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，对有丝分裂进程至关重要，但对于检查点恢复和适应 DNA 损伤和复制应激也很重要。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，模仿早期胚胎复制，我们证明 Plk1 与预复制蛋白同时被招募到染色质，在那里它在整个 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，这似乎对蛋白磷酸酶 2A 的去磷酸化敏感。Plk1 免疫耗竭的提取物显示 DNA 复制减少，由野生型重组 Plk1 拯救。相反，适度的 Plk1 过度表达加速了 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 期抑制 ATR/Chk1 依赖的 S 期检查点。此外，我们观察到磷酸化的 Plk1 水平在快速、早期的细胞周期中很高。当细胞周期和复制程序随着更活跃的检查点而减慢时，非洲爪蟾的发育会在中囊胚转变后减少。这些数据揭示了 Plk1 作为早期快速分裂胚胎中 DNA 复制的积极调节因子的作用。