Cohesin is a key organizer of chromatin folding in eukaryotic cells. The two main activities of this ring-shaped protein complex are the maintenance of sister chromatid cohesion and the establishment of long-range DNA–DNA interactions through the process of loop extrusion. Although the basic principles of both cohesion and loop extrusion have been described, we still do not understand several crucial mechanistic details. One of such unresolved issues is the question of whether a cohesin ring topologically embraces DNA string(s) during loop extrusion. Here, we show that cohesin complexes residing on CTCF-occupied genomic sites in mammalian cells do not interact with DNA topologically. We assessed the stability of cohesin-dependent loops and cohesin association with chromatin in high-ionic-strength conditions in G1-synchronized HeLa cells. We found that increased salt concentration completely displaces cohesin from those genomic regions that correspond to CTCF-defined loop anchors. Unsurprisingly, CTCF-anchored cohesin loops also dissipate in these conditions. Because topologically engaged cohesin is considered to be salt resistant, our data corroborate a non-topological model of loop extrusion. We also propose a model of cohesin activity throughout the interphase, which essentially equates the termination of non-topological loop extrusion with topological loading of cohesin. This theoretical framework enables a parsimonious explanation of various seemingly contradictory experimental findings.

中文翻译：

---

粘连蛋白-染色质关联对高盐处理的敏感性证实了环挤出的非拓扑模式


粘连蛋白是真核细胞染色质折叠的关键组织者。这种环形蛋白质复合物的两个主要活性是维持姐妹染色单体的内聚力和通过环挤出过程建立长程 DNA-DNA 相互作用。尽管已经描述了内聚和环挤出的基本原理，但我们仍然不了解几个关键的机械细节。这些未解决的问题之一是粘连环在环挤出过程中是否在拓扑上包含 DNA 串。在这里，我们表明，存在于哺乳动物细胞中 CTCF 占据的基因组位点上的粘连蛋白复合物在拓扑上不与 DNA 相互作用。我们评估了 G1 同步 HeLa 细胞中高离子强度条件下粘连蛋白依赖性环和粘连蛋白与染色质关联的稳定性。我们发现，盐浓度的增加完全取代了与 CTCF 定义的环锚相对应的基因组区域中的粘连蛋白。不出所料，CTCF 锚定的粘连蛋白环在这些条件下也会消失。由于拓扑接合的粘连蛋白被认为是耐盐的，因此我们的数据证实了环挤出的非拓扑模型。我们还提出了整个界面的粘连蛋白活性模型，该模型本质上将非拓扑环挤出的终止与粘连蛋白的拓扑负载等同起来。这个理论框架可以对各种看似矛盾的实验结果进行简洁的解释。