ACTIVITY OF DNA-LIGASE-IV STIMULATED BY COMPLEX-FORMATION WITH XRCC4 PROTEIN IN MAMMALIAN-CELLS

Mutation of the XRCC4 gene in mammalian cells(1,2) prevents the format ion of the signal and coding joints in the V(D)J recombination reactio n(3), which is necessary for production of a functional immunoglobulin gene, and renders the cells highly sensitive to ionizing radiation(4) . However, XRCC4 shares no sequence homology with other proteins, nor does it have a biochemical activity to indicate what its function migh t be(2). Here we show that DNA ligase IV (ref. 5) co-immunoprecipitate s with XRCC4 and that these two proteins specifically interact with on e another in a yeast two-hybrid system. Ligation of DNA double-strand breaks in a cell-free system by DNA ligase IV is increased fivefold by purified XRCC4 and seven- to eightfold when XRCC4 is co-expressed wit h DNA ligase IV. We conclude that the biological consequences of mutat ing XRCC4 are primarily due to the loss of its stimulatory effect on D NA ligase IV: the function of the XRCC4-DNA ligase IV complex may be t o carry out the final steps of V(D)J recombination and joining of DNA ends.