Rapid formation and high diffusibility of actin-cofilin cofilaments at low pH

Cofilin is a small actin-binding protein that is known to bind both F-actin and G-actin, severing the former. The interaction of cofilin with actin is pH-sensitive, F-actin being preferentially bound at low pH and G-actin at higher pH, within the physiological range. Diffusion coefficients of F-actin with cofilin were measured by the fluorescence recovery after photobleaching (FRAP) technique. This has the potential for simultaneous and direct measurement of average polymer length via the average diffusion coefficient of the polymers (DLM) as well as the fraction of polymerized actin, fLM, present in solution. In the range of cofilin-actin ratios up to 1 : 1 and at both pH 6.5 and pH 8.0, the diffusion coefficients of the polymers increased with the amount of cofilin present in the complex, in a co-operative manner to a plateau. We interpret this as indicating co-operative binding/severing and that filaments less than a certain length cannot be severed further. Under the conditions used here, filaments were found to be more motile at pH 6.5 than at pH 8.0. At pH 8.0, some actin is expected to be sequestered as ADP-actin-cofilin complexes, with the remaining actin being present as long slowly diffusing filaments. At pH 6.5, however, cofilin binds to F-actin to form short rapidly diffusing cofilaments. These filaments form very rapidly from cofilin-actin monomeric complexes, possibly indicating that this complex is able to polymerize without dissociation. These findings may be relevant to the nuclear import of actin-cofilin complexes.