Binding study of metal ions to S100 protein: 43Ca, 25Mg, 67Zn and 39K n.m.r

The interactions of the S100 protein (S100) with metal cations such as Ca2+, Mg2+, Zn2+ and K+ were studied by the metal n.m.r. spectroscopy. The line widths of 43Ca, 25Mg, 67Zn and 39K n.m.r. markedly increased by adding all S100s. A broad 43Ca n.m.r. band of Ca(2+)-S100a solution was not affected by Zn2+ and K+, while it was greatly decreased by adding Mg2+. The 43Ca n.m.r. spectra of Ca(2+)-S100a0 and -S100b solutions consisted of two slow-exchangeable signals which corresponded to Ca2+ bound to two environmentally different sites of the S100a0. These two 43Ca n.m.r. signals were not affected by Zn2+ and K+. The line width of broad 25Mg n.m.r. band of the Mg(2+)-S100 solution greatly decreased by adding Ca2+, while it did not change by adding Zn2+ and K+. Further, the addition of Ca2+, Mg2+ and K+ did not affect the line width of the 67Zn n.m.r. of the Zn(2+)-S100 solutions. These findings suggest that: (1) Mg2+ binds to all S100s, and at least one of the Mg2+ binding sites of S100 molecule is the same as the Ca2+ binding site; (2) Zn2+ binds to S100s, although the binding site(s) is/are different from Ca(2+)- or Mg(2+)-binding site(s), and the environment of Zn2+ nuclei will not change even though Ca2+ binds to S100s.