How chromatin enzymes work in condensed chromatin and how they maintain diffusional mobility inside remains unexplored. Here we investigated these challenges using the Drosophila ISWI remodeling ATPase, which slides nucleosomes along DNA. Folding of chromatin fibers did not affect sliding in vitro. Catalytic rates were also comparable in- and outside of chromatin condensates. ISWI cross-links and thereby stiffens condensates, except when ATP hydrolysis is possible. Active hydrolysis is also required for ISWI’s mobility in condensates. Energy from ATP hydrolysis therefore fuels ISWI’s diffusion through chromatin and prevents ISWI from cross-linking chromatin. Molecular dynamics simulations of a ‘monkey-bar’ model in which ISWI grabs onto neighboring nucleosomes, then withdraws from one before rebinding another in an ATP hydrolysis-dependent manner, qualitatively agree with our data. We speculate that monkey-bar mechanisms could be shared with other chromatin factors and that changes in chromatin dynamics caused by mutations in remodelers could contribute to pathologies.

染色质酶如何在浓缩染色质中发挥作用以及它们如何保持内部的扩散流动性仍有待探索。在这里，我们使用果蝇ISWI 重塑 ATP 酶（该酶沿着 DNA 滑动核小体）研究了这些挑战。染色质纤维的折叠不影响体外滑动。染色质凝聚体内部和外部的催化速率也相当。 ISWI 交联，从而使冷凝物变硬，除非可能发生 ATP 水解。 ISWI 在凝析油中的流动性也需要主动水解。因此，来自 ATP 水解的能量促进 ISWI 在染色质中的扩散，并防止 ISWI 交联染色质。 “猴棒”模型的分子动力学模拟在质量上与我们的数据一致，在该模型中，ISWI 抓住邻近的核小体，然后从一个核小体中退出，然后以 ATP 水解依赖性方式重新结合另一个核小体。我们推测猴棒机制可能与其他染色质因子共享，并且重塑者突变引起的染色质动力学变化可能导致病理学。