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

In this paper we introduce an alternate model for the equilibrium binding of S-l-N (S-l, subfragment l of myosin; N, nucleotide) on the troponin-tropomyosin-actin complex, including the influence of Ca2+ on this binding. In our previous model [Hill, T. L., Eisenberg, E. & Greene, L. E. (1980) Proc. Natl. Acad. Sci. USA 77, 3186-3190], we assumed that each tropomyosin unit, including one troponin-tropomyosin molecule and seven actin sites on the actin filament, could exist in two conformational states which presumably differed in the position of the tropomyosin on the actin. The binding of S-l-N or Ca2+ to the tropomyosin unit shifted the equilibrium between the two states but did not affect the intrinsic conformation of each state. In contrast, in the present model, we assume 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 two positions as in our earlier model. The binding of S-l-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-l.