F-actin fiber

The F-actin helix has 13 molecules of G-actin in six turns of the helix, repeating every 360 A. Oriented gels of actin fibers yield x-ray fiber diffraction patterns to about 6 A resolution. Knowing the atomic structure of G-actin it was possible for the group of Ken Holmes to determine its orientation in the F-actin fiber, and thus arrive at an atomic model of the actin filament that best accounted for the fiber diffraction pattern. 

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. 

Similar to the situation with 2D membranes, the basic molecular characteristics of chitosan such as DD also show great influence on the ability of chitosan scaffolds to modulate stem cell behavior. The chitosan scaffolds with a high DD can maintain the viability and pluripotency of buffalo embryonic stem-like (ES-Uke) cells [15]. However, the cell behavior on 2D and 3D environments are quite different. Comparison of MSC behavior in both 2D plates and chitosan/gelatin/chondroitin scaffolds demonstrates that the 3D microenviromnent can enhance osteogenesis and maintain the viability of cells [161]. The research by Altman et al. [162] found that the apparent elastic modulus and cytoskeleton F-actin fiber density were higher for ADSCs seeded in 3D sflk fibroin/chitosan scaffolds than on 2D glass plates (Fig. 13). 

Fig. 13 Fluorescent Images and the corresponding line profiles of the F-actin fibers red) of ADSCs seeded on (a) glass surface and (b) silk fibroin/chitosan (SFCS) scaffold. F-actin fiber density of ADSCs was quantified and confirmed by line-profile analysis of the fibers using Image software. The x-axis is the distance in microns, and the peaks correspond to the intensity of the rhodamine-phalloidin stain (red), whose peak maximum occurs at the location of the fibers along the line. Nuclei were stained with DAPI (blue) [162]...

The polymer self-assembly theory of Oosawa and Kasai (1962) provides valuable insights into the nature of the nucleation process. The polymerization nucleus is considered to form by the accretion of protomers, but the process is highly cooperative and unfavorable. Indeed, this is strongly suggested by the observation that thousands of actin or tubulin protomers are found in F-actin and microtubule structures if nucleation of self-assembly were readily accomplished and highly favorable, the consequence would be that many more fibers of shorter polymer length would be observed. The Oosawa kinetic theory for nucleation permits one to obtain information about the size of the polymerization nucleus if two basic assumptions can be satisfied in the experimental system. First, the rate of nuclei formation is assumed to be proportional to the loth power of the protomer concentration with io representing the number of protomers required to create the nucleus. Second, the treat-... 

This theory clearly predicts that the shape of the polymer length distribution curve determines the shape of the time course of depolymerization. For example Kristofferson et al. (1980) were able to show that apparent first-order depolymerization kinetics arise from length distributions which are nearly exponential. It should also be noted that the above theory helps one to gain a better feeling for the time course of cytoskeleton or mitotic apparatus disassembly upon cooling cells to temperatures which destabilize microtubules and effect unidirectional depolymerization. Likewise, the linear depolymerization kinetic model could be applied to the disassembly of bacterial flagella, muscle and nonmuscle F-actin, tobacco mosaic virus, hemoglobin S fibers, and other linear polymers to elucidate important rate parameters and to test the sufficiency of the end-wise depolymerization assumption in such cases. 

Microfilaments of F actin traverse the microvilli in ordered bundles. The microfila-ments are attached to each other by actin-as-sociated proteins, particularly fimbrin and vil-lin. Calmodulin and a myosin-like ATPase connect the microfilaments laterally to the plasma membrane. Fodrin, another microfila-ment-associated protein, anchors the actin fibers to each other at the base, as well as attaching them to the cytoplasmic membrane and to a network of intermediate filaments. In this example, the microfilaments have a mainly static function. In other cases, actin is also involved in dynamic processes. These include muscle contraction (see p. 332), cell movement, phagocytosis by immune cells, the formation of microspikes and lamellipo-dia (cellular extensions), and the acrosomal process during the fusion of sperm with the egg cell. 

The striation of the muscle fibers is characteristic of skeletal muscle. It results from the regular arrangement of molecules of differing density. The repeating contractile units, the sarcomeres, are bounded by Z lines from which thin filaments of F-actin (see p. 204) extend on each side. In the A bands, there are also thick parallel filaments of myosin. The H bands in the middle of the A bands only contain myosin, while only actin is found on each size of the Z lines. 

In nature, extended helical conformations appear to be utilized in two major ways to provide linear systems for the storage, duplication, and transmission of information (DNA, RNA), and to provide inelastic fibers for the generation and transmission of forces (F-actin, myosin, and collagen). 

The most abundant microfilaments are composed of fibrous actin (F-actin Fig. 7-10). The thin filaments of F-actin are also one of the two major components of the contractile fibers of skeletal muscle. There is actually a group of closely related actins encoded by a multigene family. At least four vertebrate actins are specific to various types of muscle, while two (P- and y-actins) are cytosolic.298 299 Actins are present in all animal cells and also in fungi and plants as part of the cytoskeleton. The microfilaments can associate to... 

Lorenz, M., Popp, D., and Holmes, K. C. (1993). Refinement of the F-actin model against X-ray fiber diffraction data by the use of a directed mutation algorithm. J. Mol. Biol. 234, 826-836. 

Oda, T., Makino, K., Yamashita, I., Namba, K., and Maeda, Y. (1998). Effect of the length and effective diameter of F-actin on the filament orientation in liquid crystalline sols measured by X-ray fiber diffraction. Biophys. J. 75, 2672-2681. 

A myofibril consists of the thick filaments (myosin) and thin filaments (F-actin with tropomyosin and troponin). The muscle cell, extending the length of the muscle, is termed a muscle fiber and contains a number of myofibrils. 

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.

Sbrissa, D., Ikonomov, O.C., Strakova, J., and Shisheva, A., 2004, Role for a novel signaling intermediate, phosphatidylinositol 5-phosphate, in insulin-regulated F-actin stress fiber breakdown and GLUT4 translocation. Endocrinology 145 4853 4865. 

The actin cytoskeleton is important in cell shape changes, polarization, motility, chemotaxis, metastasis and ceU division, and Rho family proteins play critical roles in these effects. The involvement of PLD in changes in the actin cytoskeleton was first suggested by the effects of exogenous PLD and PA, which induced actin stress fiber formation and also increased the F-actin content of fibroblasts [172, 173]. Another study utilizing endotheUal ceUs also showed that PA that was free of LPA induced actin stress fiber formation [174]. Furthermore, the effects of LPA to induce this change in the actin cytoskeleton and also PA formation were blocked by butan-l-ol, but not butan-2-ol [174] (Fig. 4.6). 

This protein was recently prepared from cod muscles and investigated by Connell (1954). Acetone-dried muscle fiber was extracted with water by the method of Feuer, et al. (1948) omitting the final treatment with Na2COa (Tsao and Bailey, 1953). The G-actin obtained polymerizes on addition of salts to a viscous solution of F-actin showing pronounced double refrac-... 

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