Interaction of Fi bronectin-coated Beads with Attached and Spread Fibroblasts

After 15 min incubations, binding of 0.8-, 6-, and 16ym fibronectin-coated latex beads occurred primarily at the margins of chick embryo tibroblasts that previously were attached and spread on tibronectin-coated glass coverslips. Extensive phagocytosis of the smallest beads and some phagocytosis of the larger beads occurred within 2 h. Following binding of the 16-urn beads, there were no changes in overall cell shape or in the distribution of several cytoskeletal proteins. There was, however, a local accumulation of actin and a-actinin patches adjacent to the sites where the beads were bound. The formation of a-actinin patches could be detected with 6or 16-urn beads shortly after initial bead binding to the cells, but a similar reorganization of a-actinin in response to the binding of 0.8~urn beads was not detected. The patches of a-actinin appeared to be associated with membrane ruffles, since such structures were observed by scanning electron microscopy @EM) to be sites of cell interaction with 6but not 0.8~nm beads. Also, two other cytoskeletal proteins normally absent from membrane ruffles, tropomyosin and vinculin, were not detected at the sites of cell-bead interaction. No reorganization of vinculin at the cell-bead interaction sites was observed even when the 16-urn beads remained bound at the cell surfaces for up to 6 h. Nevertheless, prominent vinculin plaques were observed at the marginal attachment sites on the ventral cell surfaces. Consequently, formation of mature focal adhesions may be restricted to linear regions of cell-substratum interac-