Single-stranded DNA-dependent ATP-dependent helicase. Involved in non-homologous end joining (NHEJ) DNA double strand break repair. DNA-binding is sequence-independent but has a high affinity to nicks in double-stranded DNA and to the ends of duplex DNA. Binds to naturally occurring chromosomal ends, and therefore provides chromosomal end protection. Required also for telomere recombination to repair telomeric ends in the absence of telomerase. ku70, of the ku70/ku80 heterodimer, binds to the stem loop of tlc1, the RNA component of telomerase. Involved in telomere maintenance. Interacts with telomeric repeats and subtelomeric sequences thereby controlling telomere length and protecting against subtelomeric rearrangement. Maintains telomeric chromatin, which is involved in silencing the expression of genes located at the telomere. Required for mating-type switching. ATP + H2O = ADP + H(+) + phosphate Heterodimer of Ku70 and Ku80. Belongs to the ku80 family.