Biomolecules such as proteins and RNA could organize to form condensates with distinct microenvironments through liquid–liquid phase separation (LLPS). Recent works have demonstrated that the microenvironment of biomolecular condensates plays a crucial role in mediating biological activities, such as the partition of biomolecules, and the subphase organization of the multiphasic condensates. Ions could influence the phase transition point of LLPS, following the Hofmeister series. However, the ion-specific effect on the microenvironment of biomolecular condensates remains unknown. In this study, we utilized fluorescence lifetime imaging microscopy (FLIM), fluorescence recovery after photobleaching (FRAP), and microrheology techniques to investigate the ion effect on the microenvironment of condensates. We found that ions significantly affect the microenvironment of biomolecular condensates: salting-in ions increase micropolarity and reduce the microviscosity of the condensate, while salting-out ions induce opposing effects. Furthermore, we manipulate the miscibility and multilayering behavior of condensates through ion-specific effects. In summary, our work provides the first quantitative survey of the microenvironment of protein condensates in the presence of ions from the Hofmeister series, demonstrating how ions impact micropolarity, microviscosity, and viscoelasticity of condensates. Our results bear implications on how membrane-less organelles would exhibit varying microenvironments in the presence of continuously changing cellular conditions.

中文翻译：

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离子对生物分子凝聚体微环境的影响

蛋白质和 RNA 等生物分子可以通过液-液相分离 (LLPS) 组织形成具有不同微环境的冷凝物。最近的工作表明，生物分子凝聚体的微环境在介导生物活动中起着至关重要的作用，例如生物分子的分配和多相凝聚体的亚相组织。遵循霍夫迈斯特级数，离子可以影响 LLPS 的相变点。然而，离子对生物分子凝聚体微环境的特异性影响仍然未知。在本研究中，我们利用荧光寿命成像显微镜（FLIM）、光漂白后荧光恢复（FRAP）和微流变学技术来研究离子对冷凝物微环境的影响。我们发现离子显着影响生物分子凝聚体的微环境：盐析离子会增加微极性并降低凝聚体的微粘度，而盐析离子则会产生相反的作用。此外，我们通过离子特异性效应来控制缩合物的混溶性和多层行为。总之，我们的工作首次对霍夫迈斯特系列离子存在下的蛋白质凝聚体微环境进行了定量研究，展示了离子如何影响凝聚体的微极性、微粘度和粘弹性。我们的结果对无膜细胞器在不断变化的细胞条件下如何表现出不同的微环境具有影响。