Tropomyosin stabilizes the pointed end of actin filaments by slowing depolymerization

Tropomyosin is postulated to confer stability to actin filaments in nonmuscle cells. We have found that a nonmuscle tropomyosin isolated from the intestinal epithelium can directly stabilize actin filaments by slowing depolymerization from the pointed, or slow-growing, filament end. Kinetics of elongation and depolymerization from the pointed end were measured in fluorescence assays using pyrenylactin filaments capped at the barbed end by villin. The initial pointed end depolymerization rate in the presence of tropomyosin averaged 56% of the control rate. Elongation from the pointed filament end in the presence of tropomyosin occurred at a lower free G-actin concentration, although the on rate constant, kappa p+, was not greatly affected. Furthermore, in the presence of tropomyosin, the free G-actin concentration was lower at steady state. Therefore, nonmuscle tropomyosin stabilizes the pointed filament end by lowering the off rate constant, kappa p-.