New Complex Unlocks How Telomerase Is Activated at Chromosome Ends

Telomeres cap the ends of linear chromosomes, and their maintenance by telomerase is essential for genome stability. While telomerase recruitment to telomeres is increasingly well understood, the molecular mechanisms that convert a recruited telomerase complex into one that is productively elongating DNA have remained elusive. This study addresses that gap by identifying a previously uncharacterized ternary complex—RPA–TERT–Tpz1—that is required for telomerase activation in fission yeast Schizosaccharomyces pombe.

Using a genetic screen as the entry point, the researchers found that RPA (Replication Protein A), a conserved heterotrimeric single-stranded DNA-binding complex best known for its roles in DNA replication and repair, also plays a direct role at telomeres. Through a combination of AlphaFold3-guided structural modeling and systematic mutagenesis, they mapped four key interfaces: Ssb1(RPA1)–Trt1(TERT), Ssb2(RPA2)–Trt1(TERT), Ssb2(RPA2)–Tpz1(TPP1), and a TEL-patch-mediated Trt1–Tpz1 interaction. A surprising finding was that the Tpz1 residue R81, previously annotated as part of the TEL patch that contacts TERT, actually contacts Ssb2(RPA2) in the context of this ternary complex, requiring a reassignment of its functional role.

Epistasis and suppressor analyses demonstrated that the newly identified RPA–TERT and RPA–Tpz1 interfaces work collaboratively with the established Trt1–Tpz1 interface. Mutations disrupting these interfaces did not prevent telomerase from being recruited to telomeres, but did prevent productive telomere extension—placing RPA's function downstream of recruitment and upstream of elongation. Strikingly, loss of RPA function in this context also led to a shift toward telomere maintenance via recombination-based pathways (similar to ALT in human cancers), indicating that the RPA–TERT–Tpz1 complex actively suppresses inappropriate recombination at telomeres.

The study further performed comparative AlphaFold3 modeling in budding yeast (where RPA interacts with Est3, the TPP1 ortholog) and humans (where RPA interacts with TERT directly), suggesting that a conserved mechanistic principle underlies telomerase activation across eukaryotes, even though the specific interaction partners have diversified over evolution.

These findings reframe RPA as not merely a DNA damage responder at telomeres, but as an integral component of the active telomerase machinery. The work has implications for understanding how telomere length homeostasis is maintained in normal cells and how its dysregulation contributes to aging and cancer.