Control of the activity of the MRN complex at human telomeres

Telomeres protect chromosome ends from unplanned DNA repair by assembling a protein complex (shelterin in humans) onto short DNA repeats which are synthesised by the telomerase enzyme. Among the functions of shelterin is the regulation of telomerase and the inhibition of the DNA damage response (DDR) at chromosome ends, which resemble DNA double-stranded breaks (DSBs). The MRN complex (RAD50/MRE11/NBS1) initiates repair of DSBs and activates the ATM kinase in the DDR. ATM is also implicated in telomerase activation. A key role of the telomeric complex is to regulate the activity of MRN. Previous work in the laboratory indicated that telomeric protein Rif2 in budding yeast Saccharomyces cerevisiae inhibits MRN (MRX in yeast) via a motif called MIN (MRN/X-inhibitory motif) which I found arose by convergent evolution also in the Taz1 telomeric protein of fission yeast Schizosaccharomyces pombe. I showed that the MIN motifs of Rif2 and Taz1 bind to a region in the globular domain of RAD50. Further, I showed that human shelterin protein TRF2 binds to the same region of RAD50. My results reveal that a motif in TRF2 called iDDR binds directly to RAD50 in a manner similar to the MIN motif. The MRE11 nuclease is activated by the CtIP protein at DSBs: I showed that the C-terminal region of CtIP binds to RAD50 in both human and budding yeast cells in a manner predicted to be incompatible with iDDR/MIN binding, suggesting that CtIP and the iDDR compete for binding to RAD50. Consistent with this, I showed reduction of resistance to genotoxic agents in cells over-expressing TRF2iDDR. These findings highlight the essential role of telomeric proteins in directly inhibiting the MRN complex and extend our understanding of how TRF2 regulates the DDR at chromosome ends in human cells.