Centrin is a member of the EF-hand super family that plays critical role in the centrosome duplication and separation. To investigate the role of Asp37 in the process of metal-binding and conformational change of ciliate Euplotes octocarinatus centrin (EoCen), the mutant D37K, in which aspartic acid 37 had been replaced by lysine, was first obtained by the site-directed mutagenesis. Then 2-p-toluidinylnaphthalene-6sulfonate (TNS) was used as a fluorescence probe to detect the conformational

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... ge of the protein. The results show that the metalbinding capability of the site I of EoCen was lost by the mutation of Asp37→Lys. In comparison the Tb 3+ -saturated EoCen, the hydrophobic surface of D37K, which is exposed by the binding of Tb 3+ , has shrunk sharply, suggesting that Asp37 plays an important role in maintaining the proper conformation of EoCen in the presence of Tb 3+ . Meanwhile, the conditional binding constants of TNS with Tb 3+ -loaded EoCen and D37K were obtained, K(Tb 3+ -EoCen-TNS)=(7.38 ±0.25)×10 5 M -1 and K(Tb 3+ -D37K-TNS)=(1.16± 0.05)×10 6 M -1 .