Protection of the Telomeric\nJunction by the Shelterin\nComplex

Shelterin serves critical roles in suppressing superfluous\nDNA\ndamage repair pathways on telomeres. The junction between double-stranded\ntelomeric tracts (dsTEL) and single-stranded telomeric overhang (ssTEL)\nis the most accessible region of the telomeric DNA. The shelterin\ncomplex contains dsTEL and ssTEL binding proteins and can protect\nthis junction by bridging the ssTEL and dsTEL tracts. To test this\npossibility, we monitored shelterin binding to telomeric DNA substrates\nwith varying ssTEL and dsTEL lengths and quantified its impact on\ntelomere accessibility using single-molecule fluorescence microscopy\nmethods <i>in vitro</i>. We identified the first dsTEL repeat\nnearest the junction as the preferred binding site for creating the\nshelterin bridge. Shelterin requires at least two ssTEL repeats, while\nthe POT1 subunit of shelterin that binds to ssTEL requires longer\nssTEL tracts for stable binding to telomeres and effective protection\nof the junction region. The ability of POT1 to protect the junction\nis significantly enhanced by the 5′-phosphate at the junction.\nCollectively, our results show that shelterin enhances the binding\nstability of POT1 to ssTEL and provides more effective protection\ncompared with POT1 alone by bridging single- and double-stranded telomeric\ntracts.