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 [1]; 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 [2, 3]. 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 [4, 5], 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.