Theoretical model for the cooperative equilibrium binding of myosin subfragment 1 to the actin-troponin-tropomyosin complex.
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 Ca(2+) are analyzed by using a linear Ising model. In the model, each tropomyosin-troponin unit (including seven sites on the actin filament) can be in one of two 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 Ca(2+) bound to one member of the pair. There are two sources of positive cooperativity in this system: the fact that seven 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 Ca(2+). Several extensions of this model are discussed.
Hill
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TL
National Institute of Arthritis, Metabolism and Digestive Diseases,  National Institutes of Health, Bethesda, Maryland 20205, USA.
Eisenberg
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Greene
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D000199:Actins / Q000737:chemistry*; D002118:Calcium / Q000737:chemistry; D008956:Models, Chemical; D015879:Myosin Subfragments / Q000737:chemistry*; D011485:Protein Binding; D014335:Tropomyosin / Q000737:chemistry*; D014336:Troponin / Q000737:chemistry*
D000199:Actins; D015879:Myosin Subfragments; D014335:Tropomyosin; D014336:Troponin; D002118:Calcium