TDP-43 protein travels between the cytosol and the nucleus to perform its nucleic acid binding functions through its two tandem RNA recognition motif domains (TDP-43tRRM). When exposed to various environmental stresses, it forms abnormal aggregates in the cytosol of neurons, which are the hallmarks of amyotrophic lateral sclerosis and other TDP-43 proteinopathies. However, the nature of early structural changes upon stress sensing and the consequent steps during the course of aggregation are not well understood. In this study, we show that under low-pH conditions, mimicking starvation stress, TDP-43tRRM undergoes a conformational opening reaction linked to the protonation of buried ionizable residues and grows into a metastable oligomeric assembly (called the "low-pH form" or the "L form"). In the L form, the protein molecules have disrupted tertiary structure, solvent-exposed hydrophobic patches, and mobile side chains but the native-like secondary structure remains intact. The L form structure is held by weak interactions and has a steep dependence on ionic strength. In the presence of as little as 15 mM KCl, it fully misfolds and further oligomerizes to form a β-sheet rich "β form" in at least two distinct steps. The β form has an ordered, stable structure that resembles worm-like amyloid fibrils. The unstructured regions of the protein gain structure during L ⇌ β conversion. Our results suggest that TDP-43tRRM could function as a stress sensor and support a recent model in which stress sensing during neurodegeneration occurs by assembly of proteins into metastable assemblies that are precursors to the solid aggregates.

中文翻译：

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TDP-43核酸结合域的蠕虫样淀粉样原纤维的应力诱导形成过程中的早期亚稳态组装。

TDP-43蛋白通过其两个串联的RNA识别基序域（TDP-43tRRM）在细胞质和细胞核之间传播，以执行其核酸结合功能。当暴露于各种环境压力下时，它会在神经元的胞质溶胶中形成异常聚集，这是肌萎缩性侧索硬化症和其他TDP-43蛋白病的标志。然而，对于应力感应的早期结构变化的性质及其在聚集过程中的后续步骤尚不十分了解。在这项研究中，我们表明，在低pH条件下，模拟饥饿压力，TDP-43tRRM经历了与被掩埋的可离子化残基的质子化相关的构象开放反应，并生长为亚稳态的寡聚体组装体（称为“低pH形式”或“ L形式”）。以L形式 蛋白质分子破坏了三级结构，暴露于溶剂的疏水性斑块和可移动的侧链，但天然的二级结构仍然完好无损。L型结构通过弱相互作用保持，并且对离子强度的依赖性强。在低至15 mM KCl的存在下，它至少在两个不同的步骤中完全错误折叠并进一步低聚形成富含β-折叠的“β形式”。β形式具有类似于蠕虫状淀粉样蛋白原纤维的有序稳定结构。L⇌β转化过程中蛋白质的非结构化区域获得了结构。我们的结果表明，TDP-43tRRM可以用作压力传感器并支持最近的模型，在该模型中，通过将蛋白质组装成亚稳态组装体（是固体聚集体的前体），在神经退行性过程中进行应力传感。