ON THE CRAWLING OF ANIMAL-CELLS

Cells crawl in response to external stimuli by extending and remodelin g peripheral elastic lamellae in the direction of locomotion. The remo deling requires vectorial assembly of actin subunits into linear polym ers at the lamella's leading edge and the crosslinking of the filament s by bifunctional gelation proteins. The disassembly of the crosslinke d filaments into short fragments or monomeric subunits away from the l eading edge supplies components for the actin assembly reactions that drive protrusion. Cellular proteins that respond to lipid and ionic si gnals elicited by sensory cues escort actin through this cycle in whic h filaments are assembled, crosslinked, and disassembled. One class of myosin molecules may contribute to crawling by guiding sensory recept ors to the cell surface, and another class may contribute by imposing contractile forces on actin networks in the lamellae.