CK2 与 PPM1D 动态调控 TCOF1 磷酸化，影响核仁凝聚与 RNA 聚合酶 I 活性
Dynamic phosphorylation of TCOF1 controlled by CK2 and PPM1D regulates nucleolar condensation and RNA polymerase I activity
PPM1D 通过去磷酸化支架蛋白 TCOF1 调控核仁结构，其定位依赖 C 端保守基序。研究结合超分辨显微成像与结构域定位，并用磷酸化蛋白质组学与结构建模发现，PPM1D 作用于 TCOF1 低复杂度重复域内由 CK2 磷酸化的位点，影响 TCOF1 与 RNA 聚合酶 I（Pol I）的相互作用、在纤维中心的固定及核仁凝聚。缺失 PPM1D 或抑制 CK2 打破磷酸化平衡，导致核仁解体与 rDNA 转录下降。药理抑制 PPM1D 可使细胞对 RNA Pol I 抑制剂敏感，且不依赖 p53 状态。该可逆磷酸化开关调控 TCOF1 动态与核仁功能，为靶向核糖体生物合成治疗癌症提供线索。
来源：Nucleic Acids Research · doi.org
The nucleolus is a membrane-less organelle essential for ribosomal DNA (rDNA) transcription and ribosome biogenesis, yet the mechanisms regulating its dynamic assembly remain incompletely understood. Here, we identify the phosphatase protein phosphatase magnesium-dependent 1 delta (PPM1D) as a key regulator of the nucleolar architecture via dephosphorylation of the scaffold protein TCOF1. By combining super-resolution microscopy and domain mapping, we show that PPM1D localizes to the nucleolus through a conserved motif at the C-terminus. Phosphoproteomic analysis and structural modeling revealed that PPM1D targets casein kinase 2 (CK2)-phosphorylated sites within the low-complexity repeat domain of TCOF1, which impacts its interaction with RNA polymerase I (Pol I), immobilization in the fibrillar centres, and nucleolar condensation. Disruption of the phosphorylation balance by loss of PPM1D or inhibition of CK2 leads to nucleolar disorganization and reduced rDNA transcription. Finally, we demonstrate that pharmacological inhibition of PPM1D sensitizes cells to RNA Pol I inhibitors, in both p53-dependent and -independent contexts. Together, we uncover a reversible phosphorylation switch that governs TCOF1 dynamics and its nucleolar function, with potential implications for targeting ribosome biogenesis in cancer.