Eukaryotes cytoskeleton

Although the building blocks of the eukaryotic cytoskeleton appear to be ancient, the protein domains interacting with it appear to have emerged more recendy. Several actin-binding domain families, namely calponin homology, CH, actin depolymerisation factor (ADF), the Sla2p... 

Amos LA, van den Ent F, Lowe J (2004) Structural/functional homology between the bacterial and eukaryotic cytoskeletons. Curr Opin Cell Biol 16 24-31 Andersson SG, Karlberg O, Canback B, Kurland CG (2003) On the origin of mitochondria a genomics perspective. Philos Trans R Soc Lond B Biol Sci 358 165-177 discussion 177-179... 

A three-dimensional meshwork of proteinaceous filaments of various sizes fills the space between the organelles of all eukaryotic cell types. This material is known collectively as the cytoskeleton, but despite the static property implied by this name, the cytoskeleton is plastic and dynamic. Not only must the cytoplasm move and modify its shape when a cell changes its position or shape, but the cytoskeleton itself causes these movements. In addition to motility, the cytoskeleton plays a role in metabolism. Several glycolytic enzymes are known to be associated with actin filaments, possibly to concentrate substrate and enzymes locally. Many mRNA species appear to be bound by filaments, especially in egg cells where they may be immobilized in distinct regions thereby becoming concentrated in defined tissues upon subsequent cell divisions. 

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. 

The eukaryotes these include animals, plants, fungi and protozoa, the DNA of which is enclosed in a membrane-enclosed organelle (the cell nucleus). They have a cytoskeleton (a fine membrane-like network in the interior of the cell, which provides stability) and contain mitochondria. Higher plants, as well as algae, are equipped with chloroplasts for photosynthesis. 

The cytoskeleton is one of several biological elements that define eukaryotic cells 124... 

Proteins of the cytoskeleton play a central role in the creation and maintenance of cell shapes in all tissues. They serve multiple roles in eukaryotic cells. First, they provide structural organization for the cell interior, helping to establish metabolic compartments. Second, cytoskeletal structures serve as tracks for intracellular transport, which creates and maintains differentiated cellular functions. Finally, the cytoskeleton comprises the core framework of cellular morphologies. 

A third type of bacterial toxin, diphtheria toxin, catalyzes the ADP-ribosylation of eukaryotic elongation factor (EFTU), a type of small G protein involved in protein synthesis (Table 19-2). The functional activity of the elongation factor is inhibitedby this reaction. Finally, a botulinum toxin ADP-ribosylates and disrupts the function of the small G protein Rho, which appears to be involved in assembly and rearrangement of the actin cytoskeleton (Table 19-2). These toxins maybe involved in neuropathy (see Ch. 36) and membrane trafficking (see Ch. 9). 

The family of eukaryotic Ras-like small GTPases may be divided into subfamilies, namely those of ARF, Rab, Ran, Ras, Rho, and Sar (ARF, RAB, RHO, RAS, RHO, SAR), which all contain representatives from fungi, plants, and metazoa. Consequently, these subfamilies and their cellular functions are likely to have emerged early in eukaryotic history. This implies that the last common ancestor of fungi, plants, and metazoa possessed vesicular transport (ARF and Sar), membrane trafficking (Rab), nuclear transport (Ran), signal transduction (Ras), and regulation of the actin cytoskeleton (Rho) functions. 

Hovanessian AG, Puvion-Dutilleul F, Nisole S, Svab J, Perret E, Deng JS, Krust B (2000) The cell-surface-expressed nucleolin is associated with the actin cytoskeleton. Exp Cell Res 261 312—328 Iftode C, Daniely Y, Borowiec JA (1999) Replication protein A (RPA) the eukaryotic SSB. Crit Rev Biochem Mol Biol 34 141-180... 

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. 

The cytoplasm of eukaryotic cells contains a complex network of slender rods and filaments that serve as a kind of internal skeleton. The properties of this cytoskeleton affect the shape and mechanical properties of cells. For example, the cytoskeleton is responsible... 

Some Proteins of Eukaryotic Plasma Membranes Are Connected to the Cytoskeleton... 

The cytoskeletons of other eukaryotic cells typically include both microtubules and microfilaments, which consist of long, chainlike oligomers of the proteins tubulin and actin, respectively. Bundles of microfilaments often lie just underneath the plasma membrane (fig. 17.22). They participate in processes that require changes in the shape of the cell, such as locomotion and phagocytosis. In some cells, cytoskeletal microfilaments appear to be linked indirectly through the plasma membrane to peripheral proteins on the outer surface of the cell (fig. 17.23). Among the cell surface proteins connected to this network is fibronectin, a glycoprotein believed to play a role in cell-cell interactions. The lateral diffusion of fibronectin is at least 5,000 times slower than that of freely diffusible membrane proteins. 

Some proteins of eukaryotic plasma membranes are connected to the cytoskeleton this connection inhibits their lateral mobility with the membrane. 

Actin. A protein found in combination with myosin in muscle and also found as filaments constituting an important part of the cytoskeleton in many eukaryotic cells. 

When microtubules were visualized by electron microscopy (EM), after the improvement of methods of fixation, it was realized that they formed the structural basis of flagellar axonemes and of so-called spindle fibers, as well as occurring as individual filaments in the cytoplasm. Their designation as part of the cytoskeleton suggested that they acted mainly as fixed structural supports. Subsequent research has focused more and more on their dynamic behavior and on their role as tracks for motor proteins, which may, for example, transport chromosomes during cell division. Microtubules are found in all eukaryotic cells and are essential for many cellular functions, such as motility, morphogenesis, intracellular transport, and cell division. It is that dynamic behavior that allows microtubules to fulfill all of these functions in specific places and at appropriate times in the cell cycle. 

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... 

Although the basic architecture of all eukaryotic cells is formed by membranes, organelles, and cytosol, each cell type exhibits a distinct morphology defined by cell shape and localization of organelles. The structural basis of the characteristic morphology of each cell type is the cytoskeleton, a dense network of three classes of filamentous proteins that permeate the cytosol and support the cell membrane. 

Taxol is a potent inhibitor of eukaryotic cell replication, blocking cells in the late G2, or mitotic, phase of the cell cycle. Interaction of Taxol with cells results in the formation of discrete bundles of stable microtubules as a consequence of reorganization of the microtubule cytoskeleton. Microtubules are not normally static organelles but are in a state of dynamic equilibrium with their components (i.e., soluble tubulin dimers). Taxol alters this normal equilibrium, shifting it in favor of the stable, nonfunctional microtubule polymer. 

---