Structure of F–actin solutions

Abstract

AbstractFor the purpose of investigating the structure of solutions of muscle protein F–actin and the effect of external force on this structure, simultaneous measurements of rigidity, viscosity, and birefringence were made under rotation and oscillation at very low rates of shear, and as a result a consistent picture of F–actin solutions has been obtained. The F–actin solutions of various concentrations are made by adding a small amount of magnesium ions to a salt‐free G–actin solution. Rigidity, viscosity, and degree of flow birefringence all increase in proportion to the F–actin concentration. Even at very low concentrations, F–actin solutions have a large rigidity. This rigidity is found to be due to a network structure of semiflexible F–actin filaments composed of very many G–actin molecules. The linkage‐to‐linkage length and the flexibility of filaments are estimated from experimental data. Rotation at very low rate of shear causes rigidity and viscosity to decrease rapidly. All results are reasonably explained by assuming that the filaments are elongated and then linkages are detached by shear of rotation. With a decrease in the number of linkages, F–actin filaments are completely oriented. Once attained as a result of shear, the orientation decreases very slowly on standing and gives an anomalous decay curve, probably as a result of setting of the network structure of oriented filaments. At high concentrations of F–actin, a spontaneous orientation occurs. Hysteresis phenomena of F–actin solutions observed under various mechanical treatments are all attributable to formation and detachment of linkages between semiflexible F–actin filaments.