Flat-out pushing by actin

Lamellipodia are almost flat, whereas many model systems for actin-based propulsion involve the convex surfaces of beads, vesicles, or bacteria such as Listeria. Thus, it is reassuring that actin pushing against a flat surface can lead to productive movement even in these model systems, as demonstrated by Ian Schwartz, James McGrath, and colleagues (University of Rochester, NY) using squashed beads.

Actin at the leading edge of moving cells has been suggested to work as a tethered ratchet. Thermal fluctuations lead to the bending of actin filaments away from the cell surface, freeing them for lengthening by polymerization. As these bent, and thus strained, longer filaments relax by straightening, they exert a forward force on the cell surface.

Bead-based experiments have, by contrast, led to the elastic propulsion model. In this model, new layers...