Figure 244

Thin filament organization and structure in cardiac cells, a. Immunofluorescence micrograph of a chick cardiac myocyte stained for actin (green) to show the thin filaments and for tropomodulin (red) to show the location of the slow-growing (-) ends of the thin filaments. Tropomodulin appears as regular striations because of the uniform lengths and alignments of the thin filaments in sarcomeres, b. Diagram of a thin filament. The polarity of the thin filament is indicated kidney and intestinal absorptive cells, nerve growth cones, and inner ear hair cells.

Actin filaments participate in a variety of cell functions

Actin filaments are often grouped in bundles close to the plasma membrane. Functions of these membrane-associ-ated actin filaments include

• Anchorage and movement of membrane protein. Actin filaments are distributed in three-dimensional networks throughout the cell and are used as anchors within specialized cell junctions such as focal adhesions.

• 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 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 bundles of actin filaments with their plus ends directed toward the plasma membrane.

• Extension of cell processes. These processes can be observed in many other cells that exhibit small protrusions by the fast-growing (+) end and the slow-growing (-) end. Only a portion of the entire thin filament 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 filament. (Courtesy of Drs. Velia F. Fowler and Ryan Littlefield.)

troponin complex troponin complex

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