Filament Structure

It is important to note material such as those plastics or wood that are weak in either tension or compression will also be basically weak in shear. For example, concrete is weak in shear because of its lack of strength in tension. Reinforced bars in the concrete are incorporated to prevent diagonal tension cracking and strengthen concrete beams. Similar action occurs with RPs using fiber filament structures. 

Role of the Cytoskeleton in Cell Division Formation of the Mitotic Spindle, Mitosis, and Cytokinesis Drug Effects on Microtubules Mlcrofllaments Actin Filaments Structure and Composition... 

The diversity of these subcellular actin structures is remarkable and appears to be determined by the interactions of many actin-binding proteins (ABPs) as well as by changes in the concentrations of intracellular signaling molecules such as Ca and cAMP, by small GTP-binding proteins, and by signals arising from mechanical stress. Approximately 50% of the actin molecules in most animal cells are unpolymerized subunits in the cytosolic pool and exist in a state of dynamic equilibrium with labile F-actin filamentous structures (i.e., new structures are formed while existing structures are renewed) (Hall, 1994). 

In a previous section we mentioned the significance of myosin filament structure. In nematodes two forms of myosin-II, myosin A and B, are required for proper filament stmcture (Epstein, 1988). The two forms of myosin are expressed at the proper time to allow for correct filament assembly. An accessory protein called paramyosin is also required for correct filament assembly. In vertebrate cardiac muscle, there are also two isoforms of myosin-II a-myosin and p-myosin. The proper ratio of these two proteins is of utmost importance for proper muscle activity. The incorrect synthesis of a- and P-myosins results in a severe cardiac disorder known as hypertrophic cardiomyopathy. Genetic transmission of the disease occurs in about 55% of families. The inherited condition is called familial hypertrophic cardiomyopathy (FHC), and this condition is a leading cause of sudden death in young athletes. 

Nonmuscle cells perform mechanical work, including self-propulsion, morphogenesis, cleavage, endocytosis, exocytosis, intracellular transport, and changing cell shape. These cellular functions are carried out by an extensive intracellular network of filamentous structures constimting the cytoskeleton. The cell cytoplasm is not a sac of fluid, as once thought. Essentially all eukaryotic cells contain three types of filamentous struc-mres actin filaments (7-9.5 nm in diameter also known as microfilaments), microtubules (25 nm), and intermediate filaments (10-12 nm). Each type of filament can be distinguished biochemically and by the electron microscope. 

Extrusion texturization is a process that uses mechanical shear, heat, and pressure generated in the food extruder to change the structures of food components, including proteins (Harper, 1986). Protein texturization creates filamentous structures, crumbly surfaces, or other physical formations by restructuring or realigning folded or tightly wound globular structures into stretched, layered, or cross-linked mass (Kinsella and Franzen, 1978). 

The internal physical flow along filamentous structures, made from actin, myosin, tubulin and similar proteins in later cells, can be of small or large molecules, or even of vesicles, so that movements on internal surfaces become more... 

V. Experimentally Derived Constraints on Prion Filament Structure. 151... 

A key feature of the protein-only hypothesis is that variants should represent distinct structural forms of the prion protein. A direct connection between filament structure and variants was made by Tanaka et al. (2004) using filaments made under different conditions (in this case, 4°C and 37°C). Filaments formed at 4°C were less stable against heating in 1.6% SDS and gave rise mostly to strong [PSI] variants after being transformed... 

Figure 6 Model of type VI collagen assembly. Two type VI collagen molecules assemble with 30 nm overlap " with two pairs of disulfide bonds between cysteines, one in collagenous domain and another in the C-terminal globular domain.Two dimers form a tetramer with disulfide bonds presumably in the a3(VI) chains. The tetramers assemble into the long beaded filamentous structure with 105nm periodicity.

Figure 1. Nucleation and growth of actin filaments. Nucleation is shown here as a thermodynamically unfavored process, which in the presence of sufficient actin-ATP will undergo initial elongation to form small filament structures that subsequently elongate with rate constants that do not depend on filament length. Elongation proceeds until the monomeric actin (or G-actin) concentration equals the critical concentration for actin assembly.

Over the last twenty years biophysical work on this preparation has concentrated mainly on the elucidation of the filament structure and cross-bridge conformations (Reedy et al., 1965 Squire et al., 1977 Wray, 1979 Clarke et al., 1986 Reedy et al., 1987), and on the mechanical characterisation of various equilibrium states and of the kinetics of the cross-bridge cycle (Jewell Ruegg, 1%6 White, 1970 Tregear, 1977 Gtith et al., 1981 White Thorson, 1983). The biochemistry of ATP hydrolysis by the insect proteins has received less attention than that of vertebrate muscle proteins, primarily because of shortage of tissue, but recently aspects of the biochemical kinetics have been investigated (White et al., 1986). 

Electron micrograph of a striated muscle sarcomere showing the appearance of filamentous structures when cross-sectioned at the locations illustrated below. (Electron micrograph courtesy of Dr. Hugh Huxley, Brandeis University.)... 

Kagawa, H., McLachlan, A. D., Brenner, S., Karn, J., and Gengyo-Ando, K. (1989). Paramyosin gene (unc-15) of Caenorhabditis elegans Molecular cloning, nucleotide sequence and models for thick filament structure./. Mol. Biol. 207, 311-333. 

Parry, D. A. D., and Steinert, P. M. (1995). Intermediate Filament Structure. Springer-Verlag, Heidelberg. 

Steinert, P. M., Marekov, L. N., Fraser, R. D. B., and Parry, D. A. D. (1993a). Keratin intermediate filament structure Crosslinking studies yield quantitative information on molecular dimensions and mechanism of assembly. /. Mol. Biol. 230, 436-452. 

Henderson, M., Polewski, R., Fanning, J. C., and Gibson, M. A. (1996). Microfibril-associated glycoprotein-1 (MAGP-1) is specifically located on the beads of the beaded-filament structure for fibrillin-containing microfibrils as visualized by the rotary shadowing technique./. Histochem. Cytochem. 44, 1389-1397. 

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