Ku70/Ku80 threading on DNA ends enables APLF bridging of breaks in single-molecule study
Oxford and Madrid groups showed that APLF bridges DNA ends only when at least two Ku70/Ku80 heterodimers are threaded onto each end, which limits bridging to DNA-bound Ku.
A team from the University of Oxford, the Centro Nacional de Biotecnología (CNB-CSIC) and the Spanish National Cancer Research Center (CNIO) in Madrid has described how the protein APLF brings together the two ends of a DNA double-strand break. The study was published on 3 November 2026 in Nature Communications; the corresponding authors are Fernando Moreno-Herrero and Philipp Kukura 
doi.orgNature Communicationsdoi.orgOpen source ↗. The authors combined mass photometry, atomic force microscopy, and magnetic and optical tweezers doi.orgNature Communicationsdoi.orgOpen source ↗.
The synapsis problem
Repair of double-strand breaks by non-homologous end joining requires synapsis, the bringing together of DNA ends doi.orgNature Communicationsdoi.orgOpen source ↗. APLF is emerging as an important repair factor and can form synaptic bridges in the presence of the Ku70/Ku80 heterodimer doi.orgNature Communicationsdoi.orgOpen source ↗. Until now, only a single one-to-one interaction between Ku70/Ku80 and APLF had been reported, which, according to the authors, is fundamentally insufficient for synapsis doi.orgNature Communicationsdoi.orgOpen source ↗. It also remained unclear how APLF-mediated bridging of Ku70/Ku80 heterodimers is prevented where there are no breaks doi.orgNature Communicationsdoi.orgOpen source ↗.
The single-molecule measurements showed that APLF-mediated synapsis requires threading of at least two Ku70/Ku80 heterodimers per DNA end doi.orgNature Communicationsdoi.orgOpen source ↗. According to the authors, this follows from the need for two poly(ADP-ribose)-binding zinc fingers in APLF to interact simultaneously with threaded, poly(ADP-ribosyl)ated Ku70/Ku80 on a single DNA doi.orgNature Communicationsdoi.orgOpen source ↗.
NIHCOLE and multivalency
The group also examined the long non-coding RNA NIHCOLE. They report evidence that it binds up to four Ku70/Ku80 cooperatively doi.orgNature Communicationsdoi.orgOpen source ↗. The authors describe this as consistent with a mechanism that raises synapsis efficiency by bridging two Ku70/Ku80 heterodimers bound on both sides of the break doi.orgNature Communicationsdoi.orgOpen source ↗.
From these results the authors propose a model for the physiological significance of Ku70/Ku80 threading. In it, multivalency works as a regulatory mechanism that restricts APLF-mediated bridging to DNA-bound Ku70/Ku80 doi.orgNature Communicationsdoi.orgOpen source ↗. The results were obtained with single-molecule techniques; the model is the authors' proposal.
