Polo-like kinase 1 (Plk1) is a cell cycle kinase essential for mitosis progression, but also important for checkpoint recovery and adaptation in response to DNA damage and replication stress. However, although Plk1 is expressed in S phase, little is known about its function during unperturbed DNA replication. Using Xenopus laevis egg extracts, mimicking early embryonic replication, we demonstrate that Plk1 is simultaneously recruited to chromatin with pre-replication proteins where it accumulates throughout S phase. Further, we found that chromatin-bound Plk1 is phosphorylated on its activating site T201, which appears to be sensitive to dephosphorylation by protein phosphatase 2A. Extracts immunodepleted of Plk1 showed a decrease in DNA replication, rescued by wild type recombinant Plk1. Inversely, modest Plk1 overexpression accelerated DNA replication. Plk1 depletion led to an increase in Chk1 phosphorylation and to a decrease in Cdk2 activity, which strongly suggests that Plk1 could inhibit the ATR/Chk1-dependent intra-S phase checkpoint during normal S phase. In addition, we observed that phosphorylated Plk1 levels are high during the rapid, early cell cycles of Xenopus development but decrease after the mid-blastula transition when the cell cycle and the replication program slow down along with more active checkpoints. These data shed new light on the role of Plk1 as a positive regulating factor for DNA replication in early, rapidly dividing embryos.

Polo 样激酶 1 (Plk1) 是一种细胞周期激酶，对于有丝分裂进程至关重要，而且对于检查点恢复以及响应 DNA 损伤和复制应激的适应也很重要。然而，尽管 Plk1 在 S 期表达，但对其在不受干扰的 DNA 复制过程中的功能知之甚少。使用非洲爪蟾卵提取物，模拟早期胚胎复制，我们证明 Plk1 与复制前蛋白同时被招募到染色质，并在整个 S 期积累。此外，我们发现染色质结合的 Plk1 在其激活位点 T201 上被磷酸化，该位点似乎对蛋白磷酸酶 2A 的去磷酸化敏感。 Plk1 免疫耗竭的提取物显示 DNA 复制减少，野生型重组 Plk1 可以挽救这种情况。相反，适度的 Plk1 过度表达会加速 DNA 复制。 Plk1 耗竭导致 Chk1 磷酸化增加和 Cdk2 活性降低，这强烈表明 Plk1 可以在正常 S 期期间抑制 ATR/Chk1 依赖性 S 期内检查点。此外，我们观察到磷酸化 Plk1 水平在非洲爪蟾发育的快速、早期细胞周期中很高，但在囊胚中期转变后，当细胞周期和复制程序随着检查点更活跃而减慢时，磷酸化 Plk1 水平会降低。这些数据为 Plk1 作为早期快速分裂胚胎中 DNA 复制的正调节因子的作用提供了新的线索。