Actin and the coordination of protrusion, attachment and retraction in cell crawling

To crawl over a substrate a cell must first protrude in front, establish new attachments to the substrate and then retract its rear. Protrusion and retraction utilise different subcompartments of the actin cytoskeleton and operate by different mechanisms, one involving actin polymerization and the other myosin-based contraction. Using as examples the rapidly locomoting keratocyte and the slowly moving fibroblast we illustrate how over expression of one or the other actin subcompartments leads to the observed differences in motility. We also propose, that despite these differences there is a common coordination mechanism underlying the genesis of the actin cytoskeleton that involves the nucleation of actin filaments at the protruding cell front, in the lamellipodium, and the relocation of these filaments, via polymerization and flow, to the more posterior actin filament compartments.