# Cryo-EM structures of NHEJ assemblies with nucleosomes

## TL;DR

This paper reveals how DNA repair proteins interact with DNA breaks in chromatin using cryo-EM structures, offering insights into genome stability and cancer therapy.

## Contribution

The study provides novel cryo-EM structures of NHEJ assemblies bound to nucleosomes, revealing how DNA repair proteins access DNA breaks within chromatin.

## Key findings

- Ku70/80 binds DNA ends and bends them away from the nucleosome core.
- The Ku70 SAP domain makes specific DNA contacts, aiding DSB recognition.
- Non-hydrolysable ATP stabilizes the DNA-PK dimer, promoting synapsis.

## Abstract

DNA double-strand breaks (DSBs) are highly deleterious lesions that can trigger cell death or carcinogenesis if unrepaired or misrepaired. In mammals, most DSBs are repaired by non-homologous end joining (NHEJ), which begins when Ku70/80 binds DNA ends and recruits DNA-PKcs to form the DNA-PK holoenzyme. Although recent cryo-EM studies have resolved several NHEJ assemblies, how these factors access DSBs within nucleosomes remains unclear. Here, we present cryo-EM structures of human Ku70/80 and DNA-PK bound to nucleosomes. Ku70/80 binds the DNA end and bends it away from the nucleosome core, while the Ku70 C-terminal SAP domain makes an additional, specific DNA contact. Our DNA-PK–nucleosome structure further reveals the opening of the Ku80 vWA domain, and we show that non-hydrolysable ATP promotes synapsis by stabilising the Ku80-mediated DNA-PK dimer. These structures reveal a model for DSB recognition on nucleosomal DNA and provide insights relevant to targeting NHEJ in cancer therapy.

DNA double-strand breaks endanger genome stability. Here, the authors present cryo-EM structures showing how Ku70/80 and DNA-PK bind DNA ends on nucleosomes, offering a mechanistic model for break recognition within chromatin.