Focusing on unpolymerized actin

~ scinating feature of cell locomotion is that net actin polymerization and depolymerization occur simultaneously at different locations in the cytoplasm, while the total concentration of polymerized actin remains roughly constant. Net polymerization occurs primarily at the cell front and net depolymerization occurs, depending on cell type, throughout or at the rear of the lamella (Wang, 1985; Symons and Mitchison, 1991; Theriot and Mitchison, 1992; Zigmond, 1993). The actin flux is particularly remarkable since the rates of elongation and of depolymerization can be very high. Specifically, the rate of filament elongation is at least as high as the rate of cell protrusion and the half life of monomers in a filament can be less than 1 rain in a keratocyte and 10 s in a neutrophil (Theriot and Mitchison, 1991; Cassimeris et al., 1990). In contrast, in a test tube, a concentration of monomeric actin high enough to cause rapid elongation would inhibit depolymerization. How then does a cell maintain high rates of net polymerization and net depolymerization simultaneously at different sites in its cytoplasm? The answer is probably complex involving: (a) distinct properties of actin molecules containing different intermediates of ATP hydrolysis (e.g., ATP, ADP-Pi, ADP); (b) the effects of proteins that bind to monomeric or filamentous actin; (c) the spatial distribution of monomeric actin, actin binding proteins, and their modulators in the cell. Several recent studies provide new insights into these issues. In this issue of The Journal of Cell Biology, Cao et al. report the use of a fluorescent derivative of vitamin D-binding protein as a specific probe for unpolymerized actin in cultured vertebrate cells, enabling direct comparisons of the distribution of unpolymerized actin to that of microinjected fluorescent actin (total actin) and to actin filaments stained with phalloidin (Cao et al., 1993). The remarkable observation is that some of the unpolymerized actin is localized in discrete foci as shown by labeling with vitamin D-binding protein and lack of labering with phalloidin. Similar punctate loci are also observed in the leading edge of riving epithelial cells injected with fluorescently labeled actin. These appear to move rearward as locomotion proceeds. After fixation, these foci stain with vitamin D-binding protein. Actin in discrete loci within the leading lamella of cultured cells have also been seen with antibodies specific for the ~ isotype of actin (Hoock et al., 1991). While foci of highly localized unpolymerized actin have been observed in the gametes of marine invertebrates (Tdney et al., 1973; Tilney, 1976; Spudich et al., 1988; Bonder et al., 1989), they had not been observed in vertebrate cells. Before considering the functional significance of these foci, a bit of background is required.