Concentration-dependent influence of various cytochalasins and chaetoglobosins on the phalloidin-induced polymerization of G-actin in 0.6 M potassium iodide.

Phalloidin, a bicyclic peptide from the poisonous Amanita phalloides mushroom, stimulates the viscosimetrically determined polymerization of G-actin to F-actin in 0.6 m potassium iodide, a medium in which spontaneous polymerization does not occur. The cytochalasins B, D, E, and G (CB, CD, etc.) and the chaetoglobosins A, B, C, E, F, and J (Ch-A, Ch-B, etc.) have been found to influence the rat of polymerization in different ways, depending on their chemical structure and concentrations applied. Class I cytotoxins, e.g., CB, CG, Ch-C, Ch-E, and Ch-F, which exert on F-actin a weak degradative power (DP, "Spudich effect"), increase the polymerization rate when present in a ratio of 4 mol to 1 mol of actin. Polymerization is slightly enhanced or retarded at molar ratios ranging from 0.04:1 to 0.4:1. Class III cytotoxins of strong degradative power, i.e., high affinity for actin, such as CE, Ch-B, and Ch-J, decrease the polymerization rate at a molar ratio of 4:1 during the first 30-40 min; however, they increase it at a molar ratio of 0.4:1. Members of class II (CD, Ch-A) exert an effect that can be interpreted as a combination of the effects of class I and class III cytotoxins. An explanation of this difference in behavior is offered on the basis of experiments on F-actin degradation by Ch-J and its reconstitution by phalloidin in the presence of Ch-J.