Nucleic Acids Research Advance Access published online on September 9, 2008
Nucleic Acids Research, doi:10.1093/nar/gkn552
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Genome integrity, repair and replication |
Loss of DNA ligase IV prevents recognition of DNA by double-strand break repair proteins XRCC4 and XLF
1Department of Biochemistry and Molecular Biology, 2Department of Molecular Microbiology and Immunology, Johns Hopkins University, Bloomberg School of Public Health, Baltimore, MD 21205, USA and 3International Graduate School of Arts and Sciences, Yokohama City University, 22-2 Seto, Kanazawa-ku, Yokohama 236-0027, Japan
*To whom correspondence should be addressed. Tel: +1 443 287 2515; Fax: +1 410 955 2926; Email: lhanakah{at}jhsph.edu
Received July 23, 2008. Accepted August 12, 2008.
The repair of DNA double-strand breaks by nonhomologous end-joining (NHEJ) is essential for maintenance of genomic integrity and cell viability. Central to the molecular mechanism of NHEJ is DNA ligase IV/XRCC4/XLF complex, which rejoins the DNA. During adenovirus (Ad5) infection, ligase IV is targeted for degradation in a process that requires expression of the viral E1B 55k and E4 34k proteins while XRCC4 and XLF protein levels remain unchanged. We show that in Ad5-infected cells, loss of ligase IV is accompanied by loss of DNA binding by XRCC4. Expression of E1B 55k and E4 34k was sufficient to cause loss of ligase IV and loss of XRCC4 DNA binding. Using ligase IV mutant human cell lines, we determined that the absence of ligase IV, and not expression of viral proteins, coincided with inhibition of DNA binding by XRCC4. In ligase IV mutant human cell lines, DNA binding by XLF was also inhibited. Expression of both wild-type and adenylation-mutant ligase IV in ligase IV-deficient cells restored DNA binding by XRCC4. These data suggest that the intrinsic DNA-binding activities of XRCC4 and XLF may be subject to regulation and are down regulated in human cells that lack ligase IV.