Low-complexity protein domains promote the formation of various biomolecular condensates. However, in many cases, the precise sequence features governing condensate formation and identity remain unclear. Here, we investigate the role of intrinsically disordered mixed-charge domains (MCDs) in nuclear speckle condensation. Proteins composed exclusively of arginine-aspartic acid dipeptide repeats undergo length-dependent condensation and speckle incorporation. Substituting arginine with lysine in synthetic and natural speckle-associated MCDs abolishes these activities, identifying a key role for multivalent contacts through arginine's guanidinium ion. MCDs can synergize with a speckle-associated RNA recognition motif to promote speckle specificity and residence. MCD behavior is tunable through net-charge: increasing negative charge abolishes condensation and speckle incorporation. Contrastingly, increasing positive charge through arginine leads to enhanced condensation, speckle enlargement, decreased splicing factor mobility, and defective mRNA export. Together, these results identify key sequence determinants of MCD-promoted speckle condensation and link the dynamic material properties of speckles with function in mRNA processing.

中文翻译：

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富含精氨酸的混合电荷域为核斑点凝聚提供凝聚力。

低复杂性蛋白质结构域促进各种生物分子缩合物的形成。然而，在许多情况下，控制冷凝物形成和身份的精确序列特征仍不清楚。在这里，我们研究了本质无序的混合电荷域（MCD）在核散斑凝聚中的作用。仅由精氨酸-天冬氨酸二肽重复组成的蛋白质经历长度依赖性缩合和斑点掺入。在合成和天然斑点相关 MCD 中用赖氨酸取代精氨酸消除了这些活性，从而确定了通过精氨酸的胍离子进行多价接触的关键作用。MCD 可以与斑点相关 RNA 识别基序协同作用，以促进斑点特异性和驻留。MCD 行为可通过净电荷调节：增加负电荷消除了凝结和散斑合并。相反，通过精氨酸增加正电荷会导致凝聚增强、斑点扩大、剪接因子迁移率降低和 mRNA 输出缺陷。总之，这些结果确定了 MCD 促进的斑点凝聚的关键序列决定因素，并将斑点的动态材料特性与 mRNA 处理的功能联系起来。