Repressor/activator protein 1 (RAP1) is a highly evolutionarily conserved protein found at telomeres. Although yeast Rap1 is a key telomere capping protein preventing non‐homologous end joining (NHEJ) and consequently telomere fusions, its role at mammalian telomeres in vivo is still controversial. Here, we demonstrate that RAP1 is required to protect telomeres in replicative senescent human cells. Downregulation of RAP1 in these cells, but not in young or dividing pre‐senescent cells, leads to telomere uncapping and fusions. The anti‐fusion effect of RAP1 was further explored in a HeLa cell line where RAP1 expression was depleted through an inducible CRISPR/Cas9 strategy. Depletion of RAP1 in these cells gives rise to telomere fusions only when telomerase is inhibited. We further show that the fusions triggered by RAP1 loss are dependent upon DNA ligase IV. We conclude that human RAP1 is specifically involved in protecting critically short telomeres. This has important implications for the functions of telomeres in senescent cells.

中文翻译：

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人类 RAP1 特异性保护衰老细胞的端粒免受 DNA 损伤

阻遏/激活蛋白 1 (RAP1) 是一种在端粒中发现的高度进化保守的蛋白质。尽管酵母 Rap1 是一种关键的端粒加帽蛋白，可防止非同源末端连接 (NHEJ) 并进而防止端粒融合，但其在哺乳动物体内端粒中的作用仍存在争议。在这里，我们证明 RAP1 是保护复制性衰老人类细胞中的端粒所必需的。RAP1 在这些细胞中的下调（但在年轻或分裂的衰老前细胞中则不然）会导致端粒脱帽和融合。在 HeLa 细胞系中进一步探讨了 RAP1 的抗融合作用，其中通过诱导型 CRISPR/Cas9 策略消除了 RAP1 的表达。仅当端粒酶受到抑制时，这些细胞中 RAP1 的消耗才会引起端粒融合。我们进一步表明，RAP1 丢失引发的融合依赖于 DNA 连接酶 IV。我们得出结论，人类 RAP1 特别参与保护极短的端粒。这对于衰老细胞中端粒的功能具有重要意义。