Broken DNA ends in NHEJ are recognized by
Ku heterodimer composed of 70 kDa and 80 kDa subunits forms preformed ring with central aperture approximately 30 angstroms accommodating double-stranded DNA ends without sequence specificity. Abundant nuclear protein diffuses, threading onto broken terminus within seconds, protecting end from nucleolytic degradation and acting as scaffold for non-homologous end joining machinery. Ku recruits DNA-dependent protein kinase catalytic subunit, Artemis nuclease, polymerases mu and lambda, and XRCC4-Ligase IV complex. By synapsing two Ku-bound ends, it holds broken chromosome together. Rapid binding also antagonizes resection, steering repair away from homologous recombination.
Ref: Alberts Molecular Biology of Cell Chapter 5: Ku70/Ku80 end recognition in NHEJ; Nature Struct Mol Biol 2017 Ku-DNA complex