Rif1 acts through Protein Phosphatase 1 and independent of replication timing to suppresses telomere extension in budding yeast

The Rif1 protein negatively regulates telomeric TG repeat length in the budding yeast S. cerevisiae, but how it prevents telomere over-extension is unknown. Rif1 was recently shown to control DNA replication by acting as a Protein Phosphatase 1 (PP1)-targeting subunit. Therefore we investigated whether Rif1 controls telomere length by targeting PP1 activity. We find that a Rif1 mutant that cannot interact with PP1 causes a long-telomere phenotype, similar to that of rif1∆ cells. Compromised PP1 function also causes telomere extension. Tethering PP1 at a specific telomere partially substitutes for Rif1 in limiting TG repeat length, confirming the importance of PP1 in telomere length control. Ablating Rif1-PP1 interaction leads to precocious activation of telomere-proximal replication origins and aberrantly early telomere replication. However, we find that Rif1 still limits telomere length even if nearby replication origins are deleted, indicating that effects of Rif1 on telomere length are not mediated through replication timing. Instead we find that, even at a telomere created after DNA synthesis during a mitotic block, Rif1-PP1 interaction is required to suppress telomere lengthening and prevent inappropriate recruitment of Tel1 kinase. Overall, our results show that Rif1 controls telomere length by recruiting PP1 to directly suppress telomerase-mediated TG repeat lengthening.