Department of Pharmacology and Chemical Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15213, USA.
J Biol Chem. 2011 Mar 4;286(9):7479-89. doi: 10.1074/jbc.M110.205641. Epub 2010 Dec 23.
Human telomeres are composed of duplex TTAGGG repeats and a 3' single-stranded DNA tail. The telomeric DNA is protected and regulated by the shelterin proteins, including the protection of telomeres 1 (POT1) protein that binds telomeric single-stranded DNA. The single-stranded tail can fold into G-quadruplex (G4) DNA. Both POT1 and G4 DNA play important roles in regulating telomere length homeostasis. To date, most studies have focused on individual quadruplexes formed by four TTAGGG repeats. Telomeric tails in human cells have on average six times as many repeats, and no structural studies have examined POT1 binding in competition with G4 DNA folding. Using single molecule atomic force microscopy imaging, we observed that the majority of the telomeric tails of 16 repeats formed two quadruplexes even though four were possible. The result that physiological telomeric tails rarely form the maximum potential number of G4 units provides a structural basis for the coexistence of G4 and POT1 on the same DNA molecule, which is observed directly in the captured atomic force microscopy images. We further observed that POT1 is significantly more effective in disrupting quadruplex DNA on long telomeric tails than an antisense oligonucleotide, indicating a novel POT1 activity beyond simply preventing quadruplex folding.
人类端粒由双链 TTAGGG 重复序列和 3'单链 DNA 尾巴组成。端粒 DNA 由端粒保护蛋白（包括与端粒单链 DNA 结合的端粒 1 蛋白（POT1））保护和调节。单链尾巴可以折叠成 G-四链体（G4）DNA。POT1 和 G4 DNA 都在调节端粒长度的动态平衡中发挥重要作用。迄今为止，大多数研究都集中在由四个 TTAGGG 重复序列形成的单个四链体上。人类细胞中的端粒尾巴平均有六倍那么多的重复序列，而且没有结构研究检查过 POT1 与 G4 DNA 折叠竞争时的结合情况。使用单分子原子力显微镜成像，我们观察到 16 个重复的大多数端粒尾巴形成了两个四链体，尽管有四个可能。生理端粒尾巴很少形成最大数量的 G4 单位的结果为 G4 和 POT1 共存于同一 DNA 分子提供了结构基础，这在捕获的原子力显微镜图像中直接观察到。我们进一步观察到，POT1 比反义寡核苷酸更有效地破坏长端粒尾巴上的四链体 DNA，这表明 POT1 具有一种超越简单阻止四链体折叠的新活性。
