Changes in the ultrastructure of actomyosin gel during hydrolysis of ATP under various ionic conditions.

The interaction of myosin and actin in the presence of MgATP under various ionic conditions was investigated by a simultaneous determination of changes in turbidity, liberation of inorganic phosphate, distribution of the two proteins between the supernatant and precipitate obtained after a short-time centrifugation, and by electron microscopy. The results confirm the view that the extent of association between myosin and actin filaments is low not only in the clear phase but also under the conditions when the addition of MgATP results in superprecipitation without a detectable clear phase. Extensive formation of aggregates of parallelly aligned myosin and actin filaments coincides with the depletion of ATP, indicating that these aggregates represent rigor complexes. Relation between ATP-induced turbidity changes and structural changes in the actomyosin system under various ionic conditions is discussed.