The binding of calcium+almodulin (CaM) to caldesmon (CaD) contributes to the regulation of smooth muscle contraction. It has been reported that a 17-residue synthetic peptide encompassing the residues Gly651-Ser667 of smooth muscle CaD constitutes its CaM-binding domain [Zhan, Q., Wong, S. S., & Wang, C. L. A. (1991) J. Biol. Chem. 266, 21810-218141. This peptide does not share sequence homology with the CaM-binding domains of other proteins, and in addition, the binding of C a M to CaD is

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... to be relatively weak. Here we have investigated the properties of this atypical CaM-binding domain by N M R and circular dichroism (CD) spectroscopy. Two dimensional N M R studies performed in an aqueous T F E mixture (75%/25%) showed that the peptide has the capacity to adopt an amphiphilic a-helical conformation. TRNOESY experiments and C D spectroscopy were used to determine that the CaD peptide binds in an a-helical conformation to CaM. The addition of TFE or the binding of the CaD peptide to C a M induces an a-helical structure only for the central 10 amino acid residues of the peptide. Titrations of C a M with the CaD peptide were followed by proton N M R and show the formation of a 1:l complex and that the binding is calcium-dependent. The chemical shifts of 13C-methyl groups of specifically