Mechanism of action of phalloidin on the polymerization of muscle actin

Under conditions where muscle actin only partially polymerizes, or where it does not polymerize at all, a significant enhancement of polymerization was observed if equimolar phalloidin was also present. The increased extent of polymerization in the the presence of phalloidin can be explained by the reduced critical actin concentration of partially polymerized populations at equilibrium. Under such conditions, the rate of polymerization, as judged by the length of time to reach half the viscosity plateau, was found to be essentially independent of the phalloidin concentration. Moreover, the initial rate of polymerization of actin was also found to be independent of phalloidin concentration. However, phalloidin apparently causes a reduction in the magnitude of the reverse rates in the polymerization reaction, as was demonstrated by the lack of depolymerization of phalloidin-treated actin polymers. This effect of phalloidin is also supported by the identification of actin nuclei and short polymers in populations of G-actin incubated with phalloidin in the absence of added KCl. Our conclusion, then, is that phalloidin influences the polymerization of actin by stabilizing nuclei and polymers as they are formed.