ABSTRACT
Telomeres are the ends of eukaryotic chromosomes and consist of tandem repeats of a G-rich DNA sequence, which in all vertebrates is TTAGGG (1,2). There are proteins specifically bound to mammalian telomeres (3). Some of these proteins bind to double-stranded telomeric DNA, such as TRF1 and TRF2 (4-5), which in turn recruit to the telomeric proteins important in the DNA-damage response such as tankyrase (PARP-5), Ku, DNA-PKcs, and the Mre11/Rad50/Nsb1 complex (6-11). Recently, a novel telomeric protein, POT-1, has been shown specifically to recognize the 3 single-stranded telomeric DNA or G-strand overhang (12). Telomeres protect chromosomal ends from degradation, recombination, and DNA-repair activities. Loss of telomeric function, either due to loss of telomeric sequences or to mutation of telomeric proteins, results in end-to-end fusions and loss of cell viability (9,11,13-15). The current model is that telomeres can adopt two different conformations: One is an “open” telomere that is accessible to telomerase and other cellular activities and the other is a “protected” telomere in which the single-stranded telomeric G-strand overhang invades the doublestranded telomeric DNA, forming a loop structure named the t-loop (16) (see Sec. II.D) (Fig. 1).
