Cdc13 delivers separate complexes to the telomere for end protection and replication

In Saccharomyces cerevisiae, the telomere binding protein Cdc13 mediates telomere replication by recruiting telomerase, and also performs an essential function in chromosome end protection. We show here that delivery of the Stn1 protein to the telomere, by fusing the DNA binding domain of Cdc13 (DBD(CDC13)) to Stn1, is sufficient to rescue the lethality of a cdc13 null strain and, hence, provide end protection. Telomere replication is still defective in this strain, but can be restored by delivering telomerase to the telomere as a DBD(CDC13)-telomerase fusion. These results establish Stn1 as the primary effector of chromosome end protection, whereas the principal function of Cdc13 is to provide a loading platform to recruit complexes that provide end protection and telomere replication.