Binding of amphiphilic peptides to a carboxy-terminal tryptic fragment of calmodulin

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Abstract
Calmodulin (CaM) fragments 1-77 (CaM 1-77) and 78-148 (CaM 78-148) were prepared by tryptic cleavage of CaM. CaM 78-148 exhibited Ca2+-dependent binding to mastoparan X, Polistes mastoparan, and melittin with apparent dissociation consants < 0.2 .mu.M as judged from changes in the fluorescence spectrum and anisotropy of the single trytophan residue of each of these cationic, amphiphilic peptides. This interaction was accompanied by a large spectral blue shift of the peptide fluorescence spectrum. These findings are consistent with earlier results [Malencik, D. A., and Anderson, S. R. (1984) Biochemistry 23,2420-2428] on the binding of mastoparan X to CaM fragment 72-148. The binding of the peptide to CaM 78-148 also caused a significant loss of the accessibility of the peptide trytophan to the fluorescence quencer acrylamide. The CaM 78-148 induced effects on the fluorescence spectra and tryptophan accessibility of the peptides were most pronounced for mastoparan X, a peptide with trytophan on the apolar face of the putative amphiphilic helix. The data were comparable with results from parallel experiments on the Ca2+-dependent interaction of these peptides with intact CaM. Difference circular dichroic spectra suggested that binding to CaM 78-148 was associated with the induction of considerable degrees of helicity in the amphiphilic peptides, which by themselves have predominantly random coil structures in aqueous solution. This finding is also reminiscent of the interaction of these peptides with intact CaM. Thus, the charcteristics of the high-affinity peptide binding site of CaM appear to be largely retained in CaM 78-148. CaM 1-77 exhibits a much weaker affinity (dissociation constant > 20 .mu.M) for the mastoparans.

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