Structure of the fission yeast S. pombe telomeric Tpz1-Poz1-Rap1 complex

Abstract

Telomeric shelterin complex caps chromosome ends and plays a crucial role in telomere maintenance and protection. In the fission yeast Schizosaccharomyces pombe, shelterin is composed of telomeric single- and double-stranded DNA-binding protein subcomplexes Pot1-Tpz1 and Taz1-Rap1, which are bridged by their interacting protein Poz1. However, the structure of Poz1 and how Poz1 functions as an interaction hub in the shelterin complex remain unclear. Here we report the crystal structure of Poz1 in complex with Poz1-binding motifs of Tpz1 and Rap1. The crystal structure shows that Poz1 employs two different binding surfaces to interact with Tpz1 and Rap1. Unexpectedly, the structure also reveals that Poz1 adopts a dimeric conformation. Mutational analyses suggest that proper interactions between Tpz1, Poz1, and Rap1 in the shelterin core complex are required for telomere length homeostasis and heterochromatin structure maintenance at telomeres. Structural resemblance between Poz1 and the TRFH domains of other shelterin proteins in fission yeast and humans suggests a model for the evolution of shelterin proteins.

Casein kinase 2 regulates telomere protein complex formation through Rap1 phosphorylation

“…Some of the CK2 phosphorylation sites in Rap1 are located within or near the Poz1-binding region of Rap1 (9,27,28) (Figure 3A). We thus examined whether Rap1 phosphorylation affects Rap1–Poz1 interactions in vivo .…”

“…Shelterin is a protein complex that consists of at least six subunits (4) (Figure 6A). A number of direct interactions among these six subunits have been reported (4,9,27,28,30–32), and the Rap1–Poz1 interaction is critical for the shelterin formation (27,28). We thus examined whether the rap1-7A mutation also influences the telomeric localization of shelterin components other than Poz1.…”

“…Ser496 resides within the Bqt4-interacting interface (490–513 a.a.) (29). Although Ser496 lies just outside of the Poz1-interacting interface (467–491 a.a.) (28), the longer region of Rap1 (467–496 a.a.) was required for production of high-quality crystals of the Rap1–Poz1–Tpz1 complex (27), suggesting that phosphorylation of Ser496 can influence the Rap1-Poz1 interaction. The phosphorylation of Ser496 might facilitate Rap1 interactions with Bqt4 and Poz1 through its negative charge, or it might change the three-dimensional structure of Rap1 to expose interaction domains for Bqt4 and Poz1.…”

“…The Rap1-Bqt4 and Rap1-Poz1 interactions take place at the same time during interphase. It has been shown that Taz1 forms a homodimer that binds to ds-telomere DNA (36), and that Tpz1, Poz1 and Rap1 form a ternary complex, which forms a dimerized structure (27,28). Therefore, the most likely model is that two molecules of Rap1 bind a homodimer of Taz1 to form a dimer of the ternary Tpz1–Poz1–Rap1 complex (Figure 9A).…”

TRFH domain: at the root of telomere protein evolution?

“…The results from Xue et al [9] reveal that S. pombe Poz1 is a homodimeric protein with separate interfaces for binding Rap1 and Tpz1. The expression of Poz1 mutants carrying point mutations abolishing these interfaces confirms that the shelterin bridge is required for telomerase regulation and proper sub-telomeric heterochromatin formation.…”