LAUSR

The DNA mismatch repair endonuclease MutL consists of N‐terminal ATPase and C‐terminal endonuclease domains. The endonuclease domain binds zinc ion, although the ion seems not to function as a catalytic metal ion. Here, we solved the crystal structures of the Aquifex aeolicus MutL (aqMutL) endonuclease domain complexed with a single and three zinc ions. Differences between the two structures show that binding of multiple zinc ions induces a closed‐to‐open conformational change at the catalytic site. It is also revealed that the three‐zinc‐bound form of the endonuclease domain exhibits higher endonuclease activity than the single‐zinc‐bound form. These results indicate that multiple zinc ions are required for the proper folding of the endonuclease domain, which would facilitate the endonuclease activity of aqMutL.