Fluorescence energy transfer study of troponin C-melittin complex

Fluorescence energy transfer study of rabbit skeletal troponin C (TnC), which binds a donor, dansylaziridine, to Met-25 in helix A and an acceptor, 5-(iodoacetamido)eosin, to Cys-98 in helix E revealed (i) the donor-acceptor distance did not change upon Mg2+ and Ca2+ binding to TnC, (ii) melittin binding to Ca(2+)-free TnC induced a remarkable decrease in the donor-acceptor distance both in the presence and absence of Mg2+, and (iii) Ca2+ binding to TnC in TnC-melittin complex induced a characteristic increase in the donor-acceptor distance. These results indicate that TnC assumes a contracted structure upon binding of the basic amphipathic TnC-binding peptide, melittin, and that it undergoes a characteristic structural change that includes a displacement of helix A away from helix E upon Ca2+ binding to TnC in the TnC-melittin complex.