Parking problem and negative cooperativity of binding of myosin subfragment 1 to F-actin

Previously we provided evidence that myosin subfragment 1 (S1) can bind either one (state 1) or two actin monomers (state 2) in solution and in muscle fiber. Here we present results of the kinetics study of binding of S1 to F-actin labeled with fluorescent dye pyrene. A transition from state 1 to state 2 depends on probability that the second actin is free, which is high when molar ratio of S1/actin (R) is less than 0.5, and it decreases dramatically when R > 2.0 due to the parking problem. The kinetics data obtained at different molar ratios were well fitted by two binding states model. The sequential binding of myosin head initially with one actin monomer and then with the second actin monomer in F-actin can play a key role in force generation by actin–myosin and their directed movement.

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► Myosin S1 binds one (state 1) and then two monomers (state 2) in F-actin.
► At high molar ratio S1/actin the binding of S1 in state 2 is restricted (parking problem).
► A transition from state 1 to state 2 might be associated with force generation.