Chromosome Segregation Laboratory, The Francis Crick Institute, London NW1 1AT, UK.
Mechanobiology and Biophysics Laboratory, The Francis Crick Institute, London NW1 1AT, UK; Department of Physics and Astronomy, University College London, London WC1E 6BT, UK.
Mol Cell. 2023 Nov 2;83(21):3787-3800.e9. doi: 10.1016/j.molcel.2023.09.019. Epub 2023 Oct 10.
Condensin is a structural maintenance of chromosomes (SMC) complex family member thought to build mitotic chromosomes by DNA loop extrusion. However, condensin variants unable to extrude loops, yet proficient in chromosome formation, were recently described. Here, we explore how condensin might alternatively build chromosomes. Using bulk biochemical and single-molecule experiments with purified fission yeast condensin, we observe that individual condensins sequentially and topologically entrap two double-stranded DNAs (dsDNAs). Condensin loading transitions through a state requiring DNA bending, as proposed for the related cohesin complex. While cohesin then favors the capture of a second single-stranded DNA (ssDNA), second dsDNA capture emerges as a defining feature of condensin. We provide complementary in vivo evidence for DNA-DNA capture in the form of condensin-dependent chromatin contacts within, as well as between, chromosomes. Our results support a "diffusion capture" model in which condensin acts in mitotic chromosome formation by sequential dsDNA-dsDNA capture.
凝聚素是一种结构维持染色体（SMC）复合物家族成员，被认为通过 DNA 环挤出构建有丝分裂染色体。然而，最近描述了一些不能挤出环但在染色体形成中效率很高的凝聚素变体。在这里，我们探讨了凝聚素如何替代地构建染色体。使用纯化的裂殖酵母凝聚素进行批量生化和单分子实验，我们观察到单个凝聚素依次拓扑地捕获两个双链 DNA（dsDNA）。凝聚素加载通过需要 DNA 弯曲的状态进行转换，这与相关的黏连蛋白复合物的提议一致。虽然黏连蛋白随后有利于捕获第二个单链 DNA（ssDNA），但第二个 dsDNA 捕获成为凝聚素的一个定义特征。我们以凝聚素依赖性染色质接触的形式提供了体内 DNA-DNA 捕获的补充证据，这些接触存在于染色体内部和之间。我们的结果支持了一种“扩散捕获”模型，其中凝聚素通过顺序的 dsDNA-dsDNA 捕获在有丝分裂染色体形成中发挥作用。
