Computational studies on full-length Ku70 with DNA duplexes: base interactions and a helical path.

The Ku70/80 heterodimer is among the first responding proteins to recognize and bind the DNA double strand breaks (DSBs). Once Ku is loaded at the DSB, it works as a scaffold to recruit other repair factors in non-homologous end joining thereby facilitates the following repair processes. In this work, we characterized the detailed interactions and binding free energies between a Ku70 subunit and several DNA duplexes, by using some well-established computational methods. The results reveal that the structure of the protein may suffer certain contractions without the company of Ku80, and may experience large conformational changes in the presence of different DNA duplexes. Notably, we observe the closest interactions between Ku70 and DNA can be easily strengthened to form H-bonds with the bases in the minor groove, which is unexpected. However, this finding is supported by the presence of a similar bond between Ku80 and DNA in the published crystal structure (PDB code 1JEY). We suggest that these interactions are responsible for the observed pausing sites when Ku translocates along DNA and the subtle difference in binding with AT- and GC-rich DNA ends. Additionally, simulations indicate the inner surface of the ring encircling the DNA is not flat, but contains a delicate clamp like structure, which is ideal to grip the two strands of DNA in the minor groove and confine the movement of the duplex in a unique helical path.