Binding of troponin I and the troponin I-troponin C complex to actin-tropomyosin. Dissociation by myosin subfragment 1

Troponin I (TnI) is the component of the troponin complex, TnI, TnC, TnT, t hat is responsible for inhibition of actomyosin ATPase activity. Using the fluorescence of pyrene-labeled tropomyosin (Tm), we probed the interaction of TnI and TnIC with Tm on the reconstituted muscle thin filament. The resu lts indicate that TnI and TnIC(-Ca2+) bind specifically and strongly to act in-Tm with a stoichiometry of 1 TnI or 1 TnIC/1 Tm/7 actin, in agreement wi th previous results. The binding of myosin heads (S1) to actin-Tm at low le vels of saturation caused TnI and TnIC to dissociate from actin-Tm, These r esults are interpreted in terms of the S1-binding state allosteric-cooperat ive model of the actin-Tm thin filament, closed/open. Thus, TnI and TnIC(-C a2+) bind to the closed state of actin-Tm and their binding is greatly weak ened in the SI-induced open state, indicating that they act as allosteric i nhibitors. The fluorescence change and the stoichiometry indicate that the TnI-binding site is composed of regions from both actin and Tm probably in the vicinity of Cys 190.