Cell http://doi.org/3f3 (2015)

Cells that move fast change direction less often than slower cells. In fact, only recently it has been shown that in three dimensions and in the absence of significant environmental cues and constraints, cells do not follow a random walk. Yet the underlying mechanism has remained elusive. Now, Paolo Maiuri et al. confirm by using previously published data for a variety of cell types that the persistence of cell movement in one direction depends exponentially on cell velocity, and demonstrate that this coupling between cell persistence and cell velocity is general and mediated by actin flows. The intracellular actin flows reinforce cell polarity in polarized migrating cells by enhancing the asymmetry of actin-interacting polarity cues. Although the molecules responsible for the generation of such polarity cues (which most likely depend on cell type) remain to be discerned, the researchers note that the velocity–persistence coupling of cell migration also applies to the movement of individual cell protrusions, and thus may apply to other phenomena that are also driven by actin polymerization.