Department of Biochemistry and Biophysics, University of California, San Francisco, California 94158, USA.
Nature. 2009 Dec 24;462(7276):1016-21. doi: 10.1038/nature08621.
Evenly spaced nucleosomes directly correlate with condensed chromatin and gene silencing. The ATP-dependent chromatin assembly factor (ACF) forms such structures in vitro and is required for silencing in vivo. ACF generates and maintains nucleosome spacing by constantly moving a nucleosome towards the longer flanking DNA faster than the shorter flanking DNA. How the enzyme rapidly moves back and forth between both sides of a nucleosome to accomplish bidirectional movement is unknown. Here we show that nucleosome movement depends cooperatively on two ACF molecules, indicating that ACF functions as a dimer of ATPases. Further, the nucleotide state determines whether the dimer closely engages one or both sides of the nucleosome. Three-dimensional reconstruction by single-particle electron microscopy of the ATPase-nucleosome complex in an activated ATP state reveals a dimer architecture in which the two ATPases face each other. Our results indicate a model in which the two ATPases work in a coordinated manner, taking turns to engage either side of a nucleosome, thereby allowing processive bidirectional movement. This novel dimeric motor mechanism differs from that of dimeric motors such as kinesin and dimeric helicases that processively translocate unidirectionally and reflects the unique challenges faced by motors that move nucleosomes.
均匀间隔的核小体与浓缩染色质和基因沉默直接相关。ATP 依赖性染色质组装因子（ACF）在体外形成这种结构，并且是体内沉默所必需的。ACF 通过不断将核小体朝着较长的侧翼 DNA 移动，使其移动速度快于较短的侧翼 DNA，从而产生和维持核小体的间隔。酶如何在核小体的两侧快速来回移动以实现双向运动尚不清楚。在这里，我们表明核小体运动依赖于两个 ACF 分子的协同作用，表明 ACF 作为 ATP 酶二聚体发挥作用。此外，核苷酸状态决定二聚体是否紧密结合核小体的一侧或两侧。通过对激活的 ATP 状态下的 ATP 酶-核小体复合物进行单颗粒电子显微镜重建，揭示了二聚体结构，其中两个 ATP 酶彼此相对。我们的结果表明了一种模型，其中两个 ATP 酶以协调的方式工作，轮流与核小体的任一侧结合，从而允许进行连续的双向运动。这种新的二聚体马达机制与驱动蛋白和二聚体解旋酶等二聚体马达不同，后者可连续进行单向转运，反映了移动核小体的马达所面临的独特挑战。
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