Single-stranded DNA-dependent ATP-dependent helicase. Involved in DNA non-homologous end joining (NHEJ) required for double-strand break repair. When associated with KU70, binds to double-stranded telomeric and non-telomeric DNA sequences, but not to single-stranded DNA. Plays a role in maintaining telomere length. Acts as a negative regulator of telomerase (By similarity). ATP + H2O = ADP + H(+) + phosphate Interacts with KU70. Belongs to the ku80 family.