Applied Science and Technology Graduate Group, University of California, Berkeley, CA, USA; California Institute for Quantitative Biosciences, University of California, Berkeley, CA, USA; The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
California Institute for Quantitative Biosciences, University of California, Berkeley, CA, USA; Howard Hughes Medical Institute, University of California, Berkeley, CA, USA.
Mol Cell. 2022 Aug 18;82(16):3000-3014.e9. doi: 10.1016/j.molcel.2022.06.032. Epub 2022 Jul 30.
It has been proposed that the intrinsic property of nucleosome arrays to undergo liquid-liquid phase separation (LLPS) in vitro is responsible for chromatin domain organization in vivo. However, understanding nucleosomal LLPS has been hindered by the challenge to characterize the structure of the resulting heterogeneous condensates. We used cryo-electron tomography and deep-learning-based 3D reconstruction/segmentation to determine the molecular organization of condensates at various stages of LLPS. We show that nucleosomal LLPS involves a two-step process: a spinodal decomposition process yielding irregular condensates, followed by their unfavorable conversion into more compact, spherical nuclei that grow into larger spherical aggregates through accretion of spinodal materials or by fusion with other spherical condensates. Histone H1 catalyzes more than 10-fold the spinodal-to-spherical conversion. We propose that this transition involves exposure of nucleosome hydrophobic surfaces causing modified inter-nucleosome interactions. These results suggest a physical mechanism by which chromatin may transition from interphase to metaphase structures.
有人提出，核小体阵列在体外进行液-液相分离（LLPS）的固有特性是体内染色质域组织的原因。然而，理解核小体的 LLPS 一直受到挑战，因为难以描述由此产生的异质凝聚物的结构。我们使用冷冻电子断层扫描和基于深度学习的 3D 重建/分割来确定 LLPS 各个阶段的凝聚物的分子组织。我们表明，核小体的 LLPS 涉及一个两步过程：一个旋节分解过程产生不规则的凝聚物，然后它们不利地转化为更紧凑的球形核，通过旋节材料的 accretion 或与其他球形凝聚物融合，这些球形核生长成更大的球形聚集体。组蛋白 H1 催化旋节到球形的转化超过 10 倍。我们提出，这种转变涉及暴露核小体疏水面，导致修饰的核小体间相互作用。这些结果表明了染色质可能从间相到中期结构转变的物理机制。
