MRC Laboratory of Medical Research, Cambridge, UK.
Spanish National Cancer Research Centre, Madrid, Spain.
Nat Struct Mol Biol. 2021 Apr;28(4):373-381. doi: 10.1038/s41594-021-00577-7. Epub 2021 Apr 5.
DNA mismatch repair detects and removes mismatches from DNA by a conserved mechanism, reducing the error rate of DNA replication by 100- to 1,000-fold. In this process, MutS homologs scan DNA, recognize mismatches and initiate repair. How the MutS homologs selectively license repair of a mismatch among millions of matched base pairs is not understood. Here we present four cryo-EM structures of Escherichia coli MutS that provide snapshots, from scanning homoduplex DNA to mismatch binding and MutL activation via an intermediate state. During scanning, the homoduplex DNA forms a steric block that prevents MutS from transitioning into the MutL-bound clamp state, which can only be overcome through kinking of the DNA at a mismatch. Structural asymmetry in all four structures indicates a division of labor between the two MutS monomers. Together, these structures reveal how a small conformational change from the homoduplex- to heteroduplex-bound MutS acts as a licensing step that triggers a dramatic conformational change that enables MutL binding and initiation of the repair cascade.
DNA 错配修复通过保守机制检测和消除 DNA 中的错配，将 DNA 复制的错误率降低 100 到 1000 倍。在这个过程中，MutS 同源物扫描 DNA，识别错配并启动修复。MutS 同源物如何在数百万个匹配碱基对中选择性地许可修复一个错配尚不清楚。本文呈现了四个大肠杆菌 MutS 的冷冻电镜结构，提供了从扫描同源双链 DNA 到通过中间状态结合错配和激活 MutL 的快照。在扫描过程中，同源双链 DNA 形成一个空间位阻，阻止 MutS 过渡到 MutL 结合的夹状态，只有在错配处 DNA 扭曲才能克服这种状态。四个结构中的所有结构的不对称性表明两个 MutS 单体之间存在分工。这些结构共同揭示了从同源双链结合到异源双链结合的 MutS 的小构象变化如何作为一个许可步骤，触发了一个剧烈的构象变化，从而使 MutL 结合并启动修复级联。
