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Telomerase recruitment by the telomere end binding protein-β facilitates G-quadruplex DNA unfolding in ciliates

Abstract

The telomeric G-overhangs of the ciliate Stylonychia lemnae fold into a G-quadruplex DNA structure in vivo. Telomeric G-quadruplex formation requires the presence of two telomere end binding proteins, TEBPα and TEBPβ, and is regulated in a cell-cycle dependent manner. Unfolding of this structure in S phase is dependent on the phosphorylation of TEBPβ. Here we show that TEBPβ phosphorylation is necessary but not sufficient for a G-quadruplex unfolding rate compatible with telomere synthesis. The telomerase seems to be actively involved in telomeric G-quadruplex DNA structure unfolding in vivo. Significantly, the telomerase is recruited to telomeres by phosphorylated TEBPβ, and hence telomerase recruitment is cell-cycle regulated through phosphorylation. These observations allow us to propose a model for the regulation of G-quadruplex unfolding and telomere synthesis during the cell cycle.

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Figure 1: Phosphorylation of TEBPβ is not sufficient for resolving telomeric G-quadruplexes fast enough for telomere synthesis.
Figure 2: Telomerase displaces phosphorylated TEBPβ from telomeres during replication.
Figure 3: Telomerase is involved in telomeric G-quadruplex unfolding in vivo.
Figure 4: Phosphorylated TEBPβ recruits the telomerase to the telomeres.
Figure 5: Proposed scheme for the regulation of the telomeric G-quadruplex DNA structure during the cell cycle.

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Acknowledgements

This work was supported by a grant of the Deutsche Forschungsgemeinschaft to H.J.L. and D.R. and a Human Frontiers Science Program grant to D.R. We thank C. Berger-Schaffitzel (Swiss Federal Institute of Technology, Zurich) for providing the antibodies directed against the S. lemnae telomeric G-quadruplexes.

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K.P., S.J., T.S., A.H. and H.J.L. performed experiments; K.P., D.R. and H.J.L. designed the experiments; and K.P., D.R. and H.J.L. wrote the manuscript.

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Correspondence to Daniela Rhodes or Hans Joachim Lipps.

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Paeschke, K., Juranek, S., Simonsson, T. et al. Telomerase recruitment by the telomere end binding protein-β facilitates G-quadruplex DNA unfolding in ciliates. Nat Struct Mol Biol 15, 598–604 (2008). https://doi.org/10.1038/nsmb.1422

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