Investigating potential functions of nuclear and cytoplasmic condensation of the SMN complex in RNP homeostasis

大分子复合体可以完成广泛的细胞任务。它们由不同的蛋白质或蛋白质和核酸组成，它们的忠实组装往往需要反式作用因子的帮助。剪接体USnRNPs代表了一组经过充分研究的丰富的大分子复合体，它们在体内进行辅助组装。它们在SMN-复合体的指导下形成，SMN-复合体本身构成一个大分子实体，在核和细胞质无膜室中可逆地凝聚。虽然我们在生化水平上了解了SmN-复合体在U-SnRNP中的核心功能，但它的时空调节，它在细胞内稳态中的潜在作用以及它的凝聚的功能意义仍然知之甚少。在这个应用中，我们建议研究SMN复合体响应细胞信号的相分离及其对UsnRNP动态平衡的影响，并作为生物合成途径的调节器。在第一部分中，提出了关注核SMN-复合体作用的实验。我们将检验这一假设，即聚集在Cajal小体中的核SMN-Complex作为周转中心，在那里调节细胞USnRNP的稳定水平和/或缺陷USnRNP的移除。在第二部分中，我们将利用先前发现的控制SMN凝聚的信号线索来评估细胞质SMN凝聚体在应激条件下调节U SnRNP组装的功能，以及SMN复合体在翻译控制中的新功能。我们的研究不仅将揭开SMN复合体的生物合成功能是如何整合到细胞信号和应激反应中的，而且还将为我们深入了解相分离如何启用和调节SMN复合体的新功能。

Macromolecular complexes accomplish a wide spectrum of cellular tasks. Being composed of different proteins or proteins and nucleic acids, their faithful assembly often requires assistance by trans-acting factors. Spliceosomal UsnRNPs represent a well-studied group of abundant macromolecular complexes that undergo assisted assembly in vivo. They form under the guidance of the SMN-complex, which itself constitutes a macromolecular entity that reversibly condenses in nuclear and cytoplasmatic membraneless compartments. While we comprehend core functions of the SMN-complex in U snRNP at biochemical level, its spatial and temporal regulation, its potential roles in cellular UsnRNP homeostasis and the functional significance of its condensation remain only poorly understood. In this application we propose to investigate phase separation of the SMN complex in response to cellular signaling and its impact for UsnRNP homeostasis and as a regulator for biosynthetic pathways. In the first part, experiments are proposed that focus on the role of the nuclear SMN-complex. We will test the hypothesis that nuclear SMN-complexes, condensing in Cajal bodies, serve as turnover centers, where the steady state level of cellular UsnRNP and/or the removal of defective UsnRNPs are regulated. In the second part, we will exploit previously identified signaling cues that govern SMN condensation to evaluate the function of cytoplasmic SMN condensates to regulate U snRNP assembly under stress conditions but also in novel functions of the SMN complex in translational control. Our studies will not only unravel how the biosynthetic functions of the SMN complex are integrated into cell signaling and stress response but provide insight into how phase separation enables and regulates novel functions of the SMN complex.