Certain fungi and sponges have developed toxins that target the polymerization cycle of actin and are therefore toxic to animal cells but are useful for the study of actin dynamics. Two types of these toxins have been characterized. The first class is represented by two unrelated toxins, cytochalasin D and latrunculin, that promote the depolymerization of filaments, though by different mechanisms. Cytochalasin D, a fungal alkaloid, depolymerizes actin filaments by binding to the (+) end of F-actin, where it blocks further addition of subunits. Latrunculin, a toxin secreted by sponges, binds and sequesters G-actin, inhibiting it from adding to a filament end. Exposure to either toxin thus increases the monomer pool. When cytochalasin D or latrunculin is added to live cells, the actin cytoskeleton disassembles and cell movements such as locomotion and cytokinesis are inhibited. These observations were among the first to implicate actin filaments in cell motility. Latrunculin is especially useful because it binds actin monomers and prevents any new actin assembly. Thus, if latrunculin is added to a cell, the rate at which actin-based structures disappear reflects their normal rate of turnover. This method has revealed that some actin structures have half-lives of less than a minute, whereas others are much more stable. For example, experiments with latrunculin show that the leading edges of motile cells turn over every 30–180 seconds, and that stress fibers turn over every 5–10 minutes.
In contrast, the monomer-polymer equilibrium is shifted in the direction of filaments by jasplakinolide, another sponge toxin, and by phalloidin, which is isolated from Amanita phalloides (the “angel of death” mushroom). Jasplakinolide enhances nucleation by binding and stabilizing actin dimers and thereby lowering the critical concentration. Phalloidin binds at the interface between subunits in F-actin, locking adjacent subunits together and preventing actin filaments from depolymerizing. Even when actin is diluted below its critical concentration, phalloidin-stabilized filaments will not depolymerize. Because many actin-based processes depend on actin filament turnover, the introduction of phalloidin into a cell paralyzes all these systems, and the cell dies. However, phalloidin has been very useful to researchers, as fluorescent-labeled phalloidin, which binds only to F-actin, is commonly used to stain actin filaments for light microscopy (see Figure 17-4).