[Show abstract][Hide abstract] ABSTRACT: The telomeric protein TRF2 is required to prevent mammalian telomeres from activating DNA damage checkpoints. Here we show that overexpression of TRF2 affects the response of the ATM kinase to DNA damage. Overexpression of TRF2 abrogated the cell cycle arrest after ionizing radiation and diminished several other readouts of the DNA damage response, including phosphorylation of Nbs1, induction of p53, and upregulation of p53 targets. TRF2 inhibited autophosphorylation of ATM on S1981, an early step in the activation of this kinase. A region of ATM containing S1981 was found to directly interact with TRF2 in vitro, and ATM immunoprecipitates contained TRF2. We propose that TRF2 has the ability to inhibit ATM activation at telomeres. Because TRF2 is abundant at chromosome ends but not elsewhere in the nucleus, this mechanism of checkpoint control could specifically block a DNA damage response at telomeres without affecting the surveillance of chromosome internal damage.
[Show abstract][Hide abstract] ABSTRACT: The human telomeric protein POT1 is known to bind single-stranded telomeric DNA in vitro and to participate in the regulation of telomere maintenance by telomerase in vivo. We examined the in vitro DNA binding features of POT1. We report that deleting the oligosaccharide/oligonucleotide-binding fold of POT1 abrogates its DNA binding activity. The minimal binding site (MBS) for POT1 was found to be the telomeric nonamer 5'-TAGGGTTAG-3', and the optimal substrate is [TTAGGG](n (n > or = 2)). POT1 displays exceptional sequence specificity when binding to MBS, tolerating changes only at position 7 (T7A). Whereas POT1 binding to MBS or [TTAGGG](2) was enhanced by the proximity of a 3' end, POT1 was able to bind to a [TTAGGG](5) array when positioned internally. These data indicate that POT1 has a strong sequence preference for the human telomeric repeat tract and predict that POT1 can bind both the 3' telomeric overhang and the displaced TTAGGG repeats at the base of the t-loop.