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ATP-induced dissociation of rabbit skeletal actomyosin subfragment 1. Characterization of an isomerization of the ternary acto-S1-ATP complex.

The adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) induced dissociation of actomyosin subfragment 1 (S1) has been investigated by monitoring the light scattering changes that occur on dissociation. We have shown that ATP gamma S dissociates acto-S1 by a mechanism similar to that of ATP but at a rate 10 times slower. The maximum rate of dissociation is limited by an isomerization of the ternary actin-S1-nucleotide complex, which has a rate of 500 s-1 for ATP gamma S and an estimated rate of 5000 s-1 for ATP (20 degrees C, 0.1 M KCl, pH 7.0). The activation energy for the isomerization is the same for ATP and ATP gamma S, and both show a break in the Arrhenius plot at 5 degrees C. The reaction between acto-S1 and ATP was also followed by the fluorescence of a pyrene group covalently attached to Cys-374. We show that the fluorescence of the pyrene group reports the isomerization step and not actin dissociation. The characterization of this isomerization is discussed in relation to force-generating models of the actomyosin cross-bridge cycle.