Time-resolved X-ray scattering study of actin polymerization from profilactin

The polymerization of actin in solutions of purified calf spleen actin or profilactin (1-10 mg . ml-1) was followed by synchrotron radiation X-ray solution scattering. At the concentration used, polymerization of actin from profilactin or actin occurs without any lag phase. It is shown by a combination of solution scattering, model calculations and electron microscopy that contrary to the conclusions from previous viscometry studies, filaments form without any lag phase in profilactin solution but aggregate in bundles or networks. This phenomenon is independent of the method used to induce polymerization: slow temperature increase, temperature jump in the presence of polymerizing salts or fast mixing with salt. This aggregation explains the lower final viscosity levels, as compared to actin solutions, observed during the polymerization of actin from profilactin.