Fluorescence energy transfer of complexes of skeletal muscle troponin C and melittin derivatives.

We studied Ca2+-dependent structural change of rabbit skeletal troponin C (TnC)-melittin (ME) complex as a model of TnC-troponin I complex. In previous study, we found that the distance between Met-25 and Cys-98 of TnC in TnC-ME complex increased upon binding of Ca2+ to TnC [H. Sano and T. Iio (1995) J. Biochem. 118, 996-1000]. In this study, we used a fluorescence energy transfer method. As a fluorescent donor, we used the tryptophan residue in four melittin derivatives, in which residue 2, 5, 8, or 13 was replaced with tryptophan. As acceptor, we used dansylaziridine (DANZ) bound to Met-25 of TnC, or N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine (1,5-I-AEDANS) bound to Cys-98 of TnC. For all TnCDANZ-ME complexes, the donor-acceptor distance (11.9-17.7 A) did not remarkably depend on Mg2+ or Ca2+ binding of TnC or on the position of tryptophan in ME derivatives. The same results were obtained for TnCAEDANS-ME complexes in the absence of Ca2+ (distance 15.2-21.7 A). But in the presence of Ca2+, tryptophan residues in the central region of ME were near to Cys-98 of TnC (distance much less than 10.4 A). Based on these results, we conclude that ME is enfolded by the N- and C-lobes of TnC, and the ME rod is almost perpendicular to a line connecting Met-25 and Cys-98 of TnC. The position of the ME rod shifts upon binding of Ca2+ to TnC.