Cytochalasins and phalloidins are small, naturally occurring organic molecules that bind to actin filaments, altering their polymerization. Cytochalasins, primarily fungal metabolites, permeate cell membranes, causing cells to stop ruffling, become less stiff, and nucleate. They bind to the barbed end of actin filaments, inhibiting both association and dissociation of subunits. The binding stoichiometry is about one cytochalasin per actin filament, with a dissociation constant (Kd) that varies depending on the experimental conditions. Cytochalasins also inhibit monosaccharide transport across the plasma membrane but not in some cases. The inhibition constant (Ki) measures the effect on filament growth or shortening, with CD being more effective than CB. Cytochalasins can shorten actin filaments, possibly through a process called "severing," but this mechanism is not well understood. Recent studies have shown that cytochalasins can bind to monomeric actin and promote dimer formation, which may explain their effects on actin polymerization and filament dynamics. Phalloidins, a group of bicyclic heptapeptides from poisonous mushrooms, bind tightly to actin filaments, shifting the equilibrium towards filament formation and lowering the critical concentration for polymerization. The binding stoichiometry is one phalloidin per one or two actin protomers. Phalloidins are widely used for visualizing actin filaments in living and fixed cells, but their toxicity can lead to cell death. They alter actin distribution and cell motility, suggesting additional targets beyond actin. Overall, cytochalasins and phalloidins have been crucial in elucidating the fundamental aspects of actin polymerization and its role in biological processes.Cytochalasins and phalloidins are small, naturally occurring organic molecules that bind to actin filaments, altering their polymerization. Cytochalasins, primarily fungal metabolites, permeate cell membranes, causing cells to stop ruffling, become less stiff, and nucleate. They bind to the barbed end of actin filaments, inhibiting both association and dissociation of subunits. The binding stoichiometry is about one cytochalasin per actin filament, with a dissociation constant (Kd) that varies depending on the experimental conditions. Cytochalasins also inhibit monosaccharide transport across the plasma membrane but not in some cases. The inhibition constant (Ki) measures the effect on filament growth or shortening, with CD being more effective than CB. Cytochalasins can shorten actin filaments, possibly through a process called "severing," but this mechanism is not well understood. Recent studies have shown that cytochalasins can bind to monomeric actin and promote dimer formation, which may explain their effects on actin polymerization and filament dynamics. Phalloidins, a group of bicyclic heptapeptides from poisonous mushrooms, bind tightly to actin filaments, shifting the equilibrium towards filament formation and lowering the critical concentration for polymerization. The binding stoichiometry is one phalloidin per one or two actin protomers. Phalloidins are widely used for visualizing actin filaments in living and fixed cells, but their toxicity can lead to cell death. They alter actin distribution and cell motility, suggesting additional targets beyond actin. Overall, cytochalasins and phalloidins have been crucial in elucidating the fundamental aspects of actin polymerization and its role in biological processes.