Distribution of distances between the tryptophan and the N-terminal residue of melittin in its complex with calmodulin, troponin C, and phospholipids.

We used frequency-domain measurements of fluorescence resonance energy transfer to measure the distribution of distances between Trp-19 of melittin and a 1-dimethylamino-5-sulfonylnaphthalene (dansyl) residue on the N-terminal-alpha-amino group. Distance distributions were obtained for melittin free in solution and when complexed with calmodulin (CaM), troponin C (TnC), or palmitoyloleoyl-L-alpha-phosphatidylcholine (POPC) vesicles. A wide range of donor (Trp-19)-to-acceptor (dansyl) distances was found for free melittin, which is consistent with that expected for the random coil state, characterized by a Gaussian width (full width at half maxima) of 28.2 A. In contrast, narrow distance distributions were found for melittin complexed with CaM, 8.2 A, or with POPC vesicles, 4.9 A. A somewhat wider distribution was found for the melittin complex with TnC, 12.8 A, suggesting the presence of heterogeneity in the mode of binding between melittin and TnC. For all the complexes the mean Trp-19 to dansyl distance was near 20 A. This value is somewhat smaller than expected for the free alpha-helical state of melittin, suggesting that binding with CaM or TnC results in a modest decrease in the length of the melittin molecule.