Cytochalasin and phalloidin are small organic molecules that bind to actin and alter its polymerization. They are widely used to study actin's role in biological processes and as models for actin-binding proteins. Cytochalasins mimic capping proteins by blocking actin filament ends, inhibiting polymerization, and shortening filaments. Phalloidin, on the other hand, stabilizes actin filaments, which may have been overlooked. Both have helped elucidate fundamental aspects of actin polymerization. Cytochalasins, fungal metabolites, permeate cell membranes, causing cells to round up, become less stiff, and enucleate. They bind to the barbed end of actin filaments, inhibiting subunit association and dissociation. The binding stoichiometry is about one cytochalasin per filament. Cytochalasin D (CD) is more effective than B (CB) in inhibiting filament growth. CD binds to actin monomers and dimers, inducing dimer formation. CD binds rapidly and loosely to monomers, followed by a conformational change to a tighter binding state. CD also induces dimerization of ATP-actin monomers, which may explain its effects on actin polymerization. Phalloidin binds more tightly to actin filaments than monomers, shifting the equilibrium toward filaments and lowering the critical concentration for polymerization. It prevents monomer dissociation from filament ends, stabilizing filaments. Phalloidin is useful for visualizing actin filaments in living and fixed cells. However, it does not permeate cell membranes and is not useful in experiments with living cells. It is taken up by cells, particularly hepatocytes, and can cause toxic effects. Cytochalasin D caps barbed ends of actin filaments, inhibiting growth and shortening. It does not prevent filament depolymerization in all cases. Cytochalasin D may compete with cellular capping proteins for barbed ends, potentially causing filament severing. Phalloidin stabilizes actin filaments, but its effects on cells are not fully understood. The mechanisms of action of cytochalasin and phalloidin are complex, involving interactions with actin monomers, dimers, and filaments. Further studies are needed to fully understand their effects on actin polymerization and cellular processes.Cytochalasin and phalloidin are small organic molecules that bind to actin and alter its polymerization. They are widely used to study actin's role in biological processes and as models for actin-binding proteins. Cytochalasins mimic capping proteins by blocking actin filament ends, inhibiting polymerization, and shortening filaments. Phalloidin, on the other hand, stabilizes actin filaments, which may have been overlooked. Both have helped elucidate fundamental aspects of actin polymerization. Cytochalasins, fungal metabolites, permeate cell membranes, causing cells to round up, become less stiff, and enucleate. They bind to the barbed end of actin filaments, inhibiting subunit association and dissociation. The binding stoichiometry is about one cytochalasin per filament. Cytochalasin D (CD) is more effective than B (CB) in inhibiting filament growth. CD binds to actin monomers and dimers, inducing dimer formation. CD binds rapidly and loosely to monomers, followed by a conformational change to a tighter binding state. CD also induces dimerization of ATP-actin monomers, which may explain its effects on actin polymerization. Phalloidin binds more tightly to actin filaments than monomers, shifting the equilibrium toward filaments and lowering the critical concentration for polymerization. It prevents monomer dissociation from filament ends, stabilizing filaments. Phalloidin is useful for visualizing actin filaments in living and fixed cells. However, it does not permeate cell membranes and is not useful in experiments with living cells. It is taken up by cells, particularly hepatocytes, and can cause toxic effects. Cytochalasin D caps barbed ends of actin filaments, inhibiting growth and shortening. It does not prevent filament depolymerization in all cases. Cytochalasin D may compete with cellular capping proteins for barbed ends, potentially causing filament severing. Phalloidin stabilizes actin filaments, but its effects on cells are not fully understood. The mechanisms of action of cytochalasin and phalloidin are complex, involving interactions with actin monomers, dimers, and filaments. Further studies are needed to fully understand their effects on actin polymerization and cellular processes.