Department of Molecular Biosciences, Northwestern University, Evanston, IL 60208-3500, USA.
Nucleic Acids Res. 2013 Jul;41(12):6149-60. doi: 10.1093/nar/gkt303. Epub 2013 Apr 24.
Cohesin plays a critical role in sister chromatid cohesion, double-stranded DNA break repair and regulation of gene expression. However, the mechanism of how cohesin directly interacts with DNA remains unclear. We report single-molecule experiments analyzing the interaction of the budding yeast cohesin Structural Maintenance of Chromosome (SMC)1-SMC3 heterodimer with naked double-helix DNA. The cohesin heterodimer is able to compact DNA molecules against applied forces of 0.45 pN, via a series of extension steps of a well-defined size ≈130 nm. This reaction does not require ATP, but is dependent on DNA supercoiling: DNA with positive torsional stress is compacted more quickly than negatively supercoiled or nicked DNAs. Un-nicked torsionally relaxed DNA is a poor substrate for the compaction reaction. Experiments with mutant proteins indicate that the dimerization hinge region is crucial to the folding reaction. We conclude that the SMC1-SMC3 heterodimer is able to restructure the DNA double helix into a series of loops, with a preference for positive writhe.
黏合蛋白在姐妹染色单体黏合、双链 DNA 断裂修复和基因表达调控中起着关键作用。然而，黏合蛋白如何直接与 DNA 相互作用的机制尚不清楚。我们报告了一项单分子实验，分析了芽殖酵母结构维持染色体（SMC）1-SMC3 异二聚体与裸露双链 DNA 的相互作用。黏合蛋白异二聚体能够在 0.45 pN 的外力作用下，通过一系列大小约为 130nm 的延伸步骤，压缩 DNA 分子。该反应不需要 ATP，但依赖于 DNA 超螺旋化：正扭转应力的 DNA 比负超螺旋或缺口 DNA 更快地被压缩。未缺口的扭转松弛 DNA 是压缩反应的不良底物。用突变蛋白进行的实验表明，二聚化铰链区对折叠反应至关重要。我们得出结论，SMC1-SMC3 异二聚体能够将 DNA 双螺旋重组为一系列环，偏爱正扭曲。
