ACTIN AND THE COORDINATION OF PROTRUSION, ATTACHMENT AND RETRACTION IN CELL CRAWLING

To crawl over a substrate a cell must first protrude in front, establi sh new attachments to the substrate and then retract its rear. Protrus ion and retraction utilise different subcompartments of the actin cyto skeleton and operate by different mechanisms, one involving actin poly merization and the other myosin-based contraction. Using as examples t he rapidly locomoting keratocyte and the slowly moving fibroblast we i llustrate how over expression of one or the other actin subcompartment s leads to the observed differences in motility. We also propose, that despite these differences there is a common coordination mechanism un derlying the genesis of the actin cytoskeleton that involves the nucle ation of actin filaments at the protruding cell front, in the lamellip odium, and the relocation of these filaments, via polymerization and f low, to the more posterior actin filament compartments.