MOVING AND STATIONARY ACTIN-FILAMENTS ARE INVOLVED IN SPREADING OF POSTMITOTIC PTK2 CELLS

We have investigated spreading of postmitotic PtK2 cells and the behav ior of actin filaments in this system by time-lapse microscopy and pho toactivation of fluorescence. During mitosis PtK2 cells round up and a t cytokinesis the daughter cells spread back to regain their interphas e morphology. Normal spreading edges are quite homogenous and are not comprised of two distinct areas (lamellae and lamellipodia) as found i n moving edges of interphase motile cells. Spreading edges are connect ed to a network of long, thin, actin-rich fibers called retraction fib ers. A role for retraction fibers in spreading was tested by mechanica l disruption of fibers ahead of a spreading edge. Spreading is inhibit ed over the region of disruption, but not over neighboring intact fibe rs. Using photoactivation of fluorescence to mark actin filaments, we have determined that the majority of actin filaments move forward in s preading edges at the same rate as the edge. As far as we are aware, t his is the first time that forward movement of a cell edge has been co rrelated with forward movement of actin filaments. In contrast, actin filaments in retraction fibers remain stationary with respect to the s ubstrate. Thus there are at least two dynamic populations of actin pol ymer in spreading postmitotic cells. This is supported by the observat ion that actin filaments in some spreading edges not only move forward , but also separate into two fractions or broaden with time. A small f raction of postmitotic cells have a spreading edge with a distinct lam ellipodium. In these edges, marked actin polymer fluxes backward with respect to substrate. We suggest that forward movement of actin filame nts may participate in generating force for spreading in postmitotic c ells and perhaps more generally for cell locomotion.