Actin at cell-cell junctions is composed of two dynamic and functional populations.
BACKGROUND In motile cells, protrusion of the lamellipodium (a type of cell margin) requires assembly of actin monomers into actin filaments at the tip of the lamellipodium. The importance of actin-filament disassembly in this process is less well understood, and is assessed here using the actin drug jasplakinolide, which has two known activities - inhibition of filament disassembly and induction of an increase in actin polymer. RESULTS In cells the two activities of jasplakinolide were found to be separable; 1 microM jasplakinolide could permeate cells, bind cellular filamentous actin (F-actin) and inhibit filament disassembly within 3.5 minutes, but significant increase in actin polymer was not detected until 60 minutes of treatment. In live, permeabilised cells, jasplakinolide did not inhibit filament assembly from supplied, purified actin monomers. In migrating chick fibroblasts, lamellipodium protrusion was blocked within 1-5 minutes of treatment with 1 microM jasplakinolide, without any perturbation of actin organisation. In non-migrating chick fibroblasts, there was a delay in the onset of jasplakinolide-induced inhibition of lamellipodium protrusion, during which lamellipodium length increased linearly with no increase in protrusion rate. Motility of the bacterium Listeria in infected PtK2 cells was reduced 2.3-fold within 3 minutes of treatment with 1 microM jasplakinolide. CONCLUSIONS Actin-filament disassembly is tightly coupled to lamellipodium protrusion in migrating chick fibroblasts and motility of Listeria in PtK2 cells. One simple interpretation of these data is a situation whereby ongoing actin-filament assembly uses free actin monomer derived from filament disassembly, in preference to stored monomer.