Department of Structural Biology, St. Jude Children's Research Hospital, Memphis, TN, 38105, USA.
Integrative Biomedical Sciences Program, University of Tennessee Health Sciences Center, Memphis, TN, 38163, USA.
Nat Commun. 2018 Feb 26;9(1):842. doi: 10.1038/s41467-018-03255-3.
Nucleophosmin (NPM1) is an abundant, oligomeric protein in the granular component of the nucleolus with roles in ribosome biogenesis. Pentameric NPM1 undergoes liquid-liquid phase separation (LLPS) via heterotypic interactions with nucleolar components, including ribosomal RNA (rRNA) and proteins which display multivalent arginine-rich linear motifs (R-motifs), and is integral to the liquid-like nucleolar matrix. Here we show that NPM1 can also undergo LLPS via homotypic interactions between its polyampholytic intrinsically disordered regions, a mechanism that opposes LLPS via heterotypic interactions. Using a combination of biophysical techniques, including confocal microscopy, SAXS, analytical ultracentrifugation, and single-molecule fluorescence, we describe how conformational changes within NPM1 control valency and switching between the different LLPS mechanisms. We propose that this newly discovered interplay between multiple LLPS mechanisms may influence the direction of vectorial pre-ribosomal particle assembly within, and exit from the nucleolus as part of the ribosome biogenesis process.
核仁磷酸蛋白（Nucleophosmin，NPM1）是核仁颗粒成分中一种丰富的寡聚蛋白，在核糖体生物发生中具有多种作用。五聚体 NPM1 通过与核仁成分（包括核糖体 RNA（rRNA）和具有多价精氨酸丰富线性基序（R-motif）的蛋白质）的异质相互作用发生液-液相分离（liquid-liquid phase separation，LLPS），并成为液态核仁基质的组成部分。在这里，我们表明 NPM1 还可以通过其多聚两性的无规卷曲区域的同源相互作用发生 LLPS，这一机制与通过异质相互作用发生的 LLPS 机制相反。我们使用了一系列生物物理技术，包括共聚焦显微镜、小角 X 射线散射（small-angle X-ray scattering，SAXS）、分析超速离心和单分子荧光，描述了 NPM1 中的构象变化如何控制价数以及不同 LLPS 机制之间的转换。我们提出，这种新发现的多个 LLPS 机制之间的相互作用可能会影响核糖体生物发生过程中前核糖体颗粒在核仁内的定向组装和从核仁中的输出。
