Actin globular

G-actin (globular actin) has a molecular weight of about 42 kDa. In higher vertebrates, six isoforms of G-actin, which contain 374/375 residues, are expressed in a cell-specific manner. They are present in striated muscle cells (skeletal and cardiac isoforms), smooth muscle cells (vascular and visceral isoforms) and in non-muscle cells (two isoforms). 

Contractile proteins which form the myofibrils are of two types myosin ( thick filaments each approximately 12 nm in diameter and 1.5 (im long) and actin ( thin filaments 6nm diameter and 1 (Am in length). These two proteins are found not only in muscle cells but widely throughout tissues being part of the cytoskeleton of all cell types. Filamentous actin (F-actin) is a polymer composed of two entwined chains each composed of globular actin (G-actin) monomers. Skeletal muscle F-actin has associated with it two accessory proteins, tropomyosin and troponin complex which are not found in smooth muscle, and which act to regulate the contraction cycle (Figure 7.1). 

Actin, the most abundant protein in eukaryotic cells, is the protein component of the microfilaments (actin filaments). Actin occurs in two forms—a monomolecular form (C actin, globular actin) and a polymer (F actin, filamentous actin). G actin is an asymmetrical molecule with a mass of 42 kDa, consisting of two domains. As the ionic strength increases, G actin aggregates reversibly to form F actin, a helical homopolymer. G actin carries a firmly bound ATP molecule that is slowly hydrolyzed in F actin to form ADR Actin therefore also has enzyme properties (ATPase activity). 

Figure 12.11 (a) Polymerization of globular actin (G-actin) and modification of the filament... 

As in the case of the myosin head, knowledge of actin filament structure, or thin filament structure as it is termed when tropomyosin and troponin are present, also progressed rapidly when the structure of the globular actin (G-actin) monomer was determined by protein crystallography in... 

Figure 14.19 Lever mechanism of myosin moving along an actin filament. The spheres represent globular actin monomers.

Figure 10.4 (A) A schematic of globular actin monomer forming a protein filament, called F-actin. This filament is one of the important components of muscle cells, as well as the cytoskeleton of other cells. (B) Oriented actin filaments inside a fibroblast cell, called stress fibers, seen through a fluorescence microscope. Image obtained from Nguyen et al. [148] and reprinted with permission.

CC - - SUBUNIT POLYMERIZATION OF GLOBULAR ACTIN (G-ACTIN) LEADS TO A CC STRUCTURAL FILAMENT (F-ACTIN) IN THE FORM OF A TWO-STRANDED... 

Actin is considered to be the receptor to pectenotoxins. The interaction decreases the formation of filamentous actin, and increases the amount of globular actin, the most important factor to the potency of pectenotoxins being an intact lactone ring. The interaction between actin and pectec-notoxin follows a stoichiometry of 1 1. From indirect observations, actin is a sensor to the effect of several other toxins." ... 

Helical symmetry The polymeric proteins of filamentous viruses and the cytoskel-ton possess helical symmetry, in which subunits are related by a translation, as well as a rotational component. Actin, myosin, tubulin and various other fibrous proteins all interact with helical symmetry, which is often called screw symmetry. Screw symmetry, which relates the positions of adjacent subunits, combines a translation along the helix axis with the rotation. Actin forms a two-stranded helix of globular actin subunits. However, important variations in the helix parameters occur (Egehnan et al, 1982). The rise per subunit is relatively constant, but the twist or relative rotation around the helix axis is highly variable. This polymorphic tendency is probably important for the smooth functioning of muscle contraction, which involves considerable force generation. 

The thin filaments, composed primarily of actin, are approximately 3 urn long and 8 nm in diameter. The filament is basically formed by a nonintegral double helix of globular actin monomers (M.W. 50,000 5.5 nm in diameter) which have a period of approximately 37 nm with approximately 13 actin monomers per turn per strand. The thin filaments originate at the Z line in the center of the I band and, while negatively charged, do not normally form a lattice except close to their attachment to the Z-line in which region the lattice is square. 

The monomer G-actin (globular actin) consists of 375 amino acids, has a molecular weight of 42,000 and is able to bind ATP and a doubly charged cation. G-actin exists only at low ionic strengths. The addition of singly and doubly charged cations starts the polymerization to F-actin (fibrillar actin) with the cleavage of ATP to ADP, which remains in the bound state. 

---