Actin filaments are severed by both native and recombinant dictyostelium cofilin but to different extents

Cofilin has been reported to depolymerize F-actin alternately by either severing filaments to increase the number of depolymerizing ends or by increasing the off-rate of monomers from F-actin without increasing the number of filament ends. We have compared directly the ability of native and recombinant cofilins from Dictyostelium to sever F-actin. Our results demonstrate that native cofilin has a higher level of severing activity than recombinant cofilin. Significantly, the measurement of cofilin's severing activity by two independent methods, direct visualization with an improved light microscope assay and by scoring of the number of pointed ends by DNase I binding, clearly shows that both native and recombinant cofilins sever F-actin but to different extents. The severing activity in preparations of recombinant cofilin is variable depending on the method of preparation and, in some cases, is difficult to detect by microscopy assays. This latter point is particularly significant because it may lead to the conclusion that cofilin severs weakly or not at all depending on its method of isolation.