Myosin II contributes to cell-scale actin network treadmilling through network disassembly

Abstract

In animals, most cells when on the move migrate using a crawling motion, in which the front of the cell is propelled forward by the force provided by polymerization of actin filaments. Cell biologists have generally assumed that the rear of the crawling cell is then pushed forward by a contractile force generated by non-muscle myosin II. Observations of fish keratocytes in motion now show that no actual contraction is required for rear retraction. Rather, the myosin II has a direct role in facilitating actin network treadmilling via actin disassembly. Eukaryotic cells crawl through a process in which the front of the cell is propelled forwards by the force provided by polymerization of actin filaments. These must be disassembled at the rear of the cell to allow sustained motility. It is now shown that non-muscle myosin II protein is needed for the disassembly of actin networks at the rear of crawling cells. Crawling locomotion of eukaryotic cells is achieved by a process dependent on the actin cytoskeleton1: protrusion of the leading edge requires assembly of a network of actin filaments2, which must be disassembled at the cell rear for sustained motility. Although ADF/cofilin proteins have been shown to contribute to actin disassembly3, it is not clear how activity of these locally acting proteins could be coordinated over the distance scale of the whole cell. Here we show that non-muscle myosin II has a direct role in actin network disassembly in crawling cells. In fish keratocytes undergoing motility, myosin II is concentrated in regions at the rear with high rates of network disassembly. Activation of myosin II by ATP in detergent-extracted cytoskeletons results in rear-localized disassembly of the actin network. Inhibition of myosin II activity and stabilization of actin filaments synergistically impede cell motility, suggesting the existence of two disassembly pathways, one of which requires myosin II activity. Our results establish the importance of myosin II as an enzyme for actin network disassembly; we propose that gradual formation and reorganization of an actomyosin network provides an intrinsic destruction timer, enabling long-range coordination of actin network treadmilling in motile cells.