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¹¹³Cd-NMR experiments were performed to characterize the nature of Cd²⁺ binding to calmodulin in the presence of a tetradecapeptide mastoparan or a 26-residue peptide M13 (calmodulin-binding region of skeletal muscle myosin light-chain kinase). The results indicate that binding of these peptides to calmodulin induces a positive cooperativity between Ca²⁺ binding to C- and N-terminal domains. The results imply that the activation of myosin light-chain kinase caused by the increase in Ca²⁺ concentration occurs as a result of cooperative interactions not only between two Ca²⁺ binding sites in each domain but also between the two domains. The inter-domain interaction manifests itself only in the presence of such peptides.