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

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 alpha-subunit of OnTEBP in complex with T(4)G(4) single-stranded telomeric DNA. This structure shows an (alpha-ssDNA)(2) homodimer with a large 7,000 Angstrom(2) protein protein interface in which the domains of alpha are rearranged extensively from their positions in the structure of an alpha-beta-ssDNA ternary complex. The (alpha-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 (alpha-ssDNA)(2) dimer presented here and the previously described alpha-beta-ssDNA complex demonstrate that OnTEBP forms multiple telomeric complexes that potentially mediate the assembly and disassembly of higher order telomeric structures.