Zhao J, Wang J, Chen D J, Peterson S R, Trewhella J
Chemical Science & Technology Division, Los Alamos National Laboratory, New Mexico 87545, USA.
Biochemistry. 1999 Feb 16;38(7):2152-9. doi: 10.1021/bi9825246.
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 A. 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 A, while that for the Ku/30-mer DNA is 56 A. 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.
