The rate of annealing of actin tropomyosin filaments depends strongly on the length of the filaments.

Actin tropomyosin filaments were sheared to produce short filaments. Following incubation for 0 to 10000 s annealing of the filaments was assayed by determination of the rate of polymerization of monomeric actin onto the filament ends. The rate of decrease of the concentration of filament ends was found to be proportional to its fourth power. In contrast, the rate of end-to-end association of actin filaments in the absence of tropomyosin was proportional to the square of the concentration of filament ends. The strong dependence on the filament length of the rate of annealing of actin tropomyosin filaments was interpreted by the model of Hill (Biophys. J., 44, 285-288 (1983)) who pointed out that the rate constant of end-to-end association of long rod-like filaments is expected to depend on the length of the filaments for sterical conditions.