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Dimeric structure of the Oxytricha nova telomere end-binding protein α-subunit bound to ssDNA

Abstract

Telomeres are the specialized protein–DNA complexes that cap and protect the ends of linear eukaryotic chromosomes. The extreme 3′ end of the telomeric DNA in Oxytricha nova is bound by a two-subunit sequence-specific and 3′ end-specific protein called the telomere end-binding protein (OnTEBP). Here we describe the crystal structure of the α-subunit of OnTEBP in complex with T4G4 single-stranded telomeric DNA. This structure shows an (α–ssDNA)2 homodimer with a large 7,000 Å2 protein–protein interface in which the domains of α are rearranged extensively from their positions in the structure of an α–β–ssDNA ternary complex. The (α–ssDNA)2 complex can bind two telomeres on opposite sides of the dimer and, thus, acts as a protein mediator of telomere–telomere associations. The structures of the (α–ssDNA)2 dimer presented here and the previously described α–β–ssDNA complex demonstrate that OnTEBP forms multiple telomeric complexes that potentially mediate the assembly and disassembly of higher order telomeric structures.

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Acknowledgements

We would like to thank D.M. Prescott for discussions pertaining to O. nova gene organization and providing us with O. nova macronuclei; D. Lyons and T.R. Cech for antibodies; M.P. Horvath, D.S. Classen, D.L. Theobald, and J.M. Bevilacqua for their support and advice during the course of this project; and A.V. Johnson for assistance with the macronuclear lysis experiments. This work was supported by grants from the American Cancer Society and National Institutes of Health.

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The authors declare no competing financial interests.

Correspondence to Olve B. Peersen.

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Figure 1: Structures of O. nova telomere end-binding protein complexes.
Figure 2: ssDNA register shift and protein–protein interactions inter-relating the two OnTEBP complexes.
Figure 3: Stereo image of the protein–ssDNA interactions in the (α–ssDNA)2 dimer and structural alignment of Pot1 sequence homology.
Figure 4: OnTEBP α-subunit as a mediator of higher order telomere–telomere association.