Depolymerization of actin in concentrated solutions of divalent metal chlorides

Actin transferred to concentrated (0.3-1.2 M) MgCl2 solutions depolymerizes completely. When protected by a high excess of ATP, actin in this MgCl2-depolymerized state is stable for several days in the cold. In the absence of excess ATP it slowly denatures. Chiroptical data and proteolysis experiments show that MgCl2-depolymerized actin is in a native, folded state, although its helix content is considerably decreased. By dissolving F-actin pellets or actin precipitated in paracrystalline state in concentrated MgCl2 solutions in the presence of ATP, very concentrated (100-200 mg/ml) monomeric actin solutions can be prepared. CaCl2 and MnCl2 have similar effects although these were not studied in detail.