Small-angle X-ray and neutron scattering with contrast variation has been used to study the structure of the DNA targeting component (Ku) of the DNA-dependent protein kinase and its complex with DNA. The Ku protein in solution has the approximate shape of a prolate ellipsoid with semi-axes of 24, 43, and 89 Å. In the presence of a minimal-length DNA binding sequence (a 24-base-pair duplex DNA), a 1:1 Ku/DNA complex forms. This 1:1 stoichiometry is observed when either the Ku or the DNA is in excess. Analysis of the contrast variation data on Ku complexed with either the 24- mer duplex DNA or a slightly longer 30-mer duplex DNA shows that both the DNA and Ku structures have the same overall conformations within the 1:1 complex as the uncomplexed components. The separation of the centers-of-mass for the Ku/24-mer DNA complex is 46 Å, while that for the Ku/30-mer DNA is 56 Å. The DNA binds within what appears to be a preformed channel that penetrates deeply into the Ku protein such that the entire length of the 24-mer DNA spans the protein. The slightly longer 30-mer binds in a similar fashion, but with its extra length protruding from the protein envelop. The scattering data are consistent with the idea that the Ku 'threads' onto the duplex DNA via a channel that can completely bury approximately 24 base pairs.
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