Tropomyosin binding to F-actin protects the F-actin from disassembly by brain actin-depolymerizing factor (ADF).

Brain or muscle F-actin is rapidly depolymerized to monomeric actin in vitro by actin-depolymerizing factor, a protein isolated from chick embryo brain. Binding of muscle tropomyosin to muscle F-actin protects the F-actin from depolymerization by this factor. A 8.4/1.0 molar ratio of actin subunits to tropomyosin, achieved by incubation of the F-actin with excess tropomyosin, protects 58% of the F-actin from depolymerization by excess actin-depolymerizing factor for at least 3 hr at 25 degrees C. Thus, actin-depolymerizing factor seems to be specifically directed toward actin filaments lacking tropomyosin.