Studies on the mode of Ku interaction with DNA

The Ku heterodimer plays a central role in non-homologous end-joining. The binding of recombinant Ku to DNA has been investigated by dynamic light scattering, double filter binding, fluorescence spectroscopy and band shift assays. The hydrodynamic radius of Ku in solution is 5.2 nm and does not change when a 25bp dsDNA fragment (D25) is added indicating that only one Ku molecule binds to a 25bp fragment. The dissociation constant (kd) for the binding to D25 is 3.8 0.9 nM. If both ends of the substrate are closed with hairpin loops, Ku is still able to bind with little change in the kd. The kd is not affected by ATP, Mg2+ or ionic strength. However, the addition of BSA decreases the kd by two fold. DNA substrates of 50bp can bind two Ku molecules, while three molecules are bound to a 75bp substrate. Data analysis with the Hill equation yields a value of the Hill coefficient (n) close to 1 and the kd values for the binding of Ku to both ends of these substrates are the same. Thus, we demonstrate that there is no cooperative interaction among the Ku heterodimers binding longer substrates.