Structural Basis for Shelterin Bridge Assembly

Abstract

SUMMARY Telomere elongation through telomerase enables chromosome survival during cellular proliferation. The conserved multifunctional shelterin complex associates with telomeres to coordinate multiple telomere activities, including telomere elongation by telomerase. Similar to the human shelterin, fission yeast shelterin is composed of telomeric sequence-specific double- and single-stranded DNA binding proteins, Taz1 and Pot1, respectively, bridged by Rap1, Poz1, and Tpz1. Here, we report the crystal structure of the fission yeast Tpz1475–508-Poz1-Rap1467–496 complex that provides the structural basis for shelterin bridge assembly. Biochemical analyses reveal that shelterin bridge assembly is a hierarchical process in which Tpz1 binding to Poz1 elicits structural changes in Poz1, allosterically promoting Rap1 binding to Poz1. Perturbation of the cooperative Tpz1-Poz1-Rap1 assembly through mutation of the “conformational trigger” in Poz1 leads to unregulated telomere lengthening. Furthermore, we find that the human shelterin counterparts TPP1-TIN2-TRF2 also assemble hierarchically, indicating cooperativity as a conserved driving force for shelterin assembly.

Telomeres and Telomere Length: A General Overview

“…TIN2 further recruits TPP1 forming a triple complex-TIN2-TPP1-TRF2 [46]. This interaction provides a structural basis for shelterin bridge assembly [47]. Representation of shelterin complex, heterotrimeric complex CST, and telomeric repeat containing RNA (TERRA).…”

RNA-DNA hybrids support recombination-based telomere maintenance in fission yeast

“…Moreover, loss of Taz1 and Rap1 phenocopy one another with respect to increased telomere length and de-repression of TERRA (MILLER et al 2005;GREENWOOD AND COOPER 2012), although the mechanisms at play may be distinct (MILLER et al 2005;DEHE et al 2012). We note, however, that recent evidence supports allosteric regulation of Rap1 binding to Tpz1-Poz1 to regulate telomere lengthening (KIM et al 2017), suggesting that the Rap1-Taz1 interface may also be subject to modulation. Further studies will be required to test if changes in the composition and/or conformation of shelterin are tied to the mechanisms allowing for telomere maintenance in the absence of RNA-DNA hybrids.…”

Telomere Formation Systems in Budding and Fission Yeasts

“…Poz1, the ortholog of human TIN2, links the double-strand DNA binding complex of Taz1-Rap1 and Rif1 to the singlestrand DNA binding complex of Pot1-Tpz1-Ccq1 (Figure 1). The entire complex negatively regulates telomerase as loss of Poz1 or Rap1 results in elongation of the terminal telomere repeat sequence tracts [70,71]. In addition to telomere proteins, S. pombe has also conserved heterochromatin-mediated transcriptional silencing via di-and tri-methylation of lysine 9 of histone H3 (H3K9me2 and H3K9me3), which is absent in S. cerevisiae [72,73].…”

“…The S. cerevisiae telomere formation system proved to be a highly useful tool, but was limited by the biology of this yeast species. By translating this system to S. pombe, some of these issues, such as the presence of H3K9me2 and 3 modifications and a high level of conservation of the telomeric complexes with humans were addressed (Figure 1B and C) [56,70,72,73]. The recent development of the S. pombe telomere formation system thus opens new avenues to study telomeric chromatin regulation and telomere formation.…”