Actin is the most abundant intracellular protein in most eu-karyotic cells. In muscle cells, for example, actin comprises 10 percent by weight of the total cell protein; even in non-muscle cells, actin makes up 1-5 percent of the cellular protein. The cytosolic concentration of actin in nonmuscle cells ranges from 0.1 to 0.5 mM; in special structures such as microvilli, however, the local actin concentration can be 5 mM.
M FIGURE 19-2 Actin cytoskeleton in a moving cell. Fish keratinocytes are among the fastest crawling cells. Two actin-containing structures work together to generate the force for movement. A network of actin filaments in the front of the cell pushes the membrane forward. Meanwhile, the cell body is pulled by a band of myosin and actin (bracketed). This arrangement of actin and myosin is typical in a moving cell. [From T M. Svitkina et al., 1997, J. Cell Biol. 139:397.]
To grasp how much actin cells contain, consider a typical liver cell, which has 2 X 104 insulin receptor molecules but approximately 5 X 108, or half a billion, actin molecules. The high concentration of actin compared with other cell proteins is a common feature of all cytoskeletal proteins. Because they form structures that cover large parts of the cell interior, these proteins are among the most abundant proteins in a cell.
A moderate-sized protein with a molecular weight of 42,000, actin is encoded by a large, highly conserved gene family. Actin arose from a bacterial ancestor and then evolved further as eukaryotic cells became specialized. Some single-celled organisms such as rod-shaped bacteria, yeasts, and amebas have one or two actin genes, whereas many mul-ticellular organisms contain multiple actin genes. For instance, humans have six actin genes, which encode isoforms of the protein, and some plants have more than 60 actin genes, although most are pseudogenes. In vertebrates, the four a-actin isoforms present in various muscle cells and the P-actin and 7-actin isoforms present in nonmuscle cells differ at only four or five positions. Although these differences among isoforms seem minor, the isoforms have different functions: a-actin is associated with contractile structures; 7-actin accounts for filaments in stress fibers; and p-actin is at the front, or leading edge, of moving cells where actin
▲ FIGURE 19-3 Structures of monomeric G-actin and F-actin filament. (a) Model of a p-actin monomer from a nonmuscle cell shows it to be a platelike molecule (measuring 5.5 X 5.5 X 3.5 nm) divided by a central cleft into two approximately equally sized lobes and four subdomains, numbered I—IV. ATP (red) binds at the bottom of the cleft and contacts both lobes (the yellow ball represents Mg2+). The island C-termini lie in subdomain I. (b) In the electron microscope, negatively stained actin filaments appear as long, flexible, and twisted strands of beaded subunits. Because of the twist, the filament appears alternately thinner (7 nm diameter) and thicker filaments polymerize (Figure 19-2). Sequencing of actins from different sources has revealed that they are among the most conserved proteins in a cell, comparable with histones, the structural proteins of chromatin (Chapter 10). The sequences of actins from amebas and from animals are identical at 80 percent of the positions.
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