Kinetics of acto-S1 interaction as a guide to a model for the crossbridge cycle

Recent experiments on the kinetics of the interaction between myosin subfragment 1 (S1) and F-actin in solution are summarized. It is concluded that, at every step of the ATPase cycle, the association between the two proteins takes place in two stages. The equilibrium constant of the second step and thus the affinity of S1 for actin changes from step to step during the enzymatic reaction. It is proposed that the transient kinetic evidence can be interpreted in terms of two different classes of contraction models. The first one, which is widely used at present, identifies particular steps in the enzymatic reaction as directly responsible for the conformational change which represents the power stroke of muscle contraction (direct coupling model). In the second class of model, to which we wish to draw attention, changes in affinity modulated by the enzymatic reaction result in changes in the relative amounts of time spent by parts of the myosin molecule in two different environments. These environments determine whether the molecule exists in the 'long' or 'short' state, and it is the transition between these two which constitutes the power stroke (indirect coupling model).