Actin fibrillar

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. 

Tropomyosin and troponin. Tropomyosin is apparently the most stable of the fish fibrillar proteins during frozen storage. It can be extracted long after actin and myosin become inextract-able however, it does denature gradually (90). 

Intermediate filaments are about lOnm in diameter, and are more stable (strongly bound) than actin filaments. Like actin filaments they function in the maintenance of cell shape by bearing tension. Intermediate filaments organise the internal tridimensional structure of the ceU, anchoring organelles and serving as structural components of the nuclear lamina (a dense fibrillar network inside the nucleus) and sarcomeres. They also participate in some cell-cell and cell-matrix junctions. 

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. 

The actomyosin complex can be isolated directly from muscle, and it can also be formed in vitro from its two separate components. The complex consists of three myosin molecules and one fibrillar actin molecule (F-actin), and it has marked ATPase activity which is activated by magnesium and calcium ions. ATP causes the complex to dissociate into actin and... 

Rabbit Muscle Actin. The monomeric G form of actin carries an ATP firmly bound, which is hydrolyzed in a stoichiometric reaction to ADP during polymerization to the fibrillar F form under the influence of inorganic cations, especially Mg. By removal of the triggering cations, and displacement of the ADP by fresh ATP, the protein depolymerized again to the G form. 

Maturation or aging is accompanied by morphological changes which primarily affect the cytoskeleton. Microexaminations show that the Z lines, which as cross structures (cf. 12.2.1) separate the individual sarcomeres in the muscle fibril, are broken up during aging. In addition, the fibrillar proteins titin and desmin are degraded. In comparison, the contractile proteins myosin and actin are stable. They are attacked only at temperatures above 25 °C. The connective tissue present outside the muscle cells also remains intact. 

Actin is a one-chain globular protein with a relative molecular weight of 43.5 kDa (G-actin, where G stands for globular). This monomeric form polymerises to yield a fibrous form of actin (F-actin, where stands F for fibrous). The units of G-actin are organised into a heKx composed of two monomers. This polymer binds to the protein tropomyosin (relative molecular weight 70 kDa), which has a similar structure to the fibrillar part of myosin (two unequal polypeptide chains, essentially curled into an a-hehx). The actin is further connected to the regulatory troponins proteins (troponin complexes C, I and T) with relative molecular weights of 18,24 and 37 kDa, respectively. 

Contractile Proteins of the Muscle Fibrils. The hbrUs of muscle contain, besides the insoluble and little-studied structural proteins, also a few soluble, fibrillar proteins, which participate actively in muscle contraction they are myosin, actin, and tropomyosin. 

Actin is less soluble than myosin. It occurs in two forms. Globular G-actin has a molecular weight of 70,000 (as the monomer) or 140,000 (as the dimer). The addition of salts converts it to the polymeric F-actin, a fibrillar protein that has a high molecular weight and sediments quickly in the ultracentrifi e. In solution, actin and myosin combine easily to give actomyosin. Under certain conditions, one can isolate native actomyosin from muscle. 

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