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 of actin laid down near the bead surface force expansion of and thus induce compressive stresses in the older, outer layers. The bead shoots forward like a marble squeezed between finger and thumb.
But the absence of obvious convex curvature at the leading edge made McGrath wonder if the elastic propulsion model, and the assay systems used to study it, held any relevance for moving cells. “Maybe [the beads] are operating by a completely different mechanism than what is happening in cells,” he says. “That's a problem if we want to understand the cell's mechanism.”
McGrath's new experiments do not resolve whether spherical objects move using elastic propulsion. But they do show that an in vitro system can recapitulate movement via actin action at a flat surface. The Rochester group saw squashed beads move in directions that could only result from force against the flat, not curved, portions of their beads. McGrath hopes in the future to pattern actin polymerizing proteins only on the flat surfaces. ▪