In a cell lacking cofilin, actin monomers (red) still hook onto actin fibers at the cell margin.

To keep moving, a crawling cell requires a continuous supply of actin molecules. As Kiuchi et al. show on page 465, cofilin meets this need mainly by snipping apart actin fibers.

As a cell slithers, addition of new actin molecules to the actin fibers at its leading edge nudges the membrane forward. Cofilin boosts the amount of available actin monomers in the cell. It depolymerizes actin fibers, removing individual molecules. It can also sever fibers, leaving barbed ends for actin nucleation. Scientists weren't sure which action was more important for actin polymerization and how much cofilin contributed to the actin pool.

By tracking fluorescent actin, Kiuchi et al. determined that cofilin is responsible for more than half of the available monomers. Experiments with a cofilin mutant that couldn't depolymerize actin and another that couldn't clip it revealed that most of the free actin comes from severed fibers.

Cofilin spurs fibers at the cell's margin to add monomers. That effect might occur either because cofilin hikes the amount of free actin or because it increases the number of barbed ends, which are sticky to the monomers. But actin injected into cells hooks onto the fibers even if cofilin is blocked, ruling out the second explanation. The results indicate that cofilin keeps the actin pool well-stocked mainly by dismembering existing fibers.