An actin-interacting heptapeptide in the cofilin sequence.

Cofilin, a 21-kDa actin-binding protein, has a hexapeptide sequence DAIKKK which is identical to the N-terminal portion (residues 2-7) of tropomyosin. The synthetic heptapeptide, DAIKKKL, corresponding to residues 122-128 of cofilin, inhibited the binding of cofilin to F-actin in a dose-dependent manner. The heptapeptide cosedimented with F-actin, decreased the fluorescence intensity of pyrene-labeled F-actin, and increased the rate of polymerization of G-actin. The hexapeptides, DIKKKL and DAIKKL, also inhibited the binding of cofilin to F-actin and affected the fluorescence intensity of pyrene-labeled F-actin and the rate of actin polymerization, like the heptapeptide. However, their effects were weaker than those of the heptapeptide. Moreover, the pentapeptide, DIKKL, had little or no effect. These results suggest that the heptapeptide sequence is specific for the interaction with actin and, therefore, may constitute part of the actin-binding domain of cofilin.