Cell migration is driven by local membrane protrusion through directed polymerization of F-actin at the front. However, F-actin next to the plasma membrane also tethers the membrane and thus resists outgoing protrusions. Here, we developed a fluorescent reporter to monitor changes in the density of membrane-proximal F-actin (MPA) during membrane protrusion and cell migration. Unlike the total F-actin concentration, which was high in the front of migrating cells, MPA density was low in the front and high in the back. Back-to-front MPA density gradients were controlled by higher cofilin-mediated turnover of F-actin in the front. Furthermore, nascent membrane protrusions selectively extended outward from areas where MPA density was reduced. Thus, locally low MPA density directs local membrane protrusions and stabilizes cell polarization during cell migration.
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