Cadmium inhibits mismatch repair by blocking the ATPase activity of the MSH2–MSH6 complex

Cadmium (Cd(2+)) is a known carcinogen that inactivates the DNA mismatch repair (MMR) pathway. In this study, we have tested the effect of Cd(2+) exposure on the enzymatic activity of the mismatch binding complex MSH2–MSH6. Our results indicate that Cd(2+) is highly inhibitory to the ATP binding and hydrolysis activities of MSH2–MSH6, and less inhibitory to its DNA mismatch binding activity. The inhibition of the ATPase activity appears to be dose and exposure time dependent. However, the inhibition of the ATPase activity by Cd(2+) is prevented by cysteine and histidine, suggesting that these residues are essential for the ATPase activity and are targeted by Cd(2+). A comparison of the mechanism of inhibition with N-ethyl maleimide, a sulfhydryl group inhibitor, indicates that this inhibition does not occur through direct inactivation of sulfhydryl groups. Zinc (Zn(2+)) does not overcome the direct inhibitory effect of Cd(2+) on the MSH2–MSH6 ATPase activity in vitro. However, the increase in the mutator phenotype of yeast cells exposed to Cd(2+) was prevented by excess Zn(2+), probably by blocking the entry of Cd(2+) into the cell. We conclude that the inhibition of MMR by Cd(2+) is through the inactivation of the ATPase activity of the MSH2–MSH6 heterodimer, resulting in a dominant negative effect and causing a mutator phenotype.