Calcium-binding characteristics and conformational changes in metastasin, a member of S-100 protein family

Investigation of Ca2+-binding characteristics of metastasin (Mts-1) by competition with Fluo-3 revealed two types of Ca2+-binding sites in Mts-1 with the geometric mean of their dissociation constant (<(K)over bar (d)>) value of 2.6 mu M for the two EF-sites. The Hill coefficient (n(H)) is 0.98. A substantial increase in the affinity of Mts-1 for Ca2+ and strong cooperative character of binding (<(K)over bar (d)> = 0.2 mu M, n(H) = 1.91) is observed in the presence of the target protein p37 isolated from mouse adenocarcinoma cell lines CSML-100 and CSML-0. Two different hydrophobic sites of binding with the fluorescent probe 2-(p-toluidino) naphthalene-6-sulfonate (TNS) per Mts-1 molecule have been determined. The exposure of the hydrophobic binding sites of the first type are shown to be Ca2+-dependent and the hydrophobic binding sites of the second type are exposed independently of Ca2+ concentration. A decrease in the number of Ca2+-dependent hydrophobic centers in the presence of p37 protein was detected by measurements of TNS fluorescence.