FIGURE 2.45 • Distribution of actin flaments in pulmonary artery endothelial cells in culture. Cells were fxed and stained with NDB phallacidin stain conjugated with fuorescein dye. Phallacidin binds and stabilizes actin flaments, preventing their depolymerization. Note the accumulation of actin flaments at the periphery of the cell just beneath the plasma membrane. These cells were also stained with two additional dyes: a mitochondria-selective dye (i.e., MitoTracker Red) that allows the visualization of mitochondria (red ) in the middle of the cell and DAPI stain that reacts with nuclear DNA and exhibits blue fuorescence over the nucleus. 3,000. (Courtesy of Molecular Probes, Inc., Eugene, OR.) proteins they contain. There are two types of filaments (myoflaments) present in muscle cells: 6to 8-nm actin filaments (called thin flaments; Fig. 2.47) and 15-nm filaments (called thick flaments) of myosin II, which is the predominant protein in muscle cells. Myosin II is a double-headed molecule with an elongated rodlike tail. The specific structural and functional relationships among actin, myosin, and other ABPs in muscle contraction are
FIGURE 2.46 • Polymerization of actin flaments. Actin flaments are polarized structures. Their fast-growing end is referred to as the plus ( ) or barbed end; the slow-growing end is referred to as the minus ( ) or pointed end. The dynamic process of actin polymerization requires energy in the form of an ATP molecule that is hydrolyzed to ADP after a G-actin molecule is incorporated into the flament.
 Actin flament–severing proteins cut long actin filaments into short fragments. An example of such a protein is gelsolin, a 90-kDa ABP that normally initiates actin polymerization but at high Ca2 concentrations causes severing of the actin filaments, converting an actin gel into a ﬂuid state.
 Actin-capping proteins block further addition of actin molecules by binding to the free end of an actin filament. An example is tropomodulin, which can be isolated from skeletal and cardiac muscle cells. Tropomodulin binds to the free end of actin myofilaments, regulating the length of the filaments in a sarcomere.
 Actin cross-linking proteins are responsible for cross-linking actin filaments with each other. An example of such proteins can be found in the cytoskeleton of erythrocytes. Several proteins—such as spectrin, adductin, protein 4.1,and protein 4.9—are involved in cross-linking actin filaments.
 Actin motor proteins belong to the myosin family, which hydrolyzes ATP to provide the energy for movement along the actin filament from the minus end to the plus end. Some cells, such as muscle cells, are characterized by the size, amount, and nature of the filaments and actin-motor discussed in Chapter 11 (Muscle Tissue).
In addition to myosin II, nonmuscle cells contain myosin I, a protein with a single globular domain and short tail that attaches to other molecules or organelles. Extensive studies have revealed the presence of a variety of other nonmuscle myosin isoforms that are responsible for motor functions in many specialized cells, such as melanocytes, 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-associated actin filaments include the following.
 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.