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A brief overview about the current opinions in cell motility

Cells protrude by polymerizing monomeric (G) into polymeric (F) actin at the tip of the lamellipodium. Actin filaments are depolymerized towards the rear of the lamellipodium in a treadmilling process, thereby supplementing a G-actin pool for a new round of polymerization. In this scenario the concentrations of Fand G-actin are principal parameters, but have hitherto not been directly determined. By comparing fluorescence intensities of bleached and unbleached regions of lamellipodia in B16F1 mouse melanoma cells expressing GFP-actin, before and after extraction with Triton X-100, we show that the ratio of Fto G-actin is 3.1+/0.9. Using electron microscopy to determine the F-actin content, this ratio translates into Fand G-actin concentrations in lamellipodia of approximately 500 μM and 160 μM respectively. The excess of G-actin, at several orders of magnitude above the critical concentrations at filament ends shows that the polymerization rate is not limited by diffusion and is tightly controlled by polymerization/depolymerization modulators. Introduction Eukaryotic cells move by the extension of a leaf-like structure, the lamellipodium, at the cell front (Abercrombie et al., 1970b). Protrusion occurs by polymerization of actin filaments at the tip of the lamellipodium, thereby pushing the membrane forward (Wang, 1985). Actin filaments are polar, with the barbed, fast growing ends pointing towards the direction of protrusion (Small et al., 1978). Under steady state conditions the network of actin filaments in lamellipodia maintains a constant breadth by coordinated depolymerization from the filament pointed ends towards the rear, in a treadmilling regime (Lai et al., 2008; Wang, 1985; Waterman-Storer et al., 1998; Wegner, 1976) that is tightly regulated. Treadmilling relies in the first instance on inherent differences of critical concentration for growth at the two filament ends, measured in vitro as around 0.06 μM and 0.6 μM at the plus and minus ends respectively (Le Clainche and Carlier, 2007; Pollard et al., 2000). Regulation can take