Toxins Perturb the Pool of Actin Monomers

Several toxins shift the usual equilibrium between actin monomers and filaments. Two unrelated toxins, cytochalasin D and latrunculin promote the dissociation 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 G-actin and inhibits it from adding to a filament end. Exposure to either toxin thus increases the monomer pool. When cytochalasin is added to live cells, the actin cytoskeleton disappears and cell movements such as locomotion and cytokinesis are inhibited. These observations were among the first that implicated actin filaments in cell motility.

In contrast, the monomer-polymer equilibrium is shifted in the direction of filaments by jasplakolinode, another sponge toxin, and by phalloidin, which is isolated from Amanita phalloides (the "angel of death" mushroom). Phal-loidin poisons a cell by binding at the interface between sub-units in F-actin, thereby locking adjacent subunits together and preventing actin filaments from depolymerizing. Even when actin is diluted below its critical concentration, phal-loidin-stabilized filaments will not depolymerize. Fluorescence-labeled phalloidin, which binds only to F-actin, is commonly used to stain actin filaments for light microscopy.

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