Analysis of the role of Mg2+ on conformational change and target recognition by Ciliate Euplotes octocarinatus centrin

The binding of Mg2+ with the Euplotes octocarinatus centrin (EoCen) and the effect of Mg2+ on the binding of EoCen with the peptide melittin were examined by spectroscopic methods. In this study, it was found that Mg2+ may bind with Ca2+-binding sites, at least partly, on EoCen, which displays ∼10-fold weaker affinity than Ca2+. In the presence of Mg2+, Ca2+-saturated EoCen undergoes significant conformational changes resulting in decreased exposure of hydrophobic surfaces on the protein. Additionally, excess Mg2+ did not change the stoichiometry, but rather reduced the affinity of EoCen to melittin. The Mg2+-dependent decrease in the affinities of EoCen to melittin is an intrinsic property of Mg2+, rather than a nonspecific ionic effect. The inhibitory effect of Mg2+ on the formation of complexes between EoCen and melittin may contribute to the specificity of EoCen in target activation in response to cellular Ca2+ concentration fluctuations.