DNA Repair Proteins XLF and XRCC4 Interact With Telomeric Proteins TRF1 And RAP1

Abstract

Non‐homologous end‐joining (NHEJ) is the main repair pathway for processing double‐strand DNA breaks in Eukarya. In mammalian cells, NHEJ is mediated by a complex of proteins comprised of the Ku subunits (Ku70 and Ku86), Ligase IV, XRCC4, Artemis, DNA‐PKcs and XLF. Paradoxically, several of these NHEJ proteins are also found at telomeres, despite the fact that one of the main functions of the complex structure at telomeres is to prevent NHEJ proteins from recognizing the natural ends of chromosomes as DSBs. For instance, Ku is present at telomeres through interactions with resident telomeric proteins like TRF1 and TRF2, as is the case for DNA‐PKcs. Currently, the telomeric status of several NHEJ proteins is still unknown. Using protein fragment complementation assays, we have found interactions between two NHEJ proteins, XLF and XRCC4, with TRF1 and RAP1, two proteins present at telomeres. Co‐localization studies with TRF2 indicate that TRF1 interactions with XLF and XRCC4 occur at telomeres, while XLF‐RAP1 and XRCC4‐RAP1 interactions occur throughout the nucleus. Mutations that disrupt XRCC4‐XLF filament formation did not impair XRCC4 or XLF interaction with TRF1. Similarly, XRCC4 mutations in known phosphorylation sites for ATM and DNA‐PKcs did not impair XRCC4‐TRF1 interaction nor did they impair XRCC4 localization at telomeres. Overall, these results suggest that XRCC4 and XLF can localize at telomeres through direct interaction with telomeric resident proteins.Support or Funding InformationSupported by the UST/HCCS STEM Scholars Program (P031C110128‐12) from the US Department of Education and by the University of St Thomas Cullen Smith Chair of Biology