Jasplakinolide, a cytotoxic natural product, induces actin polymerization and competitively inhibits the binding of phalloidin to F-actin

Jasplakinolide, a naturally occurring cyclic peptide from the marine sponge, Jaspis johnstoni, has both fungicidal and antiproliferative activity. We now report that this peptide is a potent inducer of actin polymerization in vitro. The peptide has a much greater effect on Mg²⁺-actin than on Ca²⁺-actin. Competitive binding studies using rhodamine-phalloidin suggest that jasplakinolide binds to F-actin competitively with phalloidin with a dissociation constant of approximately 15 nM. This compares favorably to the previously reported IC₅₀ of 35 nM for the antiproliferative effect of jasplakinolide on PC3 prostate carcinoma cells. The binding curve suggests that nearest neighbor positive cooperativity influences the binding of jasplakinolide (and perhaps also phalloidin) to F-actin. These results imply that jasplakinolide may exert its cytotoxic effect in vivo by inducing actin polymerization and/or stabilizing pre-existing actin filaments.