 Formation of the structural core of microvilli on absorptive epithelial cells. Actin filaments may also help maintain the shape of the apical cell surface (e.g., the apical terminal web of actin filaments serves as a set of tension cables under the cell surface).
 Locomotion of cells. Locomotion is achieved by the force exerted by actin filaments by polymerization at their growing ends. This mechanism is used in many migrating cells—in particular, on transformed cells of invasive tumors. As a result of actin polymerization at their leading edge, cells extend processes from their surface by pushing the plasma membrane ahead of the growing actin filaments. The leading-edge extensions of a crawling cell are called lamellipodia; they contain elongating organized
FIGURE 2.47 • Thin flament organization and structure in cardiac cells. a. Immunofuorescence micrograph of a chick cardiac myocyte stained for actin (green) to show the thin flaments and for tropomodulin (red ) to show the location of the slow-growing ( ) ends of the thin flaments. Tropomodulin appears as regular striations because of the uniform lengths and alignment of the thin flaments in sarcomeres. 320. (Courtesy of Drs. Velia F. Fowler and Ryan Littlefeld.) b. Diagram of a thin flament. The polarity of the thin flament is indicated by the fast-growing ( ) end and the slow-growing ( ) end. Only a portion of the entire thin flament is shown for clarity. Tropomodulin is bound to actin and tropomyosin at the slow-growing ( ) end. The troponin complex binds to each tropomyosin molecule every seven actin monomers along the length of the thin flament. (Courtesy of Drs. Velia F. Fowler and Ryan Littlefeld.) bundles of actin filaments with their plus ends directed to ward the plasma membrane. Extension of cell processes. These processes can be ob called flopodia, located around their surface. As in lamel lipodia, these protrusions contain loose aggregations of 10 to 20 actin filaments organized in the same direction, again with their plus ends directed toward the plasma mem brane. Actin filaments are also essential in cytoplasmic streaming (i.e., the streamlike movement of cytoplasm that can be observed in cultured cells).
Intermediate flaments play a supporting or general structural role. These ropelike filaments are called intermediate because their diameter of 8 to 10 nm is between those of actin filaments and microtubules. Nearly all intermediate filaments consist of subunits with a molecular weight of about 50 kDa. Some evidence suggests that many of the stable structural proteins in intermediate filaments evolved from highly conserved enzymes, with only minor genetic modification.
Intermediate filaments are formed from nonpolar and highly variable intermediate filament subunits.
Unlike those of microfilaments and microtubules, the protein subunits of intermediate filaments show considerable diversity and tissue specificity. In addition, they do not posses enzymatic activity and form nonpolar filaments. Intermediate filaments also do not typically disappear and re-form in the continuous manner characteristic of most microtubules and actin filaments. For these reasons, intermediate filaments are believed to play a primarily structural role within the cell and to compose the cytoplasmic link of a tissuewide continuum of cytoplasmic, nuclear, and extracellular filaments (Fig. 2.48).