Conformational requirements for Ca²⁺/calmodulin binding and activation of myosin light chain kinase

Myosin light chain kinase contains a regulatory segment consisting of an autoinhibitory region and a calmodulin-binding sequence that folds back on its catalytic core to inhibit kinase activity. It has been proposed that alpha-helix formation may be involved in displacement of the regulatory segment and activation of the kinase by Ca(2+)/calmodulin. Proline mutations were introduced at putative non-interacting residues in the regulatory segment to disrupt helix formation. Substitution of proline residues immediately N-terminal of the Trp in the calmodulin-binding sequence had most significant effects on Ca(2+)/calmodulin binding and activation. Formation of an alpha-helix in this region upon Ca(2+)/calmodulin binding may be necessary for displacement of the regulatory segment allowing phosphorylation of myosin regulatory light chain.