Focal Adhesions: Transmembrane Junctions Between the Extracellular Matrix and the Cytoskeleton

Many cells grown in tissue culture adhere tightly to the underlying substrate through discrete regions of the plasma membrane, referred to as adhesion plaques, focal contacts, or focal adhesions. In these regions where the surface of the cell comes closest to the substrate, the plasma membrane is specialized at its cytoplasmic face for anchoring stress fibers, the large bundles of microfilaments that are prominent in many cultured cells. In this review we discuss the composition and organization of focal adhesions. Many of their characteristics indicate that they are structurally and functionally equivalent to the adhesions made by many cells to the extracellular matrix (ECM) in vivo. We also consider the regulatory proteins that have been identified in focal adhesions since these regions are interesting not only as models for studying the links between the extracellular matrix and the cytoskeleton, but also as sites of transmembrane communication between the extracellular environment and the cytoplasm. Many features of focal adhesions, some of which are not considered here, have been reviewed previously (Burridge 1986; Woods and Couchman 1988).