Calmodulin-peptide interactions: Apocalmodulin binding to the myosin light chain kinase target-site

Noncovalent binding of the synthetic peptide RS20 to calmodulin in the presence of calcium was confirmed by electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry to form a complex with a 1:1:4 calmodulin/RS20/calcium stoichiometry. There was no evidence for formation of a calmodulin-RS20-Ca-2 species, The absence of calmodulin-RS20-Ca-2 would be consistent with models in which the two globular domains are coupled functionally. There was evidence that calmodulin, RS20-calmodulin without associated calcium, and calmodulin-RS20-Ca-4 existed together in solution, whereas calmodulin-calcium complexes were absent. It is proposed that calcium binding to form the calmodulin-RS20-Ca-4 complex occurs after an initial RS20-calmodulin binding event, and serves to secure the target within the calmodulin structure. The binding of more than one RS20 molecule to calmodulin was observed to induce unfolding of calmodulin