The microfilaments

Perhaps the most dynamic components of the cytoskeleton, the microfilaments are directly involved in cell movement and phagocytosis. They are 

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Microfilaments and Microtubules. There are two important classes of fibers found in the cytoplasm of many plant and animal ceUs that are characterized by nematic-like organization. These are the microfilaments and microtubules which play a central role in the determination of ceU shape, either as the dynamic element in the contractile mechanism or as the basic cytoskeleton. Microfilaments are proteinaceous bundles having diameters of 6—10 nm that are chemically similar to actin and myosin muscle ceUs. Microtubules also are formed from globular elements, but consist of hoUow tubes that are about 30 nm in diameter, uniform, and highly rigid. Both of these assemblages are found beneath the ceU membrane in a linear organization that is similar to the nematic Hquid crystal stmcture. 

The Locomotion of Amoeba The Locomotion of Fibroblastic Cell Types The Locomotion of Leukocytes The Behavior of Locomoting Cells The Role of the Cytoskeleton in Cell Locomotion The Microtubule-Based Cytoskeleton The Intermediate Filament-Based Cytoskeleton The Microfilament-Based Cytoskeleton The Organization of Microfilaments in Cells Microfilament Dynamics and Cell Locomotion Sites of Lamellar Protrusion May Be Determined by the Nucleation of Actin Polymerization... 

Two of the cytoskeletal components, the actin filaments and the microtubules have been studied with molecular rotors. The main component of the actin filaments is the actin protein, a 44 kD molecule found in two forms within the cell the monomeric globulin form (G-actin) and the filament form (F-actin). Actin binds with ATP to form the microfilaments that are responsible for cell shape and motility. The rate of polymerization from the monomeric form plays a vital role in cell movement and signaling. Actin filaments form the cortical mesh that is the basis of the cytoskeleton. The cytoskeleton has an active relationship with the plasma membrane. Functional proteins found in both structures... 

Intermediate filament a protein component of the cytoskeleton that includes filaments larger than the microfilaments and smaller than the microtubules. 

Actin, the most abundant protein in eukaryotic cells, is the protein component of the microfilaments (actin filaments). Actin occurs in two forms—a monomolecular form (C actin, globular actin) and a polymer (F actin, filamentous actin). G actin is an asymmetrical molecule with a mass of 42 kDa, consisting of two domains. As the ionic strength increases, G actin aggregates reversibly to form F actin, a helical homopolymer. G actin carries a firmly bound ATP molecule that is slowly hydrolyzed in F actin to form ADR Actin therefore also has enzyme properties (ATPase activity). 

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. 

Electron-microscope examination did not, however, reveal changes in the organization of the microfilaments or microtubules, but the endoplasmic reticulum was dilated in a sac-like fashion.541 Decreased proportions of fibronectin were observed, both for control and virally transformed cells. In cells that had heen exposed to tunicamycin,... 

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... 

In the cytosol of eukaryotic cells is an internal scaffold, the cytoskeleton (see Topic E2). The cytoskeleton is important in maintaining and altering the shape of the cell, in enabling the cell to move from one place to another, and in transporting intracellular vesicles. Three types of filaments make up the cytoskeleton microfilaments, intermediate filaments and microtubules. The microfilaments, diameter approximately 7 nm, are made of actin and have a mechanically supportive function. Through their interaction with myosin (see Topic Nl), the microfilaments form contractile assemblies that are involved... 

Streaming and microfilaments in cells have an intimate relationship in that the microfilaments most likely supply the moving force that governs streaming. Importantly, the microfilaments are in bundles and are proximal to the cytoplasmic stream and account for the relationship, and here exogenously applied CB has produced some interesting results. 

Eukaryotic cells have an internal scaffolding called the cytoskeleton or cytomatrix that maintains their cellular morphology and enables them to migrate, undergo shape changes, and transport vesicles. Microfilaments, made of actin, intermediate filaments, which are composed of laminin and other proteins, and microtubules, formed from the protein tubulin, along with many different accessory proteins, comprise the cytoskeleton. Both the microfilaments and the microtubules can assemble and disassemble rapidly in the cell, whereas disassembly of intermediate filaments may require their destruction. Although much is known about the molecular composition of the cytoskeleton, the molecular events involved in most cell movements are still unknown. 

It is interesting to note at this point that the keratins and other cytoskeletal proteins such as neurofilaments (from neurons) and vimentin (mesenchymal cells) are referred to as intermediate filaments because their diameter (10 nm) are intermediate between those of the microfilaments and microtubles. 

Rapid phosphorylation of the other detected phosphoproteins does occur but no definite roles have yet been ascribed to them. The 33 kDa protein may be the S6 ribosomal protein involved in the control of protein synthesis. The 57 kDa protein has been identified as the regulatory suhunit of the cyclic AMP-dependent protein kinase [44]. Of the other proteins the 76, 43 and 20 kDa may be connected with the microfilaments (43 kDa actin, 76 kDa myosin light chain kinase and 20 kDa myosin light chain) but this must be further investigated. These proteins may only play a permissive role in. steroidogenesis. The fact that the pattern of protein phosphorylation is very similar after stimulation of protein kinase C with phorbol esters supports this because the latter only marginally increase steroidogenesis [18]. 

Cytoskeleton is defined as the sum of the various filamentous proteins of eukaryotic cells that remain after the cells are extracted with a mild detergent. The cytoskeleton includes actin filaments, two-stranded helical polymers, which form the microfilaments and the actin-binding proteins. Other components are microtubules and intermediate filaments. The cytoskeleton has not only a role in maintaining the shape of cells, it is also actively engaged in cell division, in the organisation and the dynamic movement of ceD organelles and in the movement of cells in chemotaxis. 

Note Cellular model was intestinal cells. The actin filaments specific fluorescent marker, Oregon green phalloidin, detected with laser-scaiming cytometry allowed to quantify the microfilaments in rabbit intestinal cells... 

A direct link between cytochalasin and actin was provided by the demonstration that cytochalasin decreases the viscosity of actin filaments purified from muscle (50). This experiment led to two important conclusions. First, cytochalasin interacts directly with actin. Second, an interaction of cytochalasin with actin or actin-like proteins in vivo could account for the ability of cytochalasin to inhibit various forms of cell motility and contraction (50). Thus, actin was shown to be the molecular target of cytochalasin and implicated as a critical component of the microfilaments involved in cytochalasin-sensitive processes, including contraction of the cleavage furrow at cytokinesis. 

Mg -ATPase activity can likewise be inhibited in the canalicular side of the membrane by cholestatic factors (in particular bile acids). This metabolic pump transports bicarbonate and chloride into the canaliculi and is probably closely associated with the function of the microfilaments. 

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