Cytoskeletal filament

Forgacs G 1995 On the possible role of cytoskeletal filamentous networks in intracellular signalling J. Ceii. Sc/. 108 2131-43... 

The major types of cytoskeletal filaments are 7-nm-thick microfilaments. 25-nm-thick microtubules, and 10-nm-thick intermediate filaments (IPs). These are respectively composed of actin, tubulin, and a variety of interrelated sparsely soluble fibrous proteins termed intermediate filament proteins. In addition, thick myosin filaments are present in large numbers in skeletal and heart muscle cells and in small numbers in many other types of eukaryotic cells. 

Microtubules (MT) are the largest of the cytoskeletal filaments with an outer diameter of about 25 nm, a wall thickness of about 5 nm, and a central lumen measuring about 15 nm. They consist of tubulin and associated proteins. Vertebrate brain tissue is a rich source of extractable tubulin because of the large numbers of microtubules that are present in axons and dendrites. Tubulin obtained from such a natural source is a heterodimer of 100 kD composed of a-tubulin and P-tubulin. Brain a-tubulin is a globular polypeptide that contains 451 amino acid residues, whereas P-tubulin, which is somewhat shorter, is made up of 445 amino acid residues. These two molecular species of tubulin share in common 40% of their amino acid residues. 

Dynein, kinesin, and myosin are motor proteins with ATPase activity that convert the chemical bond energy released by ATP hydrolysis into mechanical work. Each motor molecule reacts cyclically with a polymerized cytoskeletal filament in this chemomechanical transduction process. The motor protein first binds to the filament and then undergoes a conformational change that produces an increment of movement, known as the power stroke. The motor protein then releases its hold on the filament before reattaching at a new site to begin another cycle. Events in the mechanical cycle are believed to depend on intermediate steps in the ATPase cycle. Cytoplasmic dynein and kinesin walk (albeit in opposite... 

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. 

Any of the cytoskeletal proteins that cross-link cytoskel-etal filaments into colinear arrays, such as the action of a-actinin in promoting the formation of actin stress fibers. Bundling proteins typically contain pairs of binding sites for attachment to cytoskeletal filaments. 

FIGURE 1-9 The three types of cytoskeletal filaments. The upper panels show epithelial cells photographed after treatment with antibodies that bind to and specifically stain (a) actin filaments bundled together to form "stress fibers," (b) microtubules radiating from the cell center, and (c) intermediate filaments extending throughout the cytoplasm. For these experiments, antibodies that specifically recognize actin, tubu-... 

V7. Vazquez Doval, J., and Sanchez Ibarrola, A., Defective mononuclear phagocyte function in systemic lupus erythematosus Relationship of FcRII (CD32) with intermediate cytoskeletal filaments. J. Inv. Allergol. Clin. Immunol. 3, 86-91 (1993). 

Microtubules The third type of cytoskeletal filaments, the microtubules, are hollow cylin-... 

In order to understand the logic of dynamic instability, we need to keep in mind that cytoskeletal filaments are unstable only when their ends are not attached to particular molecules that have the ability to anchor them. Every microtubule, for example, starts from an organising centre (the centrosome), and the extremity which is attached to this structure is perfectly stable, whereas the other extremity can grow longer or shorter, and becomes stable only when it encounters an anchoring molecule in the cytoplasm. If such an anchor is not found, the whole microtubule is rapidly dismantled and another is launched in another direction, thus allowing the cytoskeleton to explore all the cytoplasm s space in a short time. 

In the cytoplasm, and especially subjacent to the plasma membrane, are networks of protein filaments that stabilize the lipid membrane and thus contribute to the maintenance of cell shape. In cells that grow and divide, such as liver cells, the cytoplasm appears to be organized from a region near the nucleus that contains the cell s pair of centrioles (Chap. 5). There are three main types of cytoskeletal filaments (1) microtubules, 25 nm in diameter, composed of organized aggregates of the protein tubulin (Chap. 5) (2) actin filaments, 7 nm in diameter (Chap. 5) and (3) so-called intermediate filaments, 10 nm in diameter (Chap. 5). 

Eukaryotic cells contain three main kinds of cytoskeletal filament ... 

This could be seen as a very theoretical debate, but this has indeed very practical implications. Let us take the example of the analysis of a reticulum preparation. Endoplasmic reticulum is the place of synthesis of most secreted proteins and of most transmembrane proteins. As such, it contains many ribosomes (the Rough ER) and the reticulum vesicles are known to be associated with cytoskeletal filaments. The problem arises from the fact that the protein content of these cytoskeletal and ribosomal contaminants is concentrated in a few proteins. This means in turn that... 

Maniotis AJ, Chen CS, Ingber DE (1997) Demonstration of mechanical connections between integrins, cytoskeletal filaments, and nucleoplasm that stabilize nuclear structure. Proc Natl Acad Sci USA 94(3) 849-854... 

Bursch, W., Hochegger, K., Torok, L., Marian, B., Ellinger, A., and Hermann, R. S. (2000) Autophagic and apoptotic types of programmed cell death exhibit different fates of cytoskeletal filaments. J. Cell Sci. 113(Pt 7), 1189-1198. 

Proteins perform work when motor protein subunits bind and hydrolyze nucleotides such as ATP. The energy-induced change in the shape of a motor protein subunit causes an orderly change in the shapes of adjacent subunits. In this diagrammatic illustration, a motor protein complex moves attached cargo (e.g., a vesicle) as it walks along a cytoskeletal filament. 

Monocyte. Bone marrow-derived mononuclear phagocytic leukocyte, with bean-shaped nucleus and fine granular cytoplasm containing lysosomes, phagocytic vacuoles, and cytoskeletal filaments. Once transported to tissues, monocytes develop into macrophages. 

A FIGURE 1-15 The three types of cytoskeletal filaments have characteristic distributions within cells. Three views of the same cell. A cultured fibroblast was treated with three different antibody preparations. Each antibody binds specifically to the protein monomers forming one type of filament and is chemically linked to a differently colored fluorescent dye (green,... 

The ability to bind and translocate along a cytoskeletal filament, nucleic acid strand, or protein complex... 

In addition to these proteins, which are closely associated with the bilayer, cytoskeletal filaments are more loosely associated with the cytosolic face, usually through one or more... 

Each type of cytoskeletal filament is a polymer of protein subunits (Table 5-4). Monomeric actin subunits assemble into mlcrofllaments dimeric subunits composed of a- and p-tubulin polymerize into microtubules. Unlike mlcrofllaments and microtubules, which are assembled from one or two proteins, intermediate filaments are assembled from a large diverse family of proteins. The most common intermediate filaments, found in the nucleus, are composed of lamins. Intermediate filaments constructed from other proteins are expressed preferentially in certain tissues for example, keratin-containing filaments in epithelial cells, desmin-contalnlng filaments In muscle cells, and vimentin containing filaments In mesenchymal cells. 

Most eukaryotic cells contain all three types of cytoskeletal filaments, often concentrated in distinct locations. For example, in the absorptive epithelial cells that line the lumen of the intestine, actin mlcrofllaments are abundant in the apical region, where they are associated with cell-cell junctions and support a dense carpet of microvilli (Figure 5-30a). Actin filaments are also present in a narrow zone adjacent to the plasma membrane In the lateral regions of these cells. Keratin Intermediate filaments,... 

A FIGURE 5-30 Schematic depiction of the distribution of cytoskeletal filaments in eukaryotic cells and bacterial cells. 

Cytoskeletal Filaments Are Organized into Bundles and Networks... 

On first looking at micrographs of a cell, one Is struck by the dense, seemingly disorganized mat of filaments present In the cytosol. However, a keen eye will start to pick out areas—generally where the membrane protrudes from the cell surface or where a cell adheres to the surface or another cell—In which the filaments are concentrated Into bundles. From these bundles, the filaments continue Into the cell Interior, where they fan out and become part of a network of filaments. These two structures, bundles and networks, are the most common arrangements of cytoskeletal filaments In a cell. 

Three systems of cytoskeletal filaments exist in most eukaryotic cells. Compare them In terms of composition, function, and structure. 

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