Telomere capping in non-dividing yeast cells requires Yku and Rap1.

The assembly of a protective cap onto the telomeres of eukaryotic chromosomes suppresses genomic instability through inhibition of DNA repair activities that normally process accidental DNA breaks. We show here that the essential Cdc13-Stn1-Ten1 complex is entirely dispensable for telomere protection in non-dividing cells. However, Yku and Rap1 become crucially important for this function in these cells. After inactivation of Yku70 in G1-arrested cells, moderate but significant telomere degradation occurs. As the activity of cyclin-dependent kinases (CDK) promotes degradation, these results suggest that Yku stabilizes G1 telomeres by blocking the access of CDK1-independent nucleases to telomeres. The results indeed show that both Exo1 and the Mre11/Rad50/Xrs2 complex are required for telomeric resection after Yku loss in non-dividing cells. Unexpectedly, both asynchronously growing and quiescent G0 cells lacking Rap1 display readily detectable telomere degradation, suggesting an earlier unanticipated function for this protein in suppression of nuclease activities at telomeres. Together, our results show a high flexibility of the telomeric cap and suggest that distinct configurations may provide for efficient capping in dividing versus non-dividing cells.