In the absence of ATP, the myosin cross-bridge binds tightly to actin filaments. ATP binding brings about a rapid dissociation of the cross bridge from actin. Thus the cross bridge can bind either actin or ATP but to both only transiently. Solution kinetic observations were very important in establishing the relationship between the hydrolysis of ATP and the generation of force. Without actin, myosin is product-inhibited and is a poor ATPase.
Force is generated during the last step.
Furthermore, it gradually became clear that the proportion of cross bridges taking part in a contraction at any one time was only a small fraction of the total, making the registration of active cross bridge movement doubly difficult. The cleft separates two parts of the molecule which are referred to as the 50K upper and 50K lower domains or actin binding domain. In particular, this model establishes the spatial orientation of the S1 myosin fragment in the active complex. For example one finds that the cleft in myosin extends from the ATP binding site to the actin binding site and that the opening and closing of this cleft is very likely to provide the communication between the ATP site and the actin binding site. The actin binding site spans the 50K upper and lower domains and the ATP binding site extends from the 50K upper domain into the 50K lower domain.