Interaction of myosin filaments and minifilaments with actin: a comparative study

Various aspects of actin-myosin interaction were investigated using myosin in the form of filaments and minifilaments obtained by dialysis against citrate-Tris buffer or by adding this buffer to performed myosin filaments. Considerable similarities in the behaviour of the two systems were found. (1) Although the minifilaments are soluble structures, they form insoluble complexes with actin, which superprecipitate upon addition of MgATP. Observations in the electron microscope and from centrifugation experiments have shown that the two actomyosin systems undergo essentially similar structural changes during superprecipitation. (2) At low substrate concentrations the rate of ATP hydrolysis in both systems declines with time, which is typical of insoluble superprecipitating actomyosin. (3) In contrast to soluble myosin subfragments, both filamentous and minifilamentous myosin give biphasic actin-activation curves. (4) The Mg2+-ATPase activities of myosin minifilaments and standard myosin preparations at low KCl extrapolate to similar Vmax at infinite actin concentration. Since our values of Vmax for myosin filaments and minifilaments are in the range of those reported for myosin subfragments, the results of this investigation confirm the view that the catalytic properties of myosin subfragments and intact myosin are equivalent. Moreover, the data show that the extent of myosin aggregation in the initial preparations has no appreciable effect on the characteristic features of the interaction between intact myosin and actin at pH 8.