Effects of cytochalasin and phalloidin on actin

C YTOCHALASINS and phalloidins are two groups of small, naturally occurring organic molecules that bind to actin and alter its polymerization. They have been widely used to study the role of actin in biological processes and as models for actin-binding proteins. Functionally , cytochalasins resemble capping proteins, which block an end of actin filaments, nucleate polymerization, and shorten filaments. No known actin-binding protein stabilizes actin filaments as phalloidin does, but such proteins may have been missed. Cytochalasin and phalloidin have also helped to elucidate fundamental aspects of actin polymeriza-tion. This review briefly summarizes older studies and concentrates on recent v~rk on the mechanisms of action of cytochalasin and phalloidin. Cytochalasin Cytochalasins, a group of fungal metabolites, permeate cell membranes and cause cells to stop ruffling and translocating, round up (44, 54), become less stiff (12), and enucleate (28). In addition to binding actin, cytochalasins A and B also inhibit monosaccharide transport across the plasma mem-Cytochalasin Binding to Actin Filaments. Cytochalasins bind to the barbed end of actin filaments, which inhibits both the association and dissociation of subunits at that end. The stoichiometry of binding is about one cytochalasin per actin filament (8, 19); in these studies the filament number could not be determined accurately. Measurements of the affinity, based on different types of experiments, are compiled in Table I. The dissociation constant for binding (Kd) is determined with radiolabeled cytochalasin and characterizes the structural interaction between cytochalasin and actin. The inhibition constant (K 0 is measured from the effect of cytochalasins on the growth or shortening of the barbed end of actin filaments. CD ~ is about 10 times more effective than CB. For both cytochalasins, the binding and inhibition constants agree fairly well, which shows that binding causes inhibition of polymer-ization and depolymerization. The inhibition constant for growth with ATP-actin is quite different from the others and varies with the actin monomer concentration (9). These complications are probably attributable to the state of the nucleotide in the different experiments. The binding studies are performed with ATP-actin at steady state, where no net growth or shortening of filaments occurs. Actin monomers have mainly bound ATE and fila-1. Abbreviations used in thispaper: CB, cytochalasin B; CD, cytochalasin D. merits have mainly ADP because the ATP hydrolyzes after the monomer adds to the filament. In the functional studies in ADP all of the actin molecules have bound ADP. In ATE free monomers …