Globular actin polymerization

Actin is a 42 kDa bent dumbbell-shaped globular monomer which is found in most eukaryotic cells. It is the primary protein of the thin (or actin) filaments. Also, by mass or molarity, actin is the largest constituent of the contractile apparatus, actually reaching millimolar concentrations. Actins from different sources seem to be more similar than myosins from the same sources. Actin binds ATP which is hydrolyzed to ADP, if the monomeric actin polymerizes. The backbone structure of the actin filament is a helix formed by two linear strands of polymerized actins like two strings of actin beads entwined. 

Monomeric G-actin (43 kDa G, globular) makes up 25% of muscle protein by weight. At physiologic ionic strength and in the presence of Mg, G-actin polymerizes noncovalently to form an insoluble double helical filament called F-actin (Figure 49-3). The F-actin fiber is 6-7 nm thick and has a pitch or repeating structure every 35.5 nm. 

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

The globular G-actin polymerizes and forms a double helix made of G-actins. 

Actin, the major constituent of the thin filaments, exists in two forms. In solutions of low ionic strength it exists as a 42kDa monomer, termed G-actin because of its globular shape. As the ionic strength of the solution rises to that at the physiological level, G-actin polymerizes into a fibrous form, F-actin, that resembles the thin filaments found in muscle. Although actin, like myosin, is an ATPase, the hydrolysis of ATP is not involved in the contraction-relaxation cycle of muscle but rather in the assembly and disassembly of the actin filament. 

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

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 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. 

Actin is one of the major cytoskeletal proteins implicated in cell motility. Actin is a constituent of several cell types and is involved in many cellular processes including locomotion, secretion, cytoplasmic streaming, phagocytosis and cytokinesis. Actin polymerization involves the transformation of the globular protein G-actin into F-actin fibres. Studying the spontaneous... 

Actin, the major constituent of the thin filaments, can exist as monomeric globular G-actin or as polymerized fibrous F-actin. The actin filaments are connected to the thick filaments by cross-bridges formed by the SI heads of myosin. 

There are three main types of filament commonly found in the cytoskeletons of most cells. These are (1) microtubules (25 nm in diameter) composed principally of tubulin, (2) thin filaments (or microfilaments, 7 nm in diameter) which consist largely of polymerized actin and, (3) intermediate filaments (10 nm in diameter) which are manufactured from several classes of related proteins. Both tubulin and actin are globular proteins and are evolutionarily highly conserved. Their filaments tend to be much more dynamic than intermediate filaments (see Section 5.1). 

Actin. Rabbit muscle G-actin is globular with a molecular weight of 4.2 X 104. In the presence of salts it is polymerized into F-action (34). The principal properties of fish actin (35-37,40, 43,44), including amino acid composition (41), are similar to rabbit actin, but fish actin is more readily extracted from wet muscle by salt solutions as a viscous solution of actomyosin (22,35,36,45). 

Loss of the polymerizing ability of G-actin molecules indicates that the conformation of native G-actin, a globular molecule, must have been impaired during frozen storage. 

Unfolding of globular proteins and subunits. Data on frozen storage of HMM, actin and sarcoplasmic enzymes have led us to propose that denaturation involves unfolding of the protein chain based on a decrease in enzymatic activity (myosin, HMM, and sarcoplasmic enzymes), polymerizing ability (actin) and filament forming properties (myosin) (82,99,113-116,122). 

Actin is generally obtained from acetone-dried muscle powder by extraction with either water or, provided ATP is present, with KI solutions. Under these conditions the globular form of the protein is obtained, which polymerizes to the fibrillar form in 0.1 M KCl containing traces of Mg++ or Ca++ ions. Fibrillar actm combines with myosin to give actomyosin. Nevertheless, the exact structure and mode of action of the actomyosin complex are still far from being fully understood. 

Actin is globular, MW 42,000 monomer binds MgATP ATP and calcium promote polymerization polymerizes as a two-stranded helix 6-7 nm across each subunit interacts with four adjacent ones monomer can be added or lost at either end encoded by multigene family. 

Different smooth muscles all contain large quantities of actin ranging from about 30 to 50 mg/g wet weight, corresponding to approximately 0.9-1.6 mM in situ (Murphy et al., 1977 Hartshorne, 1987). Actin consequently accounts for between 30 and 50% of the total noncollagenous proteins in smooth muscle. As with skeletal muscle, smooth muscle actin can exist in vitro in two forms, namely, monomeric-"globular" G-actin and polymeric-filamentous F-actin. The mo-... 

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