Telomeres were originally defined as chromosome caps that prevent the natural ends of linear chromosomes from undergoing deleterious degradation and fusion events. POT1 (protection of telomeres) protein binds the single-stranded G-rich DNA overhangs at human chromosome ends and suppresses unwanted DNA repair activities. TPP1 is a previously identified binding partner of POT1 that has been proposed to form part of a six-protein shelterin complex at telomeres. Here, the crystal structure of a domain of human TPP1 reveals an oligonucleotide/oligosaccharide-binding fold that is structurally similar to the β-subunit of the telomere end-binding protein of a ciliated protozoan, suggesting that TPP1 is the missing β-subunit of human POT1 protein. Telomeric DNA end-binding proteins have generally been found to inhibit rather than stimulate the action of the chromosome end-replicating enzyme, telomerase. In contrast, we find that TPP1 and POT1 form a complex with telomeric DNA that increases the activity and processivity of the human telomerase core enzyme. We propose that POT1–TPP1 switches from inhibiting telomerase access to the telomere, as a component of shelterin, to serving as a processivity factor for telomerase during telomere extension.
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We thank Y. Chen and K. Wan for help at various stages of the project; N. F. Lue for the His-SUMO protein expression vector; Z. Songyang and T. de Lange for TPP1 cDNA; D. Yoder of beamline 23-ID at APS for assistance with data collection; and J. L. Chen and C. W. Greider for the human TERT and TER plasmids. Work in the laboratory of M.L. is supported by the American Cancer Society and the Sidney Kimmel Foundation. E.R.P., A.J.Z. and T.R.C. are supported by the Howard Hughes Medical Institute. Author Contributions F.W. is responsible for the bulk of the experiments; Y.Y. for structural determination of TPP1-OB; P.B. for crystallization of TPP1-OB; E.R.P. and A.J.Z. for the telomerase activity assays and some of the EMSA experiments; and T.R.C. and M.L. contributed to overall design and interpretation of the studies. The atomic coordinates and structure factors of TPP1-OB have been deposited in the RCSB Protein Data Bank with accession code 2146.
The atomic coordinates and structure factors of TPP1-OB have been deposited in the RCSB Protein Data Bank with accession code 2I46. Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
This file contains the Supplementary Figures 1–8, Supplementary Methods, Supplementary Table and additional references. The Supplementary Methods describes the detailed methods of protein purification, crystallization, structural determination, and snake-venom phosphodiesterase I digestion assay. (PDF 4673 kb)
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Wang, F., Podell, E., Zaug, A. et al. The POT1–TPP1 telomere complex is a telomerase processivity factor. Nature 445, 506–510 (2007). https://doi.org/10.1038/nature05454
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