Membrane-associated protein phase separation plays critical roles in cell biology, driving essential cellular phenomena from immune signaling to membrane traffic. Importantly, by reducing dimensionality from three to two dimensions, lipid bilayers can nucleate phase separation at far lower concentrations compared with those required for phase separation in solution. How might other intracellular lipid substrates, such as lipid droplets, contribute to nucleation of phase separation? Distinct from bilayer membranes, lipid droplets consist of a phospholipid monolayer surrounding a core of neutral lipids, and they are energy storage organelles that protect cells from lipotoxicity and oxidative stress. Here, we show that intrinsically disordered proteins can undergo phase separation on the surface of synthetic and cell-derived lipid droplets. Specifically, we find that the model disordered domains FUS LC and LAF-1 RGG separate into protein-rich and protein-depleted phases on the surfaces of lipid droplets. Owing to the hydrophobic nature of interactions between FUS LC proteins, increasing ionic strength drives an increase in its phase separation on droplet surfaces. The opposite is true for LAF-1 RGG, owing to the electrostatic nature of its interprotein interactions. In both cases, protein-rich phases on the surfaces of synthetic and cell-derived lipid droplets demonstrate molecular mobility indicative of a liquid-like state. Our results show that lipid droplets can nucleate protein condensates, suggesting that protein phase separation could be key in organizing biological processes involving lipid droplets.

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脂滴作为蛋白质相分离的底物

膜相关蛋白相分离在细胞生物学中发挥着关键作用，驱动着从免疫信号传导到膜运输的重要细胞现象。重要的是，通过将维度从三维降低到二维，与溶液中相分离所需的浓度相比，脂质双层可以以低得多的浓度使相分离成核。其他细胞内脂质底物（例如脂滴）如何促进相分离的成核？与双层膜不同，脂滴由围绕中性脂质核心的磷脂单层组成，它们是保护细胞免受脂毒性和氧化应激的能量储存细胞器。在这里，我们证明本质上无序的蛋白质可以在合成和细胞衍生的脂滴表面上发生相分离。具体来说，我们发现模型无序结构域 FUS LC 和 LAF-1 RGG 在脂滴表面分离成富含蛋白质和蛋白质耗尽的相。由于 FUS LC 蛋白之间相互作用的疏水性，离子强度的增加会导致其在液滴表面的相分离增加。 LAF-1 RGG 的情况正好相反，因为其蛋白质间相互作用的静电性质。在这两种情况下，合成脂滴和细胞来源的脂滴表面上的富含蛋白质的相都表现出表明液体状态的分子流动性。我们的结果表明，脂滴可以使蛋白质凝聚物成核，这表明蛋白质相分离可能是组织涉及脂滴的生物过程的关键。