Microfilaments actin filaments

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). 

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. 

The Cytoskeleton—Microtubules and Microfilaments Patterns of Arrangement of Actin Filaments in Animal Cells... 

Blood platelets are key players in the blood-clotting mechanism. These tiny fragments of cytoplasm are shed into the circulation from the surface of megakaryocytes located in the bone marrow. When the lining of a blood vessel is injured, activated platelets release clotting factors, adhere to each other and to damaged surfaces, and send out numerous filopodia. The shape changes that occur in activated platelets are the result of actin polymerization. Before activation, there are no microfilaments because profilin binds to G-actin and prevents its polymerization. After activation, profilin dissociates from G-actin, and bundles and networks of F-actin filaments rapidly appear within the platelet. 

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. 

Two major types of muscle fibers are found in humans white (anaerobic) and red (aerobic). The former are particularly used in sprints and the latter in prolonged aerobic exercise. During a sprint, muscle uses creatine phosphate and glycolysis as energy sources in the marathon, oxidation of fatty acids is of major importance during the later phases. Nonmuscle cells perform various types of mechanical work carried out by the structures constituting the cytoskeleton. These strucmres include actin filaments (microfilaments), micrombules (composed primarily of a- mbulin and p-mbulin), and intermediate filaments. The latter include keratins, vimentin-like proteins, neurofilaments, and lamins. 

Figure 4.11 Dephosphorylated synapsin, associated with SSVs, is thought to form a heteromeric complex with CAM kinase II (also partially embedded in the vesicular membrane) and actin filaments. An increase in intracellular Ca + triggers phosphorylation of S3mapsin I which dissociates from the vesicular membrane. This frees the vesicles from the fibrin microfilaments and makes them available for transmitter release at the active zone of the nerve terminal...

The cellular cytoskeleton, primarily composed of microfilaments, microtubules, and intermediate filaments, provides structural support and enables cell motility. The cytoskeleton is composed of biological polymers and is not static. Rather, it is capable of dynamic reassembly in less than a minute [136], The cytoskeleton is built from three key components, the actin filaments, the intermediate filaments, and the microtubules. The filaments are primarily responsible for maintaining cell shape, whereas the microtubules can be seen as the load-bearing elements that prevent a cell from collapsing [136], The cytoskeleton protects cellular structures and connects mechanotransductive pathways. Along with mechanical support, the cytoskeleton plays a critical role in many biological processes. 

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... 

The cytoskeleton is the collective name for all structural filaments in the cell. The cytoskeletal filaments are involved in establishing cell shape, and providing mechanical strength, locomotion, intracellular transport of organelles and chromosome separation in mitosis and meiosis. The cytoskeleton is made up of three kinds of protein filaments actin filaments (also called microfilaments), intermediate filaments and microtubules. 

Actin filaments are the thinnest of the cytoskeletal filaments, and therefore also called microfilaments. Polymerized actin monomers form long, thin fibers of about 8 nm in diameter. Along with the above-mentioned function of the cytoskeleton, actin interacts with myosin ( thick ) filaments in skeletal muscle fibers to provide the force of muscular contraction. Actin/Myosin interactions also help produce cytoplasmic streaming in most cells. 

In the resting neutrophil, about 50% of the actin is present in filaments within the cytoskeleton (and hence insoluble in detergents such as Triton X-100), whereas the remainder is detergent soluble and hence is not associated with the cytoskeleton. Data from studies of actin polymerisation in vitro predict that almost all of the actin within the cell should be F-actin (i.e. present in microfilaments). Upon stimulation of neutrophils with agonists such as fMet-Leu-Phe or PMA, actin polymerisation is activated extremely rapidly. There are two important questions Firstly, how is actin maintained in the unpolymerised state in resting cells Secondly, how is it rapidly assembled into the cytoskeleton during activation The answers to these questions lie in understanding the functions of the numerous proteins involved in the assembly and disassembly of actin filaments (Table 4.1). 

The fermentation broth of an unidentified endophytic fungus isolated from plants growing in Hawaii has afforded microcarpalide I (59), an alkyl-substituted nonenolide that is weakly cytotoxic as a result of its ability to disrupt microfilaments. Extracts from the strain 112/13 collected from the bark oiFicus microcarpa L. (Moraceae) demonstrated a strong abrogation of microfilament activity. In A-10 rat smooth muscle cells, a 5 Xg/mL dose was able to induce a 50%—75% loss of actin filaments. Microcarpalide 1... 

Actin filaments grow rapidly within cells, and the clearest evidence of this rapid growth is the ability of the cell s leading edge to move at rates of 0.5 to 1 micrometer per second. Likewise, actin-based motility of Listeria and Shigella can attain rates of nearly 0.5 micrometers per second. Because microfilaments contain about 360 actin monomers per micrometer of length, a motility rate of 0.5 to 1 micrometer per second corresponds to an apparent first-order rate constant (/.e., / apparent = on [Actin-ATP]) of about 180-360 s . The bimolecular rate constant for actin-ATP addition to the barbed end has a nominal value of 2-3 X 10 s . Therefore, one can estimate... 

Eukaryotic cells have an internal scaffold, the cytoskeleton, that controls the shape and movement of the cell. The cytoskeleton is made up of actin microfilaments, intermediate filaments and microtubules. 

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... 

Together with actin microfilaments and microtubules, keratin filaments make up the cytoskeleton of vertebrate epithelial cells. Keratins belong to a family of intermediate filament proteins that form a-helical coiled-coil dimers that associate laterally and end to end to form 10 nm diameter filaments. Keratin and actin filaments and microtubules form an integrated cytoskeleton that preserves the shape and structural integrity of the ker-atinocyte as well as serves to transmit mechanical loads. Keratins account for about 30% of the total protein in basal cells. 

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. 

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