Laboratory 4, National Centre for Biological Sciences, Bangalore, India.
PLoS One. 2010 Oct 28;5(10):e13726. doi: 10.1371/journal.pone.0013726.
The mismatch repair (MMR) pathway serves to maintain the integrity of the genome by removing mispaired bases from the newly synthesized strand. In E. coli, MutS, MutL and MutH coordinate to discriminate the daughter strand through a mechanism involving lack of methylation on the new strand. This facilitates the creation of a nick by MutH in the daughter strand to initiate mismatch repair. Many bacteria and eukaryotes, including humans, do not possess a homolog of MutH. Although the exact strategy for strand discrimination in these organisms is yet to be ascertained, the required nicking endonuclease activity is resident in the C-terminal domain of MutL. This activity is dependent on the integrity of a conserved metal binding motif. Unlike their eukaryotic counterparts, MutL in bacteria like Neisseria exist in the form of a homodimer. Even though this homodimer would possess two active sites, it still acts a nicking endonuclease. Here, we present the crystal structure of the C-terminal domain (CTD) of the MutL homolog of Neisseria gonorrhoeae (NgoL) determined to a resolution of 2.4 Å. The structure shows that the metal binding motif exists in a helical configuration and that four of the six conserved motifs in the MutL family, including the metal binding site, localize together to form a composite active site. NgoL-CTD exists in the form of an elongated inverted homodimer stabilized by a hydrophobic interface rich in leucines. The inverted arrangement places the two composite active sites in each subunit on opposite lateral sides of the homodimer. Such an arrangement raises the possibility that one of the active sites is occluded due to interaction of NgoL with other protein factors involved in MMR. The presentation of only one active site to substrate DNA will ensure that nicking of only one strand occurs to prevent inadvertent and deleterious double stranded cleavage.
错配修复（MMR）途径通过从新合成的链上去除错配碱基来维持基因组的完整性。在大肠杆菌中，MutS、MutL 和 MutH 通过一种涉及新链上缺乏甲基化的机制来协调区分子链。这促进了 MutH 在子链上创建一个切口以启动错配修复。许多细菌和真核生物，包括人类，都没有 MutH 的同源物。尽管这些生物体中链区分的精确策略尚未确定，但所需的切口内切酶活性存在于 MutL 的 C 末端结构域中。这种活性依赖于保守金属结合基序的完整性。与真核生物不同，细菌中的 MutL 以同源二聚体的形式存在。尽管这种同源二聚体具有两个活性位点，但它仍然是一种切口内切酶。在这里，我们展示了淋病奈瑟菌（NgoL）MutL 同源物的 C 末端结构域（CTD）的晶体结构，分辨率为 2.4 Å。该结构表明，金属结合基序存在于螺旋构象中，MutL 家族的六个保守基序中的四个，包括金属结合位点，共同定位在一起形成一个复合活性位点。NgoL-CTD 以长的倒置同源二聚体形式存在，由富含亮氨酸的疏水性界面稳定。倒置排列使每个亚基中的两个复合活性位点位于同源二聚体的相对侧面上。这种排列增加了一个活性位点由于与参与 MMR 的其他蛋白质因子的相互作用而被阻塞的可能性。只有一个活性位点呈现给底物 DNA 将确保只有一条链被切割，以防止意外和有害的双链断裂。
