The effects of metal ions on the structure and stability of the DNA gyrase B protein

The effects of mono- and divalent metal ions on the DNA gyrase B subunit, on its 43 kDa and 47 kDa domains, and on two mutants in the Toprim domain (D498A and D500C) were investigated by means of circular dichroism and protein melting experiments. Both types of metal ion, with the notable exception of Mn2C, did not affect the conformational properties of the enzyme subunit at room temperature, but were able to produce selective and differential effects on protein stability. In particular, monovalent (KC) ions increased the stability of the gyrase B structure, whereas destabilising effects were most prominent using Mn2C as the metal ion. Ca2C and Mg2C produced comparable changes in the gyrase B melting profile. Additionally, we found that monovalent (KC) ions were more effective in the 43 kDa N-terminal domainwhereATP binding occurs, whereas divalent ions caused large modifications in the conformational stability of the 47 kDa C-terminal domain. Our results on gyrase B mutants indicate that D498 interacts with Mn2C, whereas it has little effect on the binding of the other ions tested. A D500C mutation, in contrast, effectively impairs Mg2C affinity, suggesting effective contacts between this ion and D500 in the wild-type enzyme. Hence, the sites of metal ion complexation within the Toprim domain are modulated by the nature of the ion species. These results suggest a double role played by metal ions in the catalytic steps involving DNA gyrase B. One has to do with direct involvement of cations complexed to the Toprim domain in the DNA cutting-rejoining process, the other, until now overlooked, is connected to the dramatic changes in protein flexibility produced by ion binding, which reduces the energy required for the huge conformational changes essential for the catalytic cycle to occur.