[The domain organization of calmodulin].

Differential scanning microcalorimetry was used to study the domain organization of calmodulin and its fragments obtained by trypsin and thrombin treatment of the protein. It has been shown that (1) at physiological concentrations of Ca2+ ions (10(-6) divided by 10(-5) M) the protein structure represents three cooperative blocks, one of which contains two Ca2+-binding domains and the two others contain one Ca2+-binding domain; (2) stability of the cooperative blocks strongly depends on the Ca2+ concentration and in the presence of 2 mM EDTA the cooperative block containing Ca2+-binding domain III melts already at room temperature; (3) in the absence of Ca2+ ions the addition of Mg2+ or Na+ ions to the buffer system (to the concentration of 2 mM and 150 mM, respectively) does not lead to stabilization of the cooperative structure of the block containing Ca2+-binding domain III; (4) judging by thermodynamic parameters of melting, the structure of cooperative blocks within the peptides coincides with their structure in the intact molecule.