Ctp1 protein-DNA filaments promote DNA bridging and DNA double-strand break repair.

The Ctp1 protein in Schizosaccharomyces pombe is essential for DNA double-strand break (DSB) repair by homologous recombination. Fission yeast Ctp1 and its budding yeast (Sae2) and human (CtIP) homologs control Mre11-Rad50-Nbs1 nuclease complex activity and harbor DNA-binding and -bridging activities. However, the molecular basis for Ctp1-DNA transactions remains undefined. Here, we report atomic force microscopy (AFM) imaging of S. pombe Ctp1-DNA complexes revealing that Ctp1 polymerizes ondsDNAmolecules and forms synaptic filaments that bridge two dsDNA strands.Weobserved that Ctp1 DNA filaments are typified by an average filament length of ~180 bp of dsDNA and a Ctp1 tetramer footprint of ~15 bp. Biochemical results characterizing Ctp1 variants with impaired DNA-binding or -bridging properties were consistent with Ctp1-mediatedDNAbridging requiring the intact and correctly folded Ctp1 tetramer. Furthermore, mutations altering Ctp1 oligomerization and DNA bridging in vitro conferred cell sensitivity to DSB-producing agents. Together, these results support an important role for Ctp1-regulatedDNAstrand coordination required for DNA DSB repair in S. pombe. [ABSTRACT FROM AUTHOR]