In most eukaryotes, telomeres are the natural chromosome ends that are essentially involved in stable maintenance of chromosomes by facilitating chromosomes end replication and preventing them from degradation or end fusion. Hyper-activated telomere-extension activity, including both upregulation of positive regulators and downregulation of the negative regulators, potentially leads to uncontrolled proliferation that is required in cancer cells; inversely, degenerative disorders and premature aging are often accompanied with impaired or lost telomere extension. Similar to human, in fission yeast Schizosaccharomyces pombe, telomere structure is achieved by association of shelterin with both double-stranded and single-stranded telomeric DNA forming a nucleoprotein complex. Despite vital roles of shelterin components and telomere structure in telomere length regulation, structural characterizations of assembly and disassembly of shelterin are still limited, which in turn impedes our understanding of the mechanism in which the telomere length homeostasis is maintained.

To this end, we solved the crystal structure of the core part of fission yeast shelterin that reveals essential atomic-level information for cooperative assembly of shelterin bridge. Intriguingly, this cooperativity is also utilized in the human shelterin counterparts, suggesting a highly conserved driving force for shelterin formation. Moreover, this hierarchical formation not only exists within shelterin components, but also occurs in telomerase recruitment to telomere via a bivalent binding event between shelterin and telomerase.

Telomeres, the natural ends of linear eukaryotic chromosomes, are essential for cell viability and genome integrity. Telomeres are specialized DNA-protein complexes and function as protective caps to prevent chromosome ends from undergoing deleterious degradation and fusion events. Fission yeast shelterin, consisted of six-protein complex, exclusively associates with the telomeres. The shelterin is composed of telomeric sequence-specific double-stranded and single-stranded DNA binding partners, Rap1, Poz1, and Tpz1, which form a bridge connecting Taz1 and Pot1. Despite vital roles of the shelterin components in telomere length regulation, little is known about how they interact with each other and how these interactions contribute to telomerase regulation and telomere end protection. We demonstrate that Tpz1-mediated complete linkage within the shelterin, bridging telomeric dsDNA to ssDNA, controls the telomerase-nonextendible state. Moreover, Tpz1 participates in the activation of telomeres to the telomerase-extendible state via its interaction with Ccq1.

Although Rap1’s role in telomerase regulation is rather well-understood, the functional roles of its evolutionarily conserved BRCT domain remain largely unknown. Here we find a novel interaction between Rap1BRCT domain and γH2A. This interaction in subtelomeric chromatin prevents activation of DNA damage response (DDR) by competitively inhibiting the interaction between γH2A and checkpoint mediator Crb253BP1. As telomeres undergo progressive shortening, gradual loss of Rap1-γH2A interaction allows Crb253BP1 to access subtelomeric γH2A, activate DDR and followed by trigger cellular senescence. Therefore, we propose that Rap1 functions as a telomere length-based licensing factor for cellular senescence.