XRCC1 PROTEIN INTERACTS WITH ONE OF 2 DISTINCT FORMS OF DNA-LIGASE-III

Human DNA ligase III (103 kDa) has been shown to interact directly wit h the 70 kDa DNA repair protein, XRCC1. Here, the binding sites have b een defined. Subcloned fragments of XRCC1 have been expressed and assa yed 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 D NA 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) M el. Cell. Biol. 15, 5412-5422]. These two forms of DNA ligase III have identical N-terminal regions but differ toward their C termini and ma y be alternatively spliced products of the same gene. Antipeptide anti bodies directed against the different C termini of the two forms of th e enzyme indicate that both of them occur in vivo. The C-terminal regi on 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 l igase III acts together with XRCC1, suggesting a role for this isoform of the enzyme in base excision repair.