Dilution beyond a transition concentration and the enhanced filaments' formation of actin in the low ionic strength buffer.

Actin in the low ionic strength buffer solution G (0.1 mM ATP, 0.1 mM CaCl2, 2 mM Tris-Cl (pH 8.0), 5 mM 2-mercaptoethanol and 1 mM NaN3) was diluted to the concentration below 10 micrograms/ml at 7 degrees C. Viscometry showed associations of actin monomers and increasing the extent of dilution, maximally about 75% of actin were precipitated. Actin filaments were observed by electron microscope in the diluted solution. Ca2+ plays a role in the filaments' formation. It seems that a phase transition like change in the actin states arose at this concentration range.