Inhibition of MRN activity by a telomere protein motif
The MRN complex (MRX in Saccharomyces cerevisiae) initiates the repair of DNA double-stranded breaks (DSBs) and activates the Tel1/ATM kinase, which orchestrates the DNA damage response (DDR). Telomeres prevent DDR activation at chromosome ends, partly by keeping MRN-ATM in check. We show that the multiple activities of the MRX complex are disabled by telomeric protein Rif2 through the action of a short motif (MIN, MRN/X-inhibitory motif) at the N-terminal end of the protein. MIN executes telomeric suppression of Tel1, DDR and and non-homologous end joining (NHEJ) via direct biding to the N-terminal region of Rad50. A combination of biochemical and genetic data suggests that Rif2 promotes a transition within the MRX complex that is not conductive for endonuclease activity, DNA-end tethering or Tel1 kinase activation. We suggests that the MIN motif operates in the RIF2 paralog ORC4 (Origin Recognition Complex 4) in K. lactis and in telomeric protein Taz1 in Schizoccharomyces pombe, which is not evolutionarily related to Orc4/Rif2. These results highlight a potential Achilles heel in Rad50, the regulatory subunit of MRN, which we suggest has been targeted by different telomeric factors in multiple fungal lineages, raising the possibility that analogous approaches might be deployed in other Eukaryotes as well.