The effect of nucleotide on the binding of myosin subfragment 1 to regulated actin.

Previous studies have shown that in the presence of ADP, myosin subfragment 1 (S-1) binds with positive cooperativity to the troponin-tropomyosin-actin complex (regulated actin). This binding is much more cooperative in the absence than in the presence of Ca2+. Based on these data, we proposed a model (Hill, T. L., Eisenberg, E., and Greene, L. E. (1980) Proc. Natl. Acad. Sci. U. S. A. 77, 7209-7213) in which the tropomyosin-actin units in regulated actin exist in two states, a state which binds S-1 weakly and a state which binds S-1 strongly. This model predicts that the equilibrium constant between the weak and strong states of the tropomyosin-actin units is an intrinsic property of the regulated actin filament. Therefore, its value should not change when measured with different nucleotides bound to S-1. To test this prediction, the equilibrium constant was measured in the presence of S-1, S-1 . ADP, and S-1 . AMP-PNP, with and without Ca2+. The results show that, in all cases, S-1 binds with positive cooperativity to regulated actin. The observed cooperativity is consistent with the model since it was possible to fit the data obtained in the presence of different nucleotides with the same value for the equilibrium constant between the weak and strong states of the tropomyosin-actin units. However, the nucleotide bound to S-1 appears to affect the observed cooperativity by changing the affinity of S-1 for the strong and weak states.