Demonstration of a fluorometrically distinguishable intermediate in calcium binding by calmodulin-mastoparan complexes.

Observations on the intrinsic fluorescence of a high affinity calmodulin-binding peptide, Polistes mastoparan, reveal a spectroscopically distinct peptide complex present at maximum concentration when 2 mol Ca+2 are bound per mol calmodulin. The intermediate is detectable only in solutions where calcium is limiting. The results are consistent with cooperative binding of the first two equivalents of calcium by calmodulin.