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The solution structure of Ca²⁺-free calmodulin has been determined by NMR spectroscopy, and is compared to the previously reported structure of the Ca²⁺-saturated form. The removal of Ca²⁺ causes the interhelical angles of four EF-hand motifs to increase by 36°–44°. This leads to major changes in surface properties, including the closure of the deep hydrophobic cavity essential for target protein recognition. Concerted movements of helices A and D with respect to B and C, and of helices E and H with respect to F and G are likely responsible for the cooperative Ca²⁺-binding property observed between two adjacent EF-hand sites in the amino- and carboxy-terminal domains.