Telomeric DNA ends are essential for the localization of Ku at telomeres in fission yeast

The Ku70-Ku80 heterodimer is a conserved protein complex essential for the non-homologous end-joining pathway. Ku proteins are also involved in telomere maintenance, although their precise roles remain to be elucidated. In fission yeast, pku70(+), the gene encoding the Ku70 homologue, has been reported. Here we report the identification and characterization of pku80(+), the gene encoding Ku80. Both pku70(+) and pku80(+) are essential for efficient non-homologous end-joining. We also found that the pku70 and pku80 mutants are sensitive to methyl methanesulfonate and hydroxyurea, suggesting their roles in the S phase. The pku80 mutant shows telomere shortening and tandem amplification of a subtelomeric sequence but no defects in the telomere position effect, as was previously reported for the pku70 mutant. By using the chromatin immunoprecipitation assay, we demonstrated that Pku70 and Pku80 physically interact with telomeric repeats and subtelomeric sequences. Interestingly, this telomere association of Pku proteins is independent of Taz1, a telomeric DNA-binding protein. We also showed that the Pku proteins do not associate with ectopically integrated telomeric repeats in the internal region of circular chromosomes. These results indicate that the physical end of DNA is necessary for the localization of Pku80 at telomeres.