Cooperative binding of myosin subfragment-1 to the actin-troponin-tropomyosin complex.

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Abstract
The binding of rabbit myosin subfragment-1 (S-1) to the F-actin-troponin-tropomyosin complex (regulated F-actin) was examined in the presence of ADP (ionic strength, 0.23 M; 22.degree. C) by using the ultracentrifuge and S-1 blocked at SH1 with iodo[14C]acetamide. S-1.cntdot.ADP binds with positive cooperativity to regulated F-actin, both in the presence and absence of Ca; it binds independently to unregulated actin. With and without Ca2+ at very low levels of occupancy of the regulated actin by S-1.cntdot.ADP, S-1.cntdot.ADP binds to the regulated actin with < 1% of the strength that it binds to unregulated actin by S-1.cntdot.ADP, S-1.cntdot.ADP binds about 3-fold more strongly to the regulated actin than it does to unregulated actin. The major difference between the results obtained in the presence and absence of Ca2+ with regulated actin is that, in the absence of Ca2+, the binding of S-1.cntdot.ADP remains weak until a higher free S-1.cntdot.ADP concentration is reached and the transition to strong binding is much more cooperative. These results are consistent with a model that is basically similar to the cooperative binding model of Hill. The regulated actin filament can exist in 2 forms, a weak-binding and a strong-binding form; and Ca2+ and S-1.cntdot.ADP, acting as allosteric effectors, shift the equilibrium between the 2 forms.