Rate-limiting step in the actomyosin adenosinetriphosphatase cycle: studies with myosin subfragment 1 cross-linked to actin.

Although there is agreement that actomyosin can hydrolyze ATP without dissociation of the actin from myosin, there is still controversy about the nature of the rate-limiting step in the ATPase cycle. Two models, which differ in their rate-limiting step, can account for the kinetic data. In the four-state model, which has four states containing bound ATP or ADP . Pi, the rate-limiting step is ATP hydrolysis (A . M . ATP in equilibrium A . M . ADP . Pi). In the six-state model, which we previously proposed, the rate-limiting step is a conformational change which occurs before Pi release but after ATP hydrolysis. A difference between these models is that only the four-state model predicts that almost no acto-subfragment 1 (S-1) . ADP . Pi complex will be formed when ATP is mixed with acto . S-1. In the present study, we determined the amount of acto . S-1 . ADP . Pi formed when ATP is mixed with S-1 cross-linked to actin [Mornet, D., Bertrand, R., Pantel, P., Audemard, E., & Kassab, R. (1981) Nature (London) 292, 301-306]. The amount of acto . S-1 . ADP . Pi was determined both from intrinsic fluorescence enhancement and from direct measurement of Pi. We found that at mu = 0.013 M, the fluorescence magnitude in the presence of ATP of the cross-linked actin . S-1 preparation was about 50% of the value obtained with S-1, while at mu = 0.053 M the fluorescence magnitude was about 70% of that obtained with S-1.(ABSTRACT TRUNCATED AT 250 WORDS)