In our model of muscle regulation, regulated actin can exist in either the turned on form, which fully activates the myosin S-1 ATPase activity, or the turned off form, which shows very little activation. The lack of activation by the turned off form is postulated to be due to tropinin-tropomyosin inhibiting the release of Pi in the acto.S-1 ATPase cycle, rather than by blocking the binding of S-1.ATP (and S-1.ADP.Pi) to actin, as was suggested by the steric blocking model. We tested several aspects of our model. First our model predicts that S-1.ATP and the S-1.ATP analog, pPDM.S-1, should not turn on the regulated acto.S-1 ATPase activity in the absence of Ca-2+. In agreement with our model, we found that compared to the maximal turned on rate, neither S-1.ATP nor pPDM.S-1 significantly turns on the regulated acto.S-1 ATPase activity. Second, our model predicts that these S-1 species should significantly turn on the regulated acto.S-1 ATPase activity in the presence of Ca-2+ provided that S-1.ATP and pPDM.S-1 bind slightly stronger to the turned on form than to the turned off form of regulated actin. We find that under conditions in which pPDM.S-1 binds extensively to regulated actin, it does fully turn on the regulated acto.S-1 ATPase activity in the presence of Ca-2+. These data are consistent with our original model in which the equilibrium between the turned on and turned off forms of regulated actin is partially shifted towards the turned on form by Ca-2+. It does, however, rule out our alternate model in which regulated actin can exist in a continuum of forms, but under any given conditions, only one of these forms are in existence. Lastly our model predicts that in Ca-2+, the thin filament is only partially turned on, while it is necessary to have rigor bridges bound to the thin filament to completely turn it on. In agreement with this prediction, we found that the ATPase activity of regulated acto.S-1 was much less than the fully turned on rate.
