Membrane-induced bundling of actin filaments.

Dynamic interplay between the plasma membrane and underlying cytoskeleton is essential for cellular shape change. Spatial organization of actin filaments, whose growth generates membrane deformations during motility 1, phagocytosis 2, endocytosis 3, and cytokinesis 4, is mediated by specific protein-protein interactions that branch, crosslink, and bundle filaments into networks that interact with the membrane. Although membrane curvature has been found to influence binding of proteins with curvature-sensitive domains 5, the direct effect of membrane elasticity on cytoskeletal network organization is not clear. Here we show through in vitro reconstitution and elastic modeling that a lipid bilayer can drive the emergence of bundled actin filament protrusions from branched actin filament networks, thus playing a role normally attributed to actin-binding proteins. Formation of these filopodium-like protrusions with only a minimal set of purified proteins points to an active participation of the membrane in organizing actin filaments at the plasma membrane. In this way, elastic interactions between the membrane and cytoskeleton can cooperate with accessory proteins to drive cellular shape change.