Cytoskeleton intermediate filaments

Vlmentin a fibrous protein which makes up intermediate filaments in cells of mesenchymal origin. Subunit M, 53,000. The structure is similar to that of the a-keratins. See Keratins, Cytoskeleton (intermediate filaments). 

The cytoskeleton also contains different accessory proteins, which, in accordance with their affinities and functions, are designated as microtubule-associated proteins (MAPs), actin-binding proteins (ABPs), intermediate-filament-associated proteins (IFAPs), and myosin-binding proteins. This chapter is focused on those parts of the cytoskeleton that are composed of microfilaments and microtubules and their associated proteins. The subject of intermediate filaments is dealt with in detail in Volume 2. 

Thus far, microtubules and actin filaments and their associated proteins have been discussed to advantage as independent cytoskeletal components. In actual fact, all of the components of the cytoskeleton (including intermediate filaments) are precisely integrated with one another (Langford, 1995), as well as with various cytoplasmic organelles, the nuclear membrane, the plasma membrane, and the extracellular matrix. In its totality the cytoskeleton subserves many coordinated and regulated functions in the cell ... 

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... 

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. 

An intracellular fibrous system exists of filaments with an axial periodicity of 21 nm and a diameter of 8-10 nm that is intermediate between that of microfilaments (6 nm) and microtubules (23 nm). Four classes of intermediate filaments are found, as indicated in Table 49-13. They are all elongated, fibrous molecules, with a central rod domain, an amino terminal head, and a carboxyl terminal tail. They form a structure like a rope, and the mature filaments are composed of tetramers packed together in a helical manner. They are important structural components of cells, and most are relatively stable components of the cytoskeleton, not undergoing rapid assembly and disassembly and not... 

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. 

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. 

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. 

Although intermediate filaments are not universally associated with the cytoskeleton, neutrophils possess intermediate filaments of the vimentin type. Vimentin is a rod-shaped molecule of relative molecular mass 57 kDa that readily polymerises under physiological conditions to produce stable filaments 10-12 nm in diameter. Intermediate filaments are more robust than microfilaments and microtubules, and in neutrophils they form an open network of single filaments in the perinuclear space. 

Until the past decade, the cytoplasm was widely considered to be structurally unorganized with the main division of labor at the organellar level. Certainly, relatively little was known about the nature of the cyto-skeleton (with the notable exception of the mitotic apparatus and striated muscle), and the dynamics of cytoplasmic behavior were conceptualized vaguely in terms of sol-gel transitions without a sound molecular foundation. Substantial improvements in electron, light, and fluorescence microscopy, as well as the isolation of discrete protein components of the cytoskeleton, have led the way to a much better appreciation of the structural organization of the cytoplasm. Indeed, the lacelike network of thin filaments, intermediate filaments, and microtubules in nonmuscle cells is as familiar today as the organelles identified... 

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

The cytoplasm of eukaryotic cells is traversed by three-dimensional scaffolding structures consisting of filaments (long protein fibers), which together form the cytoskeleton. These filaments are divided into three groups, based on their diameters microfilaments (6-8 nm), intermediate filaments (ca. 10 nm), and microtubules (ca. 25 nm). All of these filaments are polymers assembled from protein components. 

Electron microscopy reveals several types of protein filaments crisscrossing the eukaryotic cell, forming an interlocking three-dimensional meshwork, the cytoskeleton. There are three general types of cytoplasmic filaments— actin filaments, microtubules, and intermediate filaments (Fig. 1-9)—differing in width (from about 6 to 22 nm), composition, and specific function. All types provide structure and organization to the cytoplasm and shape to the cell. Actin filaments and microtubules also help to produce the motion of organelles or of the whole cell. 

Cytoskeleton None Complex, with microtubules, intermediate filaments, actin filaments... 

Three principal components of the cytoskeleton are microfilaments of 6 nm diameter, microtubules of 23-25 nm diameter, and intermediate filaments of 10 nm diameter. A large number of associated proteins provide for interconnections, for assembly, and for disassembly of the cytoskeleton. Other proteins act as motors that provide motion. One of these motors is present in myosin of muscle. This protein is not only the motor for muscular work but also forms thick filaments of 12-16 nm diameter, which are a major structural component of muscle (see Fig. 19-6). 

Herrmann, H., Hesse, M., Reichenzeller, M., Aebi, U., and Magin, T. M. (2003). Functional complexity of intermediate filament cytoskeletons From structure to assembly to gene ablation. Int. Rev. Cytol. 223, 83-175. 

Foisner, R., Leichtfried, F. E., Herrmann, H., Small, J. V., Lawson, D., and Wiche, G. (1988). Cytoskeleton-associated plectin in situ localization, in vitro reconstitution, and binding to immobilized intermediate filament proteins. /. Biol. Chem. 106, 723-733. 

Franke, W. W., Hergt, M., and Grund, C. (1987). Rearrangement of the vimentin cytoskeleton during adipose conversion Formation of an intermediate filament cage around lipid globules. Cell 49, 131-141. 

Rezniczek, G. A., Pereda, J. M. d., Reipert, S., and Wiche, G. (1998). Linking integrin o 634-based cell adhesion to the intermediate filament cytoskeleton Direct interaction between the /34 subunit and plectin at multiple molecular sites. /. Biol. Chem. 141, 209-225. 

Styers, M. L., Salazar, G., Love, R., Peden, A. A., Kowalczyk, A. P., and Faundez, V. (2004). The endo-lysosomal sorting machinery interacts wtih the intermediate filament cytoskeleton. Mol. Biol. Cell. 15, 5369-5382. 

Svitkina, T. M., Verkhovsky, A. B., and Borisy, G. G. (1996). Plectin sidearms mediate interaction of intermediate filaments with microtubules and other components of the cytoskeleton. / Biol. Chem. 135, 991-1007. 

The cytosol is the soluble part of the cytoplasm where a large number of metabolic reactions take place. Within the cytosol is the cytoskeleton, a network of libers (microtubules, intermediate filaments and microfilaments) that maintain the shape of the cell. 

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... 

In addition to organelles, the cell cytoplasm contains actin filaments that make up the cellular cytoskeleton that controls shape. Myosin and a-actinin are also found in the cytoplasm and are believed to be involved in cell contraction. Other filaments including intermediate filaments, tubulin, calmodulin, and spectrin form networks within the cytoplasm that modify cell and organelle mobility and shape. 

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. 

---