Membrane-cytoskeletal interactions in cell adhesion and locomotion
How cells move in tissue culture continues to command the attention of cell biologists. Although many advances have been made in understanding the biochemistry and organization of various cytoskeletal components, the role these have in generating movement has not been established. Very different theories have been proposed to explain the migration of cultured cells (for example, compare Bretscher 1987, 1988, with Bray and White, 1988). In spite of major disagreements about how the force for cell movement is generated, most theories recognize that cell migration involves three distinct phases: extension of the cell membrane, adhesion of this newly extended region to the underlying substrate and then the bulk translocation of the cytoplasm in the direction of the cell extension. Much of the work in this laboratory has been aimed at understanding cell adhesion. Cells in culture can develop at least two types of adhesion to the underlying substrate. There are broad areas of weak adhesion known as close contacts, in which the plasma membrane is separated from the substrate by about 30–100 nm (Izzard and Lochner, 1976). Some cells also develop smaller, discrete regions that are more tightly adherent and which have a separation of 10–15 nm between the plasma membrane and the substrate (Izzard and Lochner, 1976). These latter have been known as focal adhesions, focal contacts or adhesion plaques and serve to anchor bundles of actin filaments (stress fibers) at their cytoplasmic face (Abercrombie et al., 1971; Heath and Dunn, 1978) . By comparison, much more is known about the structure and composition of focal adhesions than close contacts. In this review we will discuss briefly the organization of focal adhesions and their relationship to cell movement as well as to theories of cell migration. The organization and properties of focal adhesions have been discussed in greater detail elsewhere (Burridge, 1986; Burridge et al., 1988; Woods and Couchman, 1988).
KeywordsActin Filament Focal Adhesion Stress Fiber Motile Cell Focal Contact
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