1. XLF AND XRCC4 INTERACT WITH TELOMERIC PROTEINS Anderson, Chris1; Baidon, Mario1; Anderson, Spencer2; Tran, Phung2; Luna, Paloma2; Jain, R.2; Jabbur, J.2; Ribes-Zamora, A.1 1Biology Department, University of St. Thomas, Houston, TX 2Department of Biology and Physical Sciences, Houston Community College (Central), Houston, TX
fluorescence machine infinity 200 pro each bar represents 3 different measurements for 3 different transfections for a total of 9
1. INTRODUCTION
4. XLF AND XRCC4 INTERACT WITH RAP1
Non-homologous end-joining (NHEJ) is the main repair pathway for processing double-strand DNA breaks (DSBs) and is mediated by a complex of proteins comprised of the Ku subunits (Ku70/80), Ligase4, XRCC4, Aprataxin-and-PNK-like factor (APLF), DNA- PKcs and XLF, also known as Cernunnos. 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 provide a protective cap that prevents 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, TRF2 and RAP1, as is the case for DNA-PKcs. Currently, the telomeric status of several NHEJ proteins is still unknown. XLF and XRCC4 are both homodimers that can form not only high order multimer structures but also filaments composed of alternating XRCC4 and XLF molecules. Both proteins have been shown to act at different steps of the NHEJ pathway, including end processing and recruitment of DNA Ligase IV to sites of DNA damage.
Hypothesis: In this study we are investigating whether XLF and XRCC4 can be found localized at telomeres, similar to other NHEJ proteins, through interactions with telomeric proteins such as TRF1, TRF2 and RAP1. A
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3. XLF AND XRCC4 INTERACTION WITH TRF1 CO-LOCALIZES WITH TRF2
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Figure 3. Interaction localization confirmed by PFC assay. (A) Fluorescence microscopy of HEK293T cells transfected with XRCC4 and RAP1 constructs. Fluorescence was observed in individual cells 48 hours post-transfection. Images of XRCC4-RAP1 interaction show nuclear co-localization, suggesting that this interaction occurs at telomeres. (B) Same as in (A) but employing transfection with XLF and RAP1 constructs.
2. METHODS: PROTEIN FRAGMENT COMPLEMENTATION ASSAY
Fluorescence
5. CONCLUSIONS AND FUTURE DIRECTIONS
Protein-Fragment Complementation Assay (PCA) is a technique that uses fluorescence to visualize protein-protein interactions.
A fluorescent protein (Venus-YPF) is split into two fragments (N-terminal or V1 and C-terminal or V2) that are then fused to the proteins of interest and whose interaction reconstitutes the fluorophore. Previous studies have shown that V1 and V2 fragments can not reform Venus-YFP unless the proteins of interest are interacting.
To test our hypothesis, XRCC4, XLF, TRF1, TRF2 and RAP1 were fused to V1 and V2. RAD21 was used as a non-specific control.
Some of the advantages of using PCA over other protein binding detection techniques are that it can be done in live cells and that it provides information about the localization of the interaction .
PCA and co-localization demonstrates that XLF interacts with TRF1 and with RAP1 at telomeres.
XRCC4 also interacts with TRF1 and with RAP1 in a punctuate manner consistent with telomere localization.
Future Directions: Validation of our PCA results with Western assay; to confirm equivalent transfected protein co-expression. Interactome mapping; characterizing important amino acid residues on XLF and XRCC4 employing site-directed mutagenesis. B
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REFERENCES C
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Davis, A.J. and Chen, D.J. (2013). DNA double strand break repair via non-homologous end joining. Transl. Cancer Res. 2(3):
Lee, K.J., Jovanovic ,M., Udayakumar, D., Bladen, C.L., Dynan, W.S. (2004). Identification of DNA-PKcs phosphorylation sites and effects of mutation of these sites on DNA end joining in a cell-free system. DNA Repair (Amst). 4;3(3):
Lieber M.R. (2010). The mechanism of double-strand DNA break repair by the nonhomologous DNA end-joining pathway. Annu. Rev. Biochem. 79:181–211. 
Palm, W., and de Lange, T. (2008). How shelterin protects mammalian telomeres. Annu. Rev. Genet. 42:
Remy, I., and Michnick, S.W. (2007). Application of protein-fragment complementation assays in cell biology. Biotechniques 42, 137, 139, 141 passim
Riha, K., Heacock, M. L., Shippen, D. E. (2006). The role of the nonhomologous end-joining DNA double-strand break repair pathway in telomere biology. Annu. Rev. Genet. 40: 237–277. V2
CMV V1
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Figure 2. PFC assay measurements and interaction localization. (A) Fluorescence measurements of HEK293T cells transfected with indicated constructs indicating that XLF interacts with both TRF1 and TRF2 while XRCC4 only interacts with TRF1. Fluorescence was measured in 5 x 104 cells 48 hours after transfection using Tecan Infinity200Pro plate reader. Each bar represents three different measurements for three different transfections, for a total of 9 measurements for each different V1/V2 combination. (B) De-convoluted images of XLF-TRF1 interaction showing its co-localization with TRF2, suggesting that this interaction occurs at telomeres. (C) Same as in (B) but changing the order of Venus fragments.
CMV V2
RAP1
Figure 1. Experimental set up. (Left) Representation of the Protein Fragment Complementation Assay, or PCA. (Right) Constructs used in this study.
Supported by the UST/HCCS STEM Scholars Program (P031C ) from the US Dept. of Education