Ca2+ and Ionic Strength Dependencies of S1-ADP Binding to Actin-Tropomyosin-Troponin: Regulatory Implications

Skeletal and cardiac muscle contraction are inhibited by the actin-associated complex of tropomyosin-troponin. Binding of Ca21 to troponin or binding of ATP-free myosin to actin reverses this inhibition. Ca21 and ATP-free myosin stabilize different tropomyosin-actin structural arrangements. The position of tropomyosin on actin affects the binding of ATP-free myosin to actin but does not greatly affect myosin-ATP binding. Ca21 and ATP-free myosin alter both the affinity of ATP-free myosin for actin and the kinetics of that binding. A parallel pathway model of regulation simulated the effects of Ca21 and ATP-free myosin binding on both equilibrium binding of myosin-nucleotide complexes to actin and the general features of ATPase activity. That model was recently shown to simulate the kinetics of myosin-S1 binding but the analysis was limited to a single condition because of the limited data available. We have now measured equilibrium binding and binding kinetics of myosin-S1-ADP to actin at a series of ionic strengths and free Ca21 concentrations. The parallel pathway model of regulation is consistent with those data. In that model the interaction between adjacent regulatory complexes fully saturated with Ca21 was destabilized and the inactive state of actin was stabilized at high ionic strength. These changes explain the previously observed change in binding kinetics with increasing ionic strength. Originally published Biophysical Journal Vol. 87 No. 3 Sep 200