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Mol. Cell. Biol. doi:10.1128/MCB.00048-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Distinct functions of POT1 at telomeres

Department of Pharmacology and Cancer Biology, Department of Radiation Oncology, Duke University Medical Center, Durham, North Carolina

^* To whom correspondence should be addressed. Email: count004{at}mc.duke.edu.

	Abstract

The mammalian protein POT1 binds to telomeric single-stranded (ss)DNA, protecting chromosome ends from being detected as sites of DNA damage. POT1 is composed of an N-terminal ssDNA-binding domain and a C-terminal protein-interaction domain. In regards to the latter, POT1 heterodimerizes with the protein TPP1 to foster binding to telomeric ssDNA in vitro and binds the telomeric double-stranded (ds)DNA-binding protein TRF2. We sought to determine which of these functions – ssDNA-, TPP1-, or TRF2-binding – was required to protect chromosome ends from being detected as DNA damage. Using separation-of-function POT1 mutants which are deficient in one of these three activities, we find that binding to TRF2 is dispensable for protecting telomeres, but fosters robust loading of POT1 onto telomeric chromatin. Furthermore, we find that the telomeric ssDNA-binding activity and binding to TPP1 are required in cis for POT1 to protect telomeres. Mechanistically, binding of POT1 to telomeric ssDNA and association with TPP1 inhibits the localization of RPA, which can function as a DNA damage sensor, to telomeres. We speculate that POT1 is loaded onto telomere chromatin via association with TRF2, and further to telomeric ssDNA in conjunction with TPP1, and prevents binding of RPA and induction of a DNA damage response.