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

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Abstract
An alternate model is introduced for the equilibrium binding of S-1-N (S-1, subfragment 1 of myosin; N, nucleotide) on the troponin-tropomyosin-actin complex, including the influence of Ca2+ on this binding. A previous model (1980) assumed that each tropomyosin unit, including 1 troponin-tropomyosin molecule and 7 actin sites on the actin filament, could exist in 2 conformational states which presumably differed in the position of the tropomyosin on the actin. The binding of S-1-N or Ca2+ to the tropomyosin unit shifted the equilibrium between the 2 states but did not affect the intrinsic conformation of each state. The present model, assumes that tropomyosin can, in principle, occupy a continuum of positions on the actin filament. However, in any particular circumstance (N, Ca2+, salt, temperature), the tropomyosin occupies only a single position rather than existing in a dynamic equilibrium between 2 positions as in the earlier model. The binding of S-1-N or Ca2+ changes the position of tropomyosin on the actin filament and the exact position that the tropomyosin occupies depends on the nucleotide bound to S-1.