Ku stimulation of DNA ligase IV-dependent ligation requires inward movement along the DNA molecule.
The DNA ligase IV.XRCC4 complex (LX) functions in DNA non-homologous-end joining, the main pathway for double-strand break repair in mammalian cells. We show that, in contrast to ligation by T4 ligase, the efficiency of LX ligation of double-stranded (ds) ends is critically dependent upon the length of the DNA substrate. The effect is specific for ds ligation, and LX/DNA binding is not influenced by the substrate length. Ku stimulates LX ligation at concentrations resulting in 1-2 Ku molecules bound per substrate, whereas multiply Ku-bound DNA molecules inhibit ds ligation. The combined footprint of DNA with Ku and LX bound is the sum of each individual footprint suggesting that the two complexes are located in tandem at the DNA end. Inhibition of Ku translocation by the presence of cis-platinum adducts on the DNA substrate severely inhibits ligation by LX. Fluorescence resonance energy transfer analysis using fluorophore-labeled Ku and DNA molecules showed that, as expected, Ku makes close contact with the DNA end and that addition of LX can disrupt this close contact. Finally, we show that recruitment of LX by Ku is impaired in an adenylation-defective mutant providing further evidence that LX interacts directly with the DNA end, possibly via the 5'-phosphate as shown for prokaryotic ligases. Taken together, our results suggest that, when LX binds to a Ku-bound DNA molecule, it causes inward translocation of Ku and that freedom to move inward on the DNA is essential to Ku stimulation of LX activity.
Kysela
Boris
B
Genome Damage and Stability Centre, University of Sussex, Brighton BN1 9RQ, United Kingdom. b.kysela@sussex.ac.uk
Doherty
Aidan J
AJ
Chovanec
Miroslav
M
Stiff
Thomas
T
Ameer-Beg
Simon M
SM
Vojnovic
Borivoj
B
Girard
Pierre-Marie
PM
Jeggo
Penny A
PA
D034961:Antigens, Nuclear / Q000302:isolation & purification / Q000378:metabolism*; D001483:Base Sequence; D001665:Binding Sites; D004247:DNA / Q000096:biosynthesis* / Q000737:chemistry*; D004265:DNA Helicases; D000072481:DNA Ligase ATP; D011088:DNA Ligases / Q000302:isolation & purification / Q000378:metabolism*; D017931:DNA Primers; D004260:DNA Repair; D004268:DNA-Binding Proteins / Q000302:isolation & purification / Q000378:metabolism*; D031541:Fluorescence Resonance Energy Transfer; D005998:Glycine; D007700:Kinetics; D000072200:Ku Autoantigen; D008239:Lysine; D011994:Recombinant Proteins / Q000302:isolation & purification / Q000378:metabolism; D013379:Substrate Specificity
D034961:Antigens, Nuclear; D017931:DNA Primers; D004268:DNA-Binding Proteins; C000607893:LIG4 protein, human; D011994:Recombinant Proteins; D004247:DNA; D004265:DNA Helicases; C106893:XRCC5 protein, human; C000606509:Xrcc6 protein, human; D000072200:Ku Autoantigen; D011088:DNA Ligases; D000072481:DNA Ligase ATP; D008239:Lysine; D005998:Glycine