Dynamic Localization of G-Actin during Membrane Protrusion in Neuronal Motility

Background: Actin-based cell motility is fundamental, for development, function, and malignant events in eukaryotic organisms. During neural development, axonal growth cones depend on rapid assembly and disassembly of actin filaments (F-actin) for their guided extension to specific targets for wiring. Monomeric globular actin (G-actin) is the building block for F-actin but is not considered to play a direct role in spatiotemporal control of actin dynamics in cell motility. Results: Here we report that a pool of G-actin dynamically localizes to the leading edge of growth cones and neuroblastoma cells to spatially elevate the G-/F-actin ratio that drives membrane protrusion and cell movement. Loss of G-actin localization leads to the cessation and retraction of membrane protrusions. Moreover, G-actin localization occurs asymmetrically in growth cones during attractive turning. Finally, we identify the actin monomer-binding proteins profilin and thymosin beta 4 as key molecules that localize actin monomers to the leading edge of lamellipodia for their motility. Conclusions: Our results suggest that dynamic localization of G-actin provides a novel mechanism to regulate the spatiotemporal actin dynamics underlying membrane protrusion in cell locomotion and growth cone chemotaxis.