G‐F transformation of actin as a fibrous condensation

Abstract The physical feature of the G‐F transformation of action caused by a small amount of magnesium ions was investigated by measuring flow birefringence, light scattering, viscosity, and sedimentation of action solutions at various concentrations of actin and magnesium ions. All experimental results indicate that the G‐F transformation of actin can be regarded as a reversible fibrous condensation. When the concentration of actin is increased at a constant concentration of magnesium ions, fibrous aggregates of actin molecules, F‐actins, can be formed only above a critical actin concentration which is determined by the condition of medium and decreases rapidly with increasing concentration of magnesium ions. Above the critical concentration, all of the excess active actin molecules are transformed into F‐actins. These F‐actins are coexisting with active (G‐)actin molecules, the concentration of which is independent of the concentration of F‐actin and equal to the concentration of active actin at the critical point. The state of apparent units of F‐actins in solution is also determined by the condition of medium and independent of the concentration. With increasing concentration of magnesium ions, the apparent units of F‐actin become longer and longer and their thickness increases. Small globular aggregates are always observed below the critical point of formation of long fibrous aggregates. These experimental results show qualitative agreement with theoretical results obtained by one of the authors on fibrous and globular aggregations of charged macromolecules.