ABSTRACT
The actin cytoskeleton of polarized epithelial cells is organized into distinct apical, lateral, and basal plasma membrane domains, which are functionally and structurally distinct (Mooseker, 1985). For example, the apical membrane includes microvilli containing bundled arrays of parallel microfilaments that extend into the cytoplasm and terminate in a fibrous array of F-actin known as the terminal web (Drenckhahn and Groschel-Stewart, 1980). The basal and lateral membranes are associated with a dense cortical cytoskeleton composed of short cross-linked actin filaments and associated proteins like spectrin, protein 4.1, and ankyrin, which cross-link actin filaments and anchor them to the plasma membrane (Morrow et al., 1997). In cultured cells the basal membrane is also associated with stress fibers — contractile bundles of actin and nonmuscle myosin which terminate at the plasma membrane at integrin-based adhesion sites known as focal contacts. Finally, the lateral surface is associated with several sites of cell-cell contact (Hirokawa and Tilney, 1982). The most prominent of these is the apical junctional complex, which includes both TJs and the adherens junctions. The TJ creates a seal within the paracellular space, and delineates the apical and basal-lateral domains by forming a barrier to the diffusion of transmembrane proteins and lipids (see Chapters 8 and 14). The adherens junction, which in vertebrates is positioned more basal relative to the TJ in the lateral membrane, forms an adhesive structure of cadherin-based cell-cell contacts.
