Spatial regulation of actin dynamics: a tropomyosin-free, actin-rich compartment at the leading edge.

Rapid polymerization of a network of short, branched actin filaments takes place at the leading edge of migrating cells, a compartment enriched in activators of actin polymerization such as the Arp2/3 complex and cofilin. Actin filaments elsewhere in the cell are long and unbranched. Results reported here show that the presence or absence of tropomyosin in these different actin-containing regions helps establish functionally distinct actin-containing compartments in the cell. Tropomyosin, an inhibitor of the Arp2/3 complex and cofilin function, was localized in relation to actin filaments, the Arp2/3 complex, and free barbed ends of actin filaments in MTLn3 cells, which rapidly extend flat lamellipodia following EGF stimulation. All tropomyosin isoforms examined using indirect immunofluorescence were relatively absent from the dynamic leading edge compartment, but did colocalize with actin structures deeper in the lamellipodium and in stress fibers. An in vitro light microscopy assay revealed that tropomyosin protects actin filaments from cofilin severing. The results suggest that tropomyosin-free actin filaments under the membrane can participate in rapid, dynamic processes that depend on interactions between the activities of the Arp2/3 complex and ADF/cofilin that tropomyosin inhibits elsewhere in the cell.