Filament bursting is distinct from cofilin-mediated severing. (A) Time-lapse images of two actin filaments disassembling in 10 μM cofilin, 5 μM of actin monomer, and 2 mM ATP. Long filaments were chosen to illustrate the occurrence of multiple severing events within a single filament. Bar, 1 μm. (B) Bar graph showing the fraction of disassembly events scored as bursting (red) or severing (green), either in the full depolymerizing system (left) or in cofilin alone (right). In the full system, bursting occurred with significantly higher frequency than severing (χ2 = 190, df = 1, P < 0.01). However, in the presence of high concentrations of cofilin, severing occurred with significantly higher frequency than bursting (χ2 = 55, df = 1, P < 0.01) and was the predominant disassembly mechanism observed under these conditions. We note that mean initial filament length did not differ significantly between different experiments and was not the cause of the differences observed.
