Characterization of the equilibrium between blocked and closed states of muscle thin filaments.

We recently proposed a three-state model for the regulation of actomyosin interaction by tropomyosin and troponin (Tm.Tn). In this model, the thin filament exists in rapid equilibrium between the following three states: blocked, which cannot bind myosin significantly; closed, which can bind myosin weakly to form the A-state; open, which can bind both to form the A-state and isomerize to the strongly bound R-state. In this study, we demonstrate that the equilibrium between the blocked and closed states is calcium sensitive with an equilibrium constant, KB, of 0.3 and > or = 10 at pCa 8.9 and 4.6, respectively. The pCa dependence of KB is typical of that for calcium binding to thin filaments with a mid-point at pCa 5.6 and a Hill coefficient of 1.8. KB is independent of ionic strength over the range 0.4-0.06 M but increases dramatically below 0.05 M to > or = 10 at 0.01 M suggesting loss of the blocked state at low ionic strength. The blocked state also has reduced occupancy at high temperatures. KB, in the absence of calcium, increases from 0.2 at 5 degrees C to 0.6 at 40 degrees C.