Kinetics of the "On-Off" Change in Regulatory State of the Muscle Thin Filament

The excimer fluorescence of pyrene iodoacetamide-labeled tropomyosin bound to actin is a probe of the regulatory state change of the muscle thin filament associated with the cooperative binding of myosin subfragment 1 (S1) (Y. Ishii and S. S. Lehrer, Biochemistry 29, 1160, 1990). Tn this study, the excimer fluorescence was used to monitor the kinetics of the "on-off" state change due to the ATP-induced dissociation of S1 from actin-tropomyosin. A lag in the fluorescence change indicated that the thin filament was maintained in the "on-state" until more than half of the S1s dissociate. The on-off transition rate was slower than the dissociation of the remaining S1s. These delays in relaxation to the "off-state" may explain some observed kinetic and cooperative effects during tension measurements.