The interaction between calmodulin and its target protein is a key step in many calcium-regulated cellular functions. Melittin binds tightly to calmodulin in the presence of calcium and is a competitive inhibitor of calmodulin function. Using melittin as a model for the target peptide of calmodulin, we have found a large Ca2+-dependent conformational change of calmodulin in solution induced by peptide binding. Mg2+ does not substitute for Ca2+ in producing the conformation change. Small-angle x-ray scattering has shown that calmodulin exists as a dumbbell in solution, similar to that observed in the crystalline state. Our present measurements reveal that the overall structure of the Ca2+-calmodulin-melittin complex is not a dumbbell but a globular shape. Upon binding melittin, the radius of gyration decreases from 20.9 to 18.0 A and the largest dimension decreases from 60 to 47.5 A. In the absence of calcium, however, melittin has little effect on the solution structure of calmodulin.

Kataoka, M., Head, J. F., Seaton, B. A., & Engelman, D. M. (1989). Melittin binding causes a large calcium-dependent conformational change in calmodulin. Proceedings of the National Academy of Sciences, 86(18), 6944–6948.