Studying the binding specificity of MNase to divalent ions with MicroScale Thermophoresis
The endo-exonuclease MNase integrated in the extracellular membrane of Staphylococcus aureus digests singlestranded nucleic acids and shows its highest activity in presence of Ca2+ ions. We used MicroScale Thermophoresis to measure the binding of the MNase to Ca2+ ions and compared the binding affinity to different divalent and monovalent ions. Figure 1 shows the binding analysis of the MNase towards Ca2+, Mg2+, Mn2+, Zn2+ and Na+ ions.
MNase exhibits a strong binding affinity to divalent ions. MNase binds to Mg2+ ions with a KD value of 9.9 + 5.4 mM, to Mn2+ ions with a KD value of 0.9 + 0.4 mM, and to Zn2+ ions with a KD value of 0.2 + 0.1 mM. However the highest activity of the MNase depends on the binding of Ca2+ ions, to which it binds with a Kd value of 3.6 + 0.9 mM. MNase presents a lower binding affinity to monovalent ions. The study provides an example that MicroScale Thermophoresis allows to measure binding of ions to proteins.