SSH1L (blue) moves to the lamellipod when cells are induced to migrate (bottom).

The actin cytoskeleton at the front of migrating cells is an ever-changing species. Although actin is polymerized in the lamellipod, the filaments that are built remain dynamic. Their turnover is mediated by cofilin, which stimulates filament disassembly at or near the pointed ends, thus providing monomers for further polymerization. On page 465, Nagata-Ohashi et al. show that actin filaments take care of their own dynamic behavior via Slingshot-1L (SSH1L), a phosphatase that activates cofilin.

In vitro, F-actin activated SSH1L 10-fold. Migrating epithelial cells accumulated SSH1L (and thus dephosphorylated and active cofilin) in the lamellipod, and this depended on an intact actin assembly.

To bind to F-actin and activate cofilin, SSH1L must first be released from an interaction with 14–3-3 proteins, which sequester SSH1L in the cytoplasm. SSH1L escaped this interaction when it was dephosphorylated. This dephosphorylation could be induced with the extracellular migratory signal, NRG.

Restricting SSH1L activity to the F-actin–rich lamellipod may help to maintain polarized migration by allowing more stable actin structures, such as stress fibers, to persist at the rear of the cell. Cofilin was recently shown to be capable of defining the direction of cell migration (Ghosh et al. Science. 304:743–746). Now, the authors need to determine whether this function requires the F-actin– mediated activation of SSH1L. ▪