Comparison of the binding of heavy meromyosin and myosin subfragment 1 to F-actin

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Abstract
The binding of heavy meromyosin (HMM) [from rabbit skeletal muscle] to F-actin was examined at varying ionic strengths and temperatures and in the presence of ADP and AMPPNP [pyridine nucleotide phosphate] and then compared to the binding of subfragment 1 (S-1) under identical conditions. In the absence of nucleotide (.mu. = 0.12-0.43 M, 22.degree. C), HMM binds 100- to 1000-fold more strongly to F-actin than does S-1. In the absence of nucleotide, in both heads of HMM apparently bind to F-actin, with the 2nd head making a significant contribution to the free energy of binding. In the presence of ADP (.mu. = 0.43 M, 22.degree. C) or AMPPNP (.mu. = 0.12 M, 22.degree. C), the binding of HMM to F-actin is quite similar to the binding of S-1, indicating that the 2nd head of HMM does not make a strong contribution to the free energy of binding. In the presence of AMPPNP, HMM appears to bind to F-actin primarily with 1 head, while the detached head may be interfering with the binding of another HMM molecule at an adjacent actin site. With all of the different agents tested (ionic strength, temperature and nucleotide), the effect of the agent on the binding of HMM to F-actin is approximately the square of its effect on the binding of S-1 to F-actin, results cosistent with these various agents affecting the binding of each of the 2 HMM heads to the same extent as they affect the binding of an S-1 head.

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