Human DNA ligase III (103 kDa) has been shown to interact directly with the 70 kDa DNA repair protein, XRCC1. Here, the binding sites have been defined. Subcloned fragments of XRCC1 have been expressed and assayed for their ability to associate with DNA ligase III by far Western and affinity precipitation analyses. The C-terminal 96 amino acids of XRCC1 are necessary and sufficient for the specific interaction with DNA ligase III. A similar approach with the 103 kDa DNA ligase III has identified the C-terminal 148 amino acids of this enzyme as containing the binding site for XRCC1. An alternative 96 kDa form of DNA ligase III, abundant in testes, has been described [Chen, J., et al. (1995) Mol. Cell. Biol. 15, 5412-5422]. These two forms of DNA ligase III have identical N-terminal regions but differ toward their C termini and may be alternatively spliced products of the same gene. Antipeptide antibodies directed against the different C termini of the two forms of the enzyme indicate that both of them occur in vivo. The C-terminal region of the 96 kDa derivative of DNA ligase III is not able to interact with XRCC1. These findings indicate that only the larger form of DNA ligase III acts together with XRCC1, suggesting a role for this isoform of the enzyme in base excision repair.
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