Interaction of calmodulin and a calmodulin-binding peptide from myosin light chain kinase: major spectral changes in both occur as the result of complex formation.

Many different enzymes are activated by direct interaction with calmodulin; this interaction is thought to occur through a distinct calmodulin-binding domain in each of these enzymes. We have recently reported the sequence of a 27-residue peptide (denoted M13), derived from skeletal muscle myosin light chain kinase (MLCK), that exhibits the properties expected of a calmodulin-binding domain [Blumenthal, D. K., Takio, K., Edelman, A. M., Charbonneau, H., Titani, K., Walsh, K. A., & Krebs, E. G. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 3187-3191]. The interaction between chemically synthesized M13 peptide and calmodulin has been studied by circular dichroism (CD) and proton nuclear magnetic resonance (NMR) spectroscopy. In the presence of Ca2+, the observed ellipticity of an equimolar mixture of M13 and calmodulin is much greater than the sum of the ellipticities of the two isolated proteins. In the absence of Ca2+, the measured ellipticity of the mixture is approximately the sum of the two components. Addition of the peptide to calmodulin causes dramatic changes in the proton NMR spectrum; at a 1:1 molar ratio, no evidence of either free peptide or free calmodulin is observed. Moreover, these data demonstrate that a unique species of the M13-calmodulin complex is formed, indicating that the peptide binds to calmodulin in only one way. The many resonances affected by M13 binding include residues in both halves of the calmodulin molecule. The observed CD and NMR effects suggest that secondary and tertiary conformational changes occur both in M13 and in calmodulin upon complex formation.(ABSTRACT TRUNCATED AT 250 WORDS)