Kinetics of the structural transition of muscle thin filaments observed by fluorescence resonance energy transfer.

Fluorescence resonance energy transfer showed that troponin-I changes the position on an actin filament corresponding to three states (relaxed, closed, and open) of the thin filament (Hai et al. (2002) J. Biochem. 131, 407-418). In combination with the stopped-flow method, fluorescence resonance energy transfer between probes attached to position 1, 133, or 181 of troponin-I and Cys-374 of actin on reconstituted thin filaments was measured to follow the transition between three states of the thin filament. When the free Ca 2 + concentration was increased, the transition from relaxed to closed states occurred with a rate constant of ∼500 s - 1 . For the reverse transition, the rate constant was ∼60 s - 1 . When myosin subfragment-1 was dissociated from thin filaments in the presence of Ca 2 + by rapid mixing with ATP, the transition from open to closed states occurred with a single rate constant of ∼300 s - 1 . Light-scattering measurements showed that the ATP-induced myosin subfragment-1 dissociation occurred with a rate constant of ∼900 s - 1 . In the absence of Ca 2 + , the transition from open to relaxed states occurred with two rate constants of ∼400 and ∼80 s - 1 . These transition rates are fast enough to allow the spatial rearrangement of thin filaments to be involved in the regulation mechanism of muscle contraction.