Department of Systems Biology, University of Massachusetts Chan Medical School, Worcester, MA, USA.
Howard Hughes Medical Institute, Chevy Chase, MD, USA.
Nat Cell Biol. 2022 Oct;24(10):1516-1527. doi: 10.1038/s41556-022-00992-y. Epub 2022 Oct 6.
The ring-like cohesin complex mediates sister-chromatid cohesion by encircling pairs of sister chromatids. Cohesin also extrudes loops along chromatids. Whether the two activities involve similar mechanisms of DNA engagement is not known. We implemented an experimental approach based on isolated nuclei carrying engineered cleavable RAD21 proteins to precisely control cohesin ring integrity so that its role in chromatin looping could be studied under defined experimental conditions. This approach allowed us to identify cohesin complexes with distinct biochemical, and possibly structural, properties that mediate different sets of chromatin loops. When RAD21 is cleaved and the cohesin ring is opened, cohesin complexes at CTCF sites are released from DNA and loops at these elements are lost. In contrast, cohesin-dependent loops within chromatin domains that are not anchored at pairs of CTCF sites are more resistant to RAD21 cleavage. The results show that the cohesin complex mediates loops in different ways depending on the genomic context and suggests that it undergoes structural changes as it dynamically extrudes and encounters CTCF sites.
环型黏连复合物通过环绕姐妹染色单体对来介导姐妹染色单体之间的黏合。黏连复合物也沿着染色单体伸出环。这两种活性是否涉及 DNA 结合的相似机制尚不清楚。我们采用了一种基于携带工程化可切割 RAD21 蛋白的分离核的实验方法，精确控制黏连复合物环的完整性，以便在明确定义的实验条件下研究其在染色质环化中的作用。这种方法使我们能够鉴定出具有不同生化特性（可能还有结构特性）的黏连复合物，这些复合物介导不同的染色质环。当 RAD21 被切割并且黏连复合物环被打开时，CTCF 位点处的黏连复合物从 DNA 上释放，并且这些元件处的环丢失。相比之下，在未锚定在 CTCF 对的染色质域内的依赖黏连复合物的环对 RAD21 切割更具抗性。结果表明，黏连复合物通过依赖基因组环境的不同方式介导环，这表明它在动态伸出并遇到 CTCF 位点时会发生结构变化。
