T-cell restriction intracellular antigen 1 (TIA1) is an RNA-binding protein that is a major component of stress granules (SGs). The low complexity domain (LCD) of TIA1 plays a central role in facilitating SGs assembly through liquid–liquid phase separation (LLPS). Disruption of the LLPS process has been associated with several diseases. It has recently been shown that the proline-rich domain affects the LLPS process of some proteins (such as UBQLN2 and Tau). Thus, proline may regulate LLPS. The LCD of TIA1 contains 11 proline residues, and several proline-related mutations have been shown to cause amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Here, we demonstrated that TIA1 can undergo phase separation in cells. Additionally, disease-associated proline-to-leucine (P-L) mutations, which altered droplet morphology, facilitated the liquid-to-solid phase transition of TIA1 into solid-like amyloid fibrils. The changes in the physical properties of the P-L mutation altered the behavior of TIA1 in vivo and led to abnormal SGs kinetics, resulting in the formation of the pathological inclusions of ALS. Prolines are the key residues for regulating the LLPS of TIA1.

T 细胞限制性细胞内抗原 1 (TIA1) 是一种 RNA 结合蛋白，是应激颗粒 (SG) 的主要成分。TIA1 的低复杂度域 (LCD) 在通过液-液相分离 (LLPS) 促进 SGs 组装方面发挥着核心作用。LLPS 过程的中断与几种疾病有关。最近的研究表明，富含脯氨酸的结构域会影响某些蛋白质（如 UBQLN2 和 Tau）的 LLPS 过程。因此，脯氨酸可以调节 LLPS。TIA1 的 LCD 含有 11 个脯氨酸残基，一些脯氨酸相关突变已被证明会导致肌萎缩侧索硬化 (ALS) 和额颞叶痴呆 (FTD)。在这里，我们证明了 TIA1 可以在细胞中进行相分离。此外，与疾病相关的脯氨酸到亮氨酸 (PL) 突变会改变液滴形态，促进了 TIA1 的液固相转变为固体样淀粉样蛋白原纤维。PL突变物理性质的变化改变了TIA1的行为在体内并导致 SGs 动力学异常，导致 ALS 病理性包涵体的形成。脯氨酸是调节 TIA1 的 LLPS 的关键残基。