Cells migrate by directing Ras-related C3 botulinum toxin substrate 1 (Rac1) and cell division control protein 42 (Cdc42) activities and by polymerizing actin toward the leading edge of the cell. Previous studies have proposed that this polarization process requires a local positive feedback in the leading edge involving Rac small GTPase and actin polymerization with PI3K likely playing a coordinating role. Here, we show that the pleckstrin homology and RhoGEF domain containing G3 (PLEKHG3) is a PI3K-regulated Rho guanine nucleotide exchange factor (RhoGEF) for Rac1 and Cdc42 that selectively binds to newly polymerized actin at the leading edge of migrating fibroblasts. Optogenetic inactivation of PLEKHG3 showed that PLEKHG3 is indispensable both for inducing and for maintaining cell polarity. By selectively binding to newly polymerized actin, PLEKHG3 promotes local Rac1/Cdc42 activation to induce more local actin polymerization, which in turn promotes the recruitment of more PLEKHG3 to induce and maintain cell front. Thus, autocatalytic reinforcement of PLEKHG3 localization to the leading edge of the cell provides a molecular basis for the proposed positive feedback loop that is required for cell polarization and directed migration.
Polarized cell migration plays a pivotal role in the development and repair of tissues. PI3K, RhoGTPases, and actin filaments are known to be involved in a positive feedback loop that induces and maintains cell polarity. Here, we show that the pleckstrin homology and RhoGEF domain containing G3 (PLEKHG3) selectively binds to newly polymerized actin and that this interaction exerts a positive regulatory effect on PLEKHG3 activity that enhances and sustains the cell front. A lack of PLEKHG3 ablates cell polarity, resulting in a decrease in cell migration. These findings provide the missing link that explains how Ras-related C3 botulinum toxin substrate 1 (Rac1) and actin polymerization are coupled by a positive feedback loop to ensure the stability of cell polarity.