Ca2+-induced conformational shift of the COOH-domain of eel skeletal muscle troponin C in the presence of physiological concentrations of Mg2

Thespectroscopic properties of Trp152 of eel skeletal muscletroponin C have been studied under conditions in which the COOH-domain is depleted of metal ions, titrated with Mg2+ andsubsequently with Ca2+. This spectroscopic study clearly showsthat the Mg2+ or Ca2+-bound states substitution lead to distinctconformations of the COOH-domain. The analysis of eel troponin Cabsorption and Trp152 fluorescence emission spectra indicates amore polar environment in the Mg2+-bound state of the protein.Steady state tryptophan polarization and lifetime distributiondata indicate that the motion of the indole moiety is morerestricted in the Mg2+ state of the protein than in the Ca2+-bound state. However, fluorescence quenching data using I-and Cs+ show that Trp152 is more accessible to the solvent in theMg2+-bound state of eel troponin C. This spectroscopic analysisof the distinct Ca2+ and Mg2+-bound states of eel troponin C isconsistent with the description of the three-dimensionalstructure of the corresponding states of pike (pI = 4.10)parvalbumin which is structurally homologous to the COOH-domainof troponin C. Since it appears that during muscular contraction,magnesium ions, which occupy the binding sites of the COOH-domainof troponin C in the resting cell are replaced by calcium ions,the structural shift occurring upon Mg2+/Ca2+ substitution, musthave a physiological significance. The role of this domain isprobably not limited to a structural role[PUBLICATION ABSTRACT]