Cell-cycle-regulated association of RAD50/MRE11/NBS1 with TRF2 and human telomeres

Telomeres allow cells to distinguish natural chromosome ends from damaged D NA and protect the ends from degradation and fusion. In human cells, telome re protection depends on the TTAGGG repeat binding factor, TRF2 (refs 1-4), which has been proposed to remodel telomeres into large duplex loops(5) (t -loops). Here we show by nanoelectrospray tandem mass spectrometry that RAD 50 protein is present in TRF2 immunocomplexes. Protein blotting showed that a small fraction of RAD50, MRE11 and the third component of the MRE11 doub le-strand break (DSB) repair complex, the Nijmegen breakage syndrome protei n (NBS1), is associated with TRF2. Indirect immunofluorescence demonstrated the presence of RAD50 and MRE11 at interphase telomeres. NBS1 was associat ed with TRF2 and telomeres in S phase, but not in G1 or G2. Although the MR E11 complex accumulated in irradiation-induced foci (IRIFs) in response to gamma-irradiation, TRF2 did not relocate to IRIFs and irradiation did not a ffect the association of TRF2 with the MRE11 complex, arguing against a rol e for TRF2 in TSB repair. Instead, we propose that the MRE11 complex functi ons at telomeres. possibly by modulating t-loop formation.