Progress in protrusion: the tell-tale scar

The crawling movement of a cell involves protrusion of its leading edge, in coordination with the translocation of its cell body, and depends upon a c ytoplasmic machinery able to respond to signals from the environment. Protr usion is now understood to be driven by actin polymerization, and signallin g from membrane receptors to actin has been shown to be mediated by the Rho family of GTPases. However, a major gap in our understanding of regulated motility has been how to connect the signalling pathway to the motile machi nery itself. Recent structural, biochemical and genetic studies have identi fied some of the missing links and provided a strong working model for the pathways and mechanisms by which the signals are interpreted and implemente d.