RADX: A novel single-stranded DNA-binding protein regulating telomere recombination

Telomeres are specialized nucleoprotein structures present at the ends of linear chromosome. The telomeric DNA part is comprised of 5-15 kilo base pairs of double stranded TTAGGG repeats and it contains at the 3' end a single-stranded G-rich overhang of 50-400 nucleotides. Telomeric DNA associates with a large number of proteins, which inhibit DNA damage checkpoint activation, DNA repair activities and nucleolytic degradation. To better understand the complex roles which telomeres play in genome stability, premature aging and cancer development we explored the telomeric proteomic environment with BioID. We employed a CRISPR/Cas9 based knock-in approach to integrate the promiscuous biotin ligase BirA at the genomic loci of the telomeric components TRF1, TRF2 and POT1. Thus, we expressed the fusion proteins from native promoters aiming at retaining native expression levels. Upon incubation with Biotin, the promiscuous biotinylase can label proximal proteins, which can be selectively purified and identified via mass spectrometry. Our BioID results indicated that TRF1, TRF2 and POT1 share a large number of common protein partners. In addition to already known telomere components, we identified a significant number of novel telomeric proteins. Among those newly identified proteins, we found RADX, an RPA-like single-stranded DNA-binding-protein, which counteracts RAD51 at stalled replication forks. Even though RADX had so far only been characterized for its role in DNA replication we observed colocalization of ectopically expressed 3xFlag-RADX with telomeres throughout the cell cycle. The colocalization decreased when the RADXs single-stranded (ss)DNA-binding domain was mutated. We therefore hypothesize that RADX binds the single-stranded G-rich telomeric strand either as displaced strand in the t-loop or as single-stranded telomeric overhang. Chromatin-immunoprecipitation experiments suggest that RADX plays a distinct role in telomere replication, since upon treatment with the replication stress inducing reagent hydroxyurea we observed increased binding of RADX specifically to telomeres. Notably, RADX depletion on its own did not alter telomere integrity and telomere length but upon co-depletion with POT1, telomere fragility, sister-chromatid associations and telomere length increased strikingly. POT1 and POT1/RADX depletions lead to increased associations of the recombinase RAD51 with telomeres and consequently silencing of RAD51 rescued telomere integrity and elongation implying the regulatory role of RADX for RAD51. In summary my thesis describes the proteomic microenvironment of different telomeric proteins, identifies the novel telomeric protein RADX and characterizes its role at telomeres. My findings provide important insights into how homologous recombination and RAD51 loading is suppressed at telomeres.