Cation binding sites on actin: a structural relationship between antigenic epitopes and cation exchange.

Divalent cations such as Mg2+ and Ca2+, which bind specifically to actin, induce conformational changes that affect its antigenic structure. The distribution of antigenic epitopes on the sequence shows that these structural modifications involve epitopes related to monomer-monomer interfaces. In the N-terminal part, the 1-7 acidic extremity is not affected, in contrast with sequence 18-28. The ability of polycations such as diamine to modify the actin structure at concentrations below 0.1 microM strengthens the hypothesis that in vivo these compounds act locally and specifically on actin polymerization.