Abstract
The initial stage of actin polymerization caused by KCl and MgCl2 was studied main-ly by means of the light scattering. Below a certain critical salt concentration, the mole-cular weight, of G-actin is a constant value of dimer, 117, 000, regardless of the salt con-centration, while the virial coefficient becomes smaller with the increase in the salt concent-ration. These results indicate that the unit of active actin is a dimer, having two poly-merization sites, and that the depression of charge effects is an essential condition of the G-F transformation. Above this critical salt concentration, the G-F transformation begins at a critical actin concentration. Below this critical concentration of actin, the apparent molecular weight is greater than that of the dimer. This suggests that some parts of actin have already polymerized to a certain degree before the initiation of the G-F transformation; when the concentration of these polymerized actins reaches a critical value, then G-F transformation starts.
