Theoretical model for the cooperative equilibrium binding of myosin subfragment 1 to the actin-troponin-tropomyosin complex.

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Abstract
Recent experimental data on the equilibrium binding of myosin subfragment 1 (S-1) to regulated actin filaments in the presence and in the absence of Ca2+ are analyzed by using a linear Ising model. In the model, each tropomyosin-troponin unit (including 7 sites on the actin filament) can be in 1 of 2 possible states, which have different intrinsic free energies and different binding constants for S-1. Bound S-1 molecules do not interact with each other. There are nearest-neighbor (pair) interactions between these units that depend on the state of each member of the pair and on the number of Ca2+ bound to 1 member of the pair. There are 2 sources of positive cooperativity in this system: the fact that 7 actin sites change state together as part of a single unit; and the existence of attractive nearest-neighbor interactions between units. Parameters in the model are evaluated by fitting the data, both in the presence and in the absence of Ca2+. Several extensions of this model are discussed.