Sister chromatid cohesion, which is essential for mitosis, is mediated by a multi-subunit\ud protein complex called cohesin whose Scc1, Smc1, and Smc3 subunits form a tripartite\ud ring structure. It has been proposed that cohesin holds sister DNAs together by trapping\ud them inside its ring. To test this, we used site-specific cross-linking to create chemical\ud connections at the three interfaces between the ring’s three constituent polypeptides,\ud thereby creating covalently closed cohesin rings. As predicted by the ring entrapment\ud model, this procedure produces dimeric DNA/cohesin structures that are resistant to\ud protein denaturation. We conclude that cohesin rings concatenate individual sister\ud minichromosome DNAs
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