Double helix, actin

In the smooth muscle cell, CD is incorporated into the thin filaments in the "contractile domain" of the cell (Furst et al., 1986 North et al., 1994a). Ultrastruc-tural studies presented in Chapter 4 (this volume) have shown that CD is located in the thin filament in an extended form beside TM along the axis of the actin double helix. The model (Fig. 2) places CD in potential contact with actin and TM throughout its length and allows a possible end-to-end interaction. These structural arrangements form the basis of caldesmon function in the thin filament. 

Ultrastructural studies of isolated chicken gizzard thin filaments localized caldesmon on the thin filament beside tropomyosin, arranged continuously along the axis of the actin double helix (Moody et al 1990, Vibert et al 1993, I hman et al 1997). In smooth muscle filaments derived from vascular or visceral tissue, the stoichiometry of caldesmon to tropomyosin and actin has been determined to be 1 2 14 (Lehman et al 1989, Marston 1990, Lehman et al 1993). Marston and Redwood (1991) proposed that each caldesmon molecule is placed in register with tropomyosin and extends for 78 nm, the length of two tropomyosin molecules. Each caldesmon molecule interacts with 14 actin monomers. This would result in a filament without radial symmetry such that different parts of the caldesmon molecule would appear on the same side of the actin filament. 

Solutions of F-actin and myosin at high ionic strength = 0.6) in vitro form a complex called actomyosin. The formation of the complex is reflected by an increase in viscosity and occurs in a deflnite molar ratio 1 molecule of myosin per 2 molecules of G-actin, the basic unit of the double-helical F-actin strand. It appears that a spike-like structure is formed, which consists of myosin molecules embedded in a backbone made of the F-actin double helix. Addition of ATP to actomyosin causes a sudden drop in viscosity due to dissociation of the complex. When this addition of ATP is followed by addition of Ca +, the myosin ATPase is activated, ATP is hydrolyzed and the actomyosin complex again restored after the ATP concentration decreases. Upon spinning of an actomyosin solution into water, flbers are obtained which, analogous to muscle flbers, contract in the presence of ATP. Glycerol extraction of muscle fibers removes all the soluble components and abolishes the semipermeability of the membrane. Such a model muscle system shows all the reactions of in vivo muscle contraction after the readdition of ATP and Ca +. This and similar model studies demonstrate that the muscle contraction mechanism is understood in principle, although some molecular details are still not clarified. 

Examples of the various helical forms found in nature are the single helix (RNA), the double helix (DNA), the triple helix (collagen fibrils), and complex multiple helices (myosin, F-actin). Generally, these single and double helices are fairly readily soluble in dilute aqueous salt solution. The triple and complex helices are soluble only if the secondary bonds are broken. 

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

The polymerization process in the presence of ADP is shown in Fig. 5-26. When the salt concentration is raised there is a characteristic lag period, usually of many minutes, before F-actin appears. The lag period results from the fact that sufficient nuclei (containing 2-4 monomers) must form before polymerization takes place. The nuclei are unstable, probably because they do not contain all of the subunit-subunit contacts that stabilize monomers in a filament (see Fig. 5-8). Once sufficient nuclei are available, polymerization proceeds rapidly from both ends of the nuclei. The filaments themselves can contain several hundred actin monomers, each with a molecule of ADP bound. The structure can be regarded as a right-handed double helix with 13.5 monomers per turn giving a helical repeat of 36 nm (see Fig. 5-8). 

Both smooth and skeletal muscle actin filaments are saturated with tropomyosin (Sobieszek and Bremel 1975). Both exhibit the same characteristic stoichiometry of binding of 1 molecule of tropomyosin interacting with 7 monomeric units of F-actin on each of the two strands of F-actin (Hartshorne 1987). The length of tropomyosin molecules (284 amino acids) and their periodicity in smooth and striated muscles is the same (Matsumura and Lin 1982). In both tissues, tropomyosin exists as a dimeric a-helical coil (Caspar et al 1969). Individual tropomyosin molecules bind in an end to end fashion to form a continuous strand on the thin filament that lies along the long-pitch of the double helix formed by the actin monomers (Moore et al 1970, OBrien et al 1971, Spudich et al 1972, Milligan et al 1990). 

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. 

F-actin (also called microfilament or actin filament) is a double-stranded, right-handed helix with 14 actin molecules per strand and turn. F-actin has a diameter of 8 nM and is polarized with a pointed (minus) and a barbed (plus) end. 

Actin filaments are thought to exist in a double-stranded, right-hand helix with 14 subunits (per strand) per complete turn (Fig. 4.4), and a crossover distance of 38 nm. This strings of beads appearance is 70 A in diameter and thought to represent the structure of thin filaments. As the new fila-... 

In the twisting double-helical actin chain, the nucleation step involves only one monomer-monomer interaction while each monomer placed on the chain following nucleation involves two monomer-monomer bonds. In the helix-coil theory, the formation of two consecutive helical states represents the nucleation step that allows for a third helical state and hydrogen bond formation. 

The thin filament consists of a double tranded helix of actin molecules. The filament appears to be relatively thin, as determined by electron microscopy, accounting for the name. Troponin is part of the thin filament. It consists of a polymer of actin subunits. Each subunit has a molecular weight of 42,000. Troponin occurs as a compiex with tropomyosin, another protein of the thin filament. There is one molecule of troponin for every seven actin moiecuies in the thin fiiament- The thick filament is composed of a network of myosin molecules. Myosin has a molecular weight of 460,000 It consists of two Identical polypeptide chains. Myosin is long rather than globular or spherical. Each myosin heavy chain is associated with two myosin light chains. 

F-actin in the thin filaments (1 1000 nm, d 8 nm) is in the form of a double-stranded helix in which the G-actin beads are stabilized by two... 

Actin filaments organize themselves to give cell robustness. They are made of two protofilaments, eaeh of whieh is 5-9 nm in diameter, intertwined to form a double stranded helix. Aetin is responsible for eell eon-traction and movement of actin filaments help eells change shapes by adding subunits at one end while removing at another end. The dynamie nature of actin filaments faeilitates rapid regulation of various cellular proeesses and... 

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