The leading edge is a lipid diffusion barrier

Actin polymerization drives many cellular events, including endocytosis, pathogen rocketing, and cell spreading. Force generation and polymerization regulation are intimately linked where an actin meshwork attaches to, and pushes against, an interface. We reasoned that interaction with actin filament plus-ends might stabilize the position of components within the plasma membrane at the leading edge, thereby slowing the diffusion of lipids within the bilayer where filament growth occurs. To test this hypothesis we focally labeled the outer membrane leaflet of migrating keratocytes and compared the initial diffusion of carbocyanine dyes in the dorsal and ventral lamellipodium membranes using sequential TIRF and epi-fluorescent imaging. Global diffusion analysis shows that lateral mobility of lipids in the outer membrane leaflet is blocked at the leading edge during protrusion. Cytochalasin treatment abolished this diffusion barrier, but we found no evidence to support the involvement of membrane microdomains. Our results demonstrate the immobilization of membrane components at the leading edge, and suggest that interaction between actin filaments and the plasma membrane is mediated by densely packed molecular complexes. We propose that actin polymerization traps regulatory proteins at the leading edge in a positive-feedback loop.