PHYSIOCHEMICAL STUDIES OF F-ACTIN-HEAVY MEROMYOSIN SOLUTIONS

In the absence of ATP an F-actin-heavy meromyosin complex was prepared at room temperature and in a solution of 60 mM KCl containing 10 mM Tris-HCl buffer (pH 8.0). A constant concentration of F-actin was mixed with various concentrations of heavy meromyosin. 1. 1. Centrifugation experiments revealed that one heavy meromyosin molecule can bind to one actin monomer in F-actin. 2. 2. The relationship of the turbidity change of F-actin-heavy meromyosin solution to the ratio of components is linear until the binding between heavy meromyosin and F-actin becomes saturated. The turbidity experiment gave the same value of the stoichiometric binding ratio for heavy meromyosin and actin as given in (1). 3. 3. The degree of flow birefringence of the F-actin-heavy meromyosin solution decreased as the concentration of heavy meromyosin in the F-actin-heavy meromyosin solution increased. A minimum point was reached at a molar ratio of 1 6, and then the flow birefringence increased again, reaching a plateau beyond the saturation point of binding where the degree of flow birefringence is about 50% larger than that of F-actin. This abnormal behaviour of the flow birefringence was not dependent upon either the protein concentration or the shear rate of flow. The ionic strength of the solvent also had little effect on this abnormality. The extinction angle also showed similar features but reached a maximum at a molar ratio of 1 6. 4. 4. The degree of flow birefringence and the extinction angle of the F-actin-heavy meromyosin solution at every ratio studied, when dissociated by Mg2+-ATP, took the value corresponding to the pure F-actin solution before mixing with heavy meromyosin. 5. 5. The ultraviolet absorption of the F-actin-heavy meromyosin solution was hypochromically smaller than the sum of the ultraviolet absorptions of heavy meromyosin and F-actin only when the molar ratio lay between 0 and 1 2. This difference was also greatest at a molar ratio of 1 6. 6. 6. From these findings it is suggested that there are at least two types of binding between heavy meromyosin and F-actin. © 1960.