for efficient meiotic recombination.

Farah JA ，Cromie GA ，Smith GR 《-》

A conserved Ctp1/CtIP C-terminal peptide stimulates Mre11 endonuclease activity.

Zdravković A ，Daley JM ，Dutta A ，Niwa T ，Murayama Y ，Kanamaru S ，Ito K ，Maki T ，Argunhan B ，Takahashi M ，Tsubouchi H ，Sung P ，Iwasaki H ... -  《-》