Myosin II-independent F-actin flow contributes to cell locomotion in Dictyostelium

While the treadmilling and retrograde flow of F-actin are believed to be re sponsible for the protrusion of leading edges, little is known about the me chanism that brings the posterior cell body forward. To elucidate the mecha nism for global cell locomotion, we examined the organizational changes of filamentous (F-) actin in live Dictyostelium discoideum. We labeled F-actin with a trace amount of fluorescent phalloidin and analyzed its dynamics in nearly two-dimensional cells by using a sensitive, high-resolution charge- coupled device. We optically resolved a cyclic mode of tightening and loose ning of fibrous cortical F-actin and quantitated its flow by measuring temp oral and spatial intensity changes. The rate of F-actin flow was evaluated with respect to migration velocity and morphometric changes. In migrating m onopodial cells, the cortical F-actin encircling the posterior cell body gr adually accumulated into the tail end at a speed of 0.35 mu m/minute, We sh ow qualitatively and quantitatively that the F-actin flow is closely associ ated with cell migration, Similarly, in dividing cells, the cortical F-acti n accumulated into the cleavage furrow Although five times slower than the wild type, the F-actin also flows rearward in migrating mhcA(-) cells demon strating that myosin II ('conventional' myosin) is not absolutely required for the observed dynamics of F-actin. Yet consistent with the reported tran sportation of ConA-beads, the direction of observed F-actin flow in Dictyos telium is conceptually opposite from a barbed-end binding to the plasma mem brane. This study suggests that the posterior end of the cell has a unique motif that tugs the cortical actin layer rearward by means of a mechanism i ndependent from myosin II; this mechanism may be also involved in cleavage furrow formation.