Department of Biochemistry, Gene Center, LMU Munich, Munich, Germany.
Cryo EM Facility, Max Planck Institute of Biochemistry, Martinsried, Germany.
Nat Struct Mol Biol. 2018 Jan;25(1):101-108. doi: 10.1038/s41594-017-0005-5. Epub 2017 Dec 11.
Nucleosomes, the basic units of chromatin, package and regulate expression of eukaryotic genomes. Although the structure of the intact nucleosome is well characterized, little is known about structures of partially unwrapped, transient intermediates. In this study, we present nine cryo-EM structures of distinct conformations of nucleosome and subnucleosome particles. These structures show that initial DNA breathing induces conformational changes in the histone octamer, particularly in histone H3, that propagate through the nucleosome and prevent symmetrical DNA opening. Rearrangements in the H2A-H2B dimer strengthen interaction with the unwrapping DNA and promote nucleosome stability. In agreement with this, cross-linked H2A-H2B that cannot accommodate unwrapping of the DNA is not stably maintained in the nucleosome. H2A-H2B release and DNA unwrapping occur simultaneously, indicating that DNA is essential in stabilizing the dimer in the nucleosome. Our structures reveal intrinsic nucleosomal plasticity that is required for nucleosome stability and might be exploited by extrinsic protein factors.
核小体是染色质的基本单位，可对真核基因组的表达进行包装和调控。尽管完整核小体的结构已得到很好的描述，但人们对部分展开的瞬态中间产物的结构知之甚少。在这项研究中，我们呈现了九个不同构象的核小体和亚核小体颗粒的低温电镜结构。这些结构表明，初始的 DNA 呼吸会引起组蛋白八聚体的构象变化，特别是组蛋白 H3 的构象变化，这些变化会在核小体中传播，并阻止对称的 DNA 打开。H2A-H2B 二聚体的重排加强了与展开 DNA 的相互作用，并促进核小体的稳定性。与此一致的是，不能容纳 DNA 展开的交联 H2A-H2B 不能在核小体中稳定地维持。H2A-H2B 的释放和 DNA 的展开同时发生，表明 DNA 对于稳定二聚体在核小体中是必需的。我们的结构揭示了内在的核小体可塑性，这对于核小体的稳定性是必需的，并且可能被外在的蛋白因子所利用。
