Structural and Functional Interaction between the Human DNA Repair Proteins DNA Ligase IV and XRCC4

Abstract

Nonhomologous end-joining represents the major pathway used by human cells to repair DNA double-strand breaks. It relies on the XRCC4/DNA ligase IV complex to reseal DNA strands. Here we report the high-resolution crystal structure of human XRCC4 bound to the carboxy-terminal tandem BRCT repeat of DNA ligase IV. The structure differs from the homologous Saccharomyces cerevisiae complex and reveals an extensive DNA ligase IV binding interface formed by a helix-loop-helix structure within the inter-BRCT linker region, as well as significant interactions involving the second BRCT domain, which induces a kink in the tail region of XRCC4. We further demonstrate that interaction with the second BRCT domain of DNA ligase IV is necessary for stable binding to XRCC4 in cells, as well as to achieve efficient dominant-negative effects resulting in radiosensitization after ectopic overexpression of DNA ligase IV fragments in human fibroblasts. Together our findings provide unanticipated insight for understanding the physical and functional architecture of the nonhomologous end-joining ligation complex.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://mcb.asm.org/ .

ACKNOWLEDGMENTS

We are indebted to T. Lindahl (Cancer Research UK London Research Institute) and D. Biard (Commissariat à l'Energie Atomique) for the gift of plasmids, P. R. Hamann (Wyeth Research) for the gift of calicheamicin-γ1, and A. Sarasin (Institut Gustave Roussy) for the gift of immortalized MRC5 cells.

This work was supported by a grant (MOP-53209) to M.S.J. from the Canadian Institutes of Health Research and, partly, by grants from the Association pour la Recherche sur le Cancer, the Ligue Nationale Contre le Cancer (équipe labelisée), the Commissariat à l'Energie Atomique, and a Radiobiology grant from Electricité de France. M.M. was supported by the Association for International Cancer Research (grant 01-215). Patrick Calsou is a scientist from INSERM, France.