The role of substrate curvature in actin-based pushing forces.

The extension of the plasma membrane during cell crawling or spreading is known to require actin polymerization; however, the question of how pushing forces derive from actin polymerization remains open. A leading theory (herein referred to as elastic propulsion) illustrates how elastic stresses in networks growing on curved surfaces can result in forces that push particles. To date all examples of reconstituted motility have used curved surfaces, raising the possibility that such squeezing forces are essential for actin-based pushing. By contrast, other theories, such as molecular ratchets, neither require nor consider surface curvature to explain pushing forces. Here, we critically test the requirement of substrate curvature by reconstituting actin-based motility on polystyrene disks. We find that disks move through extracts in a manner that indicates pushing forces on their flat surfaces and that disks typically move faster than the spheres they are manufactured from. For a subset of actin tails that form on the perimeter of disks, we find no correlation between local surface curvature and tail position. Collectively the data indicate that curvature-dependent mechanisms are not required for actin-based pushing.